Transcript: Materials & Metallurgy

The full text of the narration of all 24 chapters. It was generated automatically from the on-screen captions, so small errors are possible. Back to the video course

0. Map of the World of Materials

Here is a simple bracket. Same shape, same dimensions, same drawing. Now let's make it three times: in steel, in aluminum, and in titanium. The steel bracket weighs one kilogram. The same bracket in aluminum weighs about a third of that, and in titanium a little over half.

But weight is only the beginning. The three brackets will bend differently under the same load, react differently to heat and environment, cost very different amounts, and take a completely different path through the shop. The drawing is identical. The material changes everything. That is what this course is about. All engineering materials fall into four large families: metals, polymers, ceramics, and composites.

In this course we focus on metals, the materials most machined parts are made of. Metals are divided into two groups. Ferrous metals, where iron is the main element: all kinds of steel, and cast iron. And non-ferrous metals: aluminum, copper, titanium, nickel, and more. Steel is by far the most widely used metal in industry.

It is cheap, it is strong, and its properties can be changed over a very wide range with alloying elements and heat treatment. How do we compare materials? By their properties. Let's start with the mechanical properties, the ones that decide how a part behaves under load. Yield strength is the stress at which the material starts to deform permanently. Up to that point, when the load is removed, the part springs back. Beyond it, the part stays bent.

Tensile strength is the maximum stress the material can carry before it breaks. For most parts, yield strength is the more important number, because a part that has deformed no longer works. Ductility is the ability of a material to deform a lot before it breaks. It is measured as the elongation of a tensile specimen. Hardness is the resistance of the surface to indentation and wear.

It is measured by pressing an indenter into the material, using the Brinell, Rockwell, and Vickers methods. And the property people tend to forget: stiffness, measured by the modulus of elasticity. It tells you how much a part bends under load, long before it reaches yield. The modulus of steel is about 200 gigapascals, titanium about 110, and aluminum about 70.

And here is a fact that surprises many people: all steels, from the cheapest to the hardest tool steel, have almost the same stiffness. Heat treatment raises strength, but it does not reduce how much the part bends under load. If a bracket bends too much, replacing the steel with a stronger steel will not help. You need to change the shape, or move to a material with a higher modulus.

Another mechanical property is toughness: the ability to absorb energy and resist fracture, for example under impact. It is measured with the Charpy test. Here lies one of the basic trade-offs of metallurgy: the harder you make a material, the more brittle it usually becomes. A very hard steel resists wear very well, but it can crack under impact. A softer steel will bend, but it will not break. That is why, in the chapters on heat treatment, we will see that the goal is never the highest hardness, but the right combination for the job the part has to do.

Beyond the mechanical properties there are physical properties, which matter just as much when choosing a material. Density: steel is about 7.85 grams per cubic centimeter, titanium about 4.4, and aluminum about 2.7. Divide strength by density and you get specific strength, and here titanium and the strong aluminum alloys win. That is why they dominate aerospace. Thermal expansion: for the same temperature change, aluminum grows about twice as much as steel.

In a precision part, or an assembly that combines the two materials, this can be the difference between an assembly that works and one that jams. Thermal and electrical conductivity: copper and aluminum conduct very well, stainless steel and titanium far less. This affects how heat spreads through the part, and also how much heat stays in the cutting zone during machining. And finally, resistance to the environment: corrosion, high temperatures, and contact with chemicals.

A part that is strong enough on day one but rusts after a year is the wrong material choice. This is where metallurgy comes in. It answers the question: where do the properties of a metal come from? The answer is a chain of three links: the chemical composition, the internal structure, and the properties we measure. Composition decides what is possible. But processing, heat treatment, and cooling decide which internal structure actually forms, and the structure is what decides the properties.

An example: 4140 steel. The same bar, exactly the same composition. In the soft condition its hardness is about 200 Brinell, and it machines easily. After hardening it can reach more than 50 Rockwell C, and then it is very hard to machine with ordinary tools. The composition did not change. The structure inside the grains did.

That is why a material cannot be defined by its name alone. 4140 steel without a heat treatment condition is an incomplete specification, and so is 6061 aluminum without the T that follows it. And now the question every estimator asks: how much does it cost? Price per kilogram is the first part. Carbon steel is the cheapest. Aluminum and stainless steel cost several times more. Titanium and nickel alloys can reach twenty times as much per kilogram, or more.

But for a machined part, price per kilogram does not tell the whole story. First, an aluminum bar weighs a third of a steel bar of the same size, so the difference in material cost is smaller than it looks. Second, and usually the bigger factor: machining time. Aluminum is cut at very high speeds. Titanium and nickel alloys are cut at speeds several times lower, and they wear out tools quickly.

And there are more factors: availability in standard sizes, minimum order quantity, the heat treatment and coating the material needs, and the material certificates the customer requires. A good material choice weighs all of these together, not just the price per kilogram. So how do you actually choose a material? You work like a funnel. Start with the requirements of the part: loads, weight, operating temperature, environment, and the standards the customer requires.

From the requirements comes a short list of candidate materials, all of which can do the job. Then check manufacturing: is the material easy to machine, can it be welded, and what heat treatment and coating does it need. And finally, decide on cost and availability. Very often the best choice is not the strongest material, but the simplest material that meets all the requirements.

And here is the map of the course. We start with the fundamentals of metallurgy: crystal structure, properties and testing, phase diagrams, and the ways to strengthen a metal. Then we go through the different steels and their heat treatments, and cast iron. From there we move on to the non-ferrous metals: aluminum, copper, titanium, and the superalloys.

We will learn how materials fail in service, through corrosion and wear, and how the material affects machining, welding, and the form it is supplied in. And we finish with standards, material certificates, and choosing a material in practice, with examples of real parts. Because in the end, every good part starts with the right material.

1. Crystal Structure and Grains

When you look at a metal part, it seems uniform and solid. But magnify the surface a hundred times, and you see that it is built from thousands of tiny crystals packed tightly together. These are the grains. And if we zoom in much further, into a single grain, we see that the atoms inside are arranged in straight rows, in a pattern that repeats over and over. This order, the crystal lattice, is the foundation of everything we will learn in this course.

It explains why metal can be bent and machined, why heat treatment changes it, and why one steel shatters in the cold while aluminum does not. What holds the atoms together? In metals, the atoms release some of their electrons, and these electrons move freely among all the atoms, like a shared cloud. This is the metallic bond. It explains two properties that set metals apart from other materials. First, the free electrons conduct electricity and heat.

Second, the bond is not tied to one particular neighbor. When a row of atoms slides to a new position, it simply bonds to its new neighbors, and the material does not break. That is why metal can be bent, forged, and drawn, while glass or ceramics crack. Most engineering metals arrange themselves in one of three structures. The first is BCC, a cube with an atom at each corner and one atom in the center. Iron at room temperature, chromium, molybdenum, and tungsten have this structure.

The second is FCC, a cube with an atom at each corner and in the center of each face. Aluminum, copper, nickel, and austenitic stainless steel have this structure. The third is HCP, a close-packed hexagonal structure. Titanium at room temperature, magnesium, and zinc have this structure. And the structure has practical consequences. FCC has many planes the atoms can slide on, so these metals are very ductile, and they stay ductile even at very low temperatures.

In BCC, slip depends more on temperature, and in the cold it becomes difficult. That is why ordinary carbon steels can become brittle at low temperatures. HCP has few slip planes, so magnesium and zinc are hard to form cold, and titanium is also less formable than aluminum. lron has a special property, and the whole theory of steel heat treatment is built on it: it changes its crystal structure with temperature. Up to 912 degrees Celsius, iron is BCC.

Above 912 degrees it changes to FCC, and this structure is called austenite. Above 1,394 degrees it returns to BCC, all the way to melting at 1,538 degrees. This change happens in the solid state, without melting the part. And when the structure changes, so does the amount of carbon the iron can hold inside it.

This is exactly the transformation that hardening relies on, as we will see in chapter 6. Now for the important question: how does a metal deform? The natural assumption is that a whole plane of atoms slides at once over the plane beneath it. But if you calculate the force that would require, you get a strength a hundred times or more higher than what we actually measure.

The reason is defects in the lattice called dislocations. A dislocation is a row of atoms that is missing, or squeezed in, inside the lattice. Think of a heavy rug you want to move. Pulling the whole thing is very hard. But if you make a ruck in it and push the ruck along, the rug moves easily, a little at a time.

That is exactly how a dislocation moves: it shifts only one row of bonds at any moment, so the force needed is much smaller. The plastic deformation of a metal, every bend, every forging, and every chip, is the movement of billions of dislocations. From this comes the central idea of metal strength: to strengthen a metal, you have to make it harder for dislocations to move. One way to do that is deformation itself.

When a metal is deformed cold, the dislocations multiply, run into each other, and tangle, and every new dislocation finds it harder to move. This is work hardening. We all know it: bend a paper clip back and forth, it gets harder at the bend, and eventually it breaks there. This effect explains why stainless steel hardens under the cutting tool, and why bent sheet metal becomes harder along the bend line. And work hardening can be removed by heating, which rebuilds the grains.

Now back to the grains. When molten metal cools, crystals start growing at many points at once. Each crystal grows in its own direction, until it meets its neighbors. Each such crystal is a grain, and the line where two grains meet is a grain boundary. Inside each grain the rows are straight and orderly, but from grain to grain the direction changes.

And here is the point: a dislocation moving inside a grain reaches the boundary, and there the rows suddenly turn in another direction. The dislocation cannot keep going straight, and it stops. In other words, every grain boundary is a barrier. So grain size directly affects strength. The smaller the grains, the more boundaries there are, the more barriers, and the stronger the metal.

And there is a rare bonus here: a fine grain also improves toughness. It is almost the only way to strengthen a metal without paying for it in brittleness. Grain size is measured under the microscope and expressed as a grain size number, according to the standard ASTM E112. Note that the number works in reverse: a higher number means a finer grain. And grain size is not fixed. Long heating at high temperature makes the grains grow.

This happens, for example, in a heat treatment done at too high a temperature, or in the zone next to a weld. When sheet or bar is rolled, the grains and the inclusions in the material are stretched along the rolling direction. As a result, the properties are no longer the same in every direction. Along the rolling direction, ductility and toughness are better, and across it they are lower. So when bending sheet, it is better for the bend line to be perpendicular to the rolling direction.

A bend whose line runs parallel to the rolling direction can crack, especially with a small radius and a hard material. The same principle works in forging. In a forged part, the flow lines of the material follow the shape of the part. In a part machined from bar, the cutting slices through these lines. This is one reason why highly loaded critical parts, like crankshafts, are forged. When the direction matters, it is marked on the drawing.

What does all this mean in the shop? FCC material, such as austenitic stainless steel or aluminum, or a steel that has passed a low- temperature impact test. A part that was overheated in heat treatment can come out with coarse grains and low toughness, even when its hardness is fine. And what drives the cost? A grain direction requirement on the drawing limits how parts are nested on the sheet, so there is more scrap.

A forged part needs a die, and that only pays off in quantity. And a requirement for fine grain or a low-temperature impact test adds testing and certificates. In the next chapter we will meet the tools used to measure all these properties: the tensile, hardness, impact, and fatigue tests.

2. Mechanical Properties and Testing

Every material certificate and every serious drawing carries numbers: minimum yield strength, a hardness range, impact energy at a given temperature. Each one is measured in a defined test, to a standard, on a specimen of a defined size. To read them correctly, and to know what they guarantee and what they do not, you need to understand how each test works. In this chapter we go through the main tests: tensile, hardness, impact, fatigue, and creep.

The tensile test is the most basic of all. You take a round or flat specimen, mark a gauge length on it, and pull it slowly until it breaks, while measuring the force and the elongation. Divide the force by the cross-sectional area of the specimen and you get stress, in megapascals, which is newtons per square millimeter. At first the line is straight: this is the elastic region. Most metals have no sharp point where the line bends, so yield strength is defined by convention.

You draw a line parallel to the straight part, offset by 0.2 percent elongation, and the stress where that line meets the curve is the yield strength. On certificates it is marked Rp0.2. Soft structural steels sometimes have a sharp yield point, marked ReH. The highest point on the curve is the tensile strength, Rm. After it, the specimen starts to neck down in one spot, and shortly afterwards it breaks. You fit the two halves together, measure the gauge length, and get the elongation in percent, A.

You also measure the reduction of area at the fracture, Z. These two numbers describe the ductility. Now let's put a few curves on top of each other. Structural steel S355: a minimum yield of 355 megapascals, which is exactly what the number in its name says. After that it keeps stretching a long way before it breaks. Hardened steel: the curve climbs much higher, sometimes above 1,500 megapascals, but it breaks after very little elongation.

A strong aluminum alloy: medium strength, and a gentler slope because of its lower modulus. And one more important point: the numbers on the certificate are minimum values, and they were measured on a specimen from the same batch. Your part, after machining, welding, or heat treatment, is a different story. A hardness test is quick, barely damages the part, and is done on the part itself.

The principle is simple: you press a hard indenter into the material with a known force, and measure the impression it leaves. In the Brinell method you press a ball, usually ten millimeters in diameter, and measure the diameter of the impression. The impression is large, so the method suits coarse materials like castings and forgings, and gives a good average over many grains. The value is written HB, or HBW. In the Rockwell method you measure not the width of the impression but the depth of penetration, and the machine shows the number directly. It is the most common method on the shop floor.

It has several scales: HRC, with a diamond cone, for hardened steels, and HRB, with a small ball, for softer materials. In the Vickers method you press a diamond pyramid and measure the diagonals of the impression under a microscope. The same method covers a very wide range of hardness, and at low loads it can measure thin layers. The value is written HV. A few practical rules.

First, you can convert between the methods, but only approximately, using conversion tables from a standard such as ASTM E140. The tables are valid for steels, not for every material. WEVUVIIU, LEIS IS fo Gt el od al MATIIGALS TEIN WMELYVOCTE PG Uitmoo Gi OLLI yt f. PEE OLS lo, LEIS LETIOHI oOtitll yee Lie megapascals is about 3.4 times the Brinell value, so a hardness check quickly confirms that a heat treatment worked. Third, the part must be thick enough: in Rockwell, the thickness should be at least ten times the depth of the impression, otherwise the anvil underneath affects the result. And you measure far enough from the edge, on a clean, smooth surface.

And fourth, on a part that has been surface hardened, a measurement on the surface gives only the hardness of the case. Its depth is measured on a cross-section, with a series of Vickers readings from the surface inward. The impact test checks something the tensile test cannot see: how the material behaves under a blow, when it has a notch. The standard Charpy specimen is a bar ten by ten millimeters, 55 millimeters long, with a V-shaped notch in the middle.

A heavy pendulum is released from a known height and breaks it in one blow. From the height the pendulum loses, you calculate the energy the specimen absorbed, in joules. The result depends strongly on temperature, so it is always stated. In carbon steels, as the temperature drops, the energy stays high down to a certain range, and there it collapses. This is the transition from ductile to brittle behavior that we met in chapter 1.

You can also see the difference on the fracture surface: a ductile fracture is dull and fibrous, a brittle fracture is bright and crystalline. A familiar example: in S355J2 steel, the letter J and the digit 2 mean that at least 27 joules are guaranteed at minus 20 degrees. Now to the most common failure in machine parts: fatigue. A part that works under a load that repeats again and again, like a rotating shaft, a spring, or a bolt in an engine, can break at a stress far below its yield strength.

It does not happen all at once. A tiny crack starts at one point, grows a little with every load cycle, and finally the remaining section can no longer carry the load, and the part breaks. On the fracture surface you see beach marks, curved lines spreading out from the origin, and a final rough zone where the part finally broke. In a fatigue test you count how many cycles each specimen survives at each load, and you get an S-N curve. Steels usually have a fatigue limit, about half the tensile strength for a polished specimen, and below it the part lasts almost indefinitely.

Aluminum has no such limit, so it is designed for a defined number of cycles. PMID REID PEEUOR PEPER CHEER POWEEER EME REINS OLIN. & PGLIYUS VIGUN GHIVOL GIWVGYo VIG ho GL LIS OUIIGUL, Gh usually at a stress concentration, such as a sharp corner, a groove, the start of a thread, or a deep tool mark. PMID REID PEEUOR PEPER CHEER POWEEER EE REIS OPI. & PCRIQUS VIGUNR GHIVOL GIG Yo ViGilo GL LIS OUIIGULY, Gh usually at a stress concentration, such as a sharp corner, a groove, the start of a thread, or a deep tool mark. That is why a corner radius, a good surface finish, and shot peening greatly extend fatigue life.

The last test is creep. At high temperature, a metal under a constant load keeps stretching slowly over time, even when the stress is below its yield strength. At room temperature this hardly happens in steel. But from around 400 degrees upward, and in aluminum at a few hundred degrees less, it becomes a design consideration. In a creep test a specimen is held in a furnace under a constant load, for hundreds and thousands of hours, and you measure the rate of elongation and the time to rupture.

The problem is familiar in places like turbines, furnaces, steam piping, and hot flange bolts. This is exactly where the nickel alloys fit, which we will meet in chapter 15. When material arrives, the certificate shows the test results, but a quick hardness check at receiving is the cheap way to confirm you got what you ordered. A heat-treated part is checked for hardness, and sometimes you need to leave a dedicated test surface for that, one that will not be machined. And on a part that works in fatigue, the radii and surface finish on the drawing are not cosmetic, they are part of its strength.

And what drives the cost? Low-temperature impact tests, or tests taken from a specific part, need specimens, a lab, and time, and a hardness check on every part adds time to every unit. And fatigue requirements bring finer finishes, precise radii, shot peening, and crack inspection. To sum up: tensile for strength and ductility, hardness for a quick check, impact for blows and cold, and fatigue and creep for what happens over time. In the next chapter we go back inside the material, and learn to read the map that explains where these properties come from: phase diagrams.

3. Alloys and Phase Diagrams

Almost no metal used in industry is pure. Pure iron is very soft, and you can bend a bar of it by hand. But add less than one percent of carbon and you get steel, several times stronger. A metal mixed with other elements to change its properties is called an alloy. In this chapter we will see what happens to the atoms when elements are mixed, and learn to read the map that shows which structure forms at each composition and each temperature: the phase diagram.

When an element is added to a metal, the simplest way it can enter is as a solid solution. There are two kinds. HEE CE OUNMOLICULIVEIGH OUIULIUEE, SEIS PUP EUTE CARWIEEE AGAR LEIS MICU UE ETE RCE CARWIETE FEE REINS PCARRIUN. TE ENO happens when the atoms are similar in size, for example nickel in copper, or chromium and nickel in iron. HEE CE OUNMOLILULIVEIGH OUIUILIUEE, LEIS PUP EUTE CARWEEE CGA LEIS MICU UE GE TEI ACHE CARWIETE TEE REINS TCARRIUN. TET happens when the atoms are similar in size, for example nickel in copper, or chromium and nickel in iron. In an interstitial solution, a very small atom squeezes into the gaps between the metal atoms. That is how carbon, nitrogen, and hydrogen enter iron.

In both cases the foreign atom distorts the lattice around it. And as we learned in chapter 1, every such distortion makes it harder for dislocations to move. This is solid solution strengthening, and it is the first reason an alloy is stronger than the pure metal. But every metal has a limit to how many foreign atoms it can hold in solution. In iron at room temperature this limit is tiny: less than three hundredths of a percent of carbon.

What happens to the extra carbon? It forms a new structure, with its own composition and lattice: a compound of iron and carbon called cementite. Cementite is very hard and very brittle. A region of the material with a uniform structure and a uniform composition is called a phase. Most alloys contain more than one phase, and the amount, shape, and distribution of the phases are what decide the properties. To know which phases will form, we use a phase diagram. The horizontal axis is composition, and the vertical axis is temperature.

Each region of the diagram shows which phases are stable at that composition and temperature, when cooling is slow enough. Let's start with the simplest example: copper and nickel, which dissolve in each other in any proportion. At the top everything is liquid, and at the bottom everything is solid. Between the two lines there is a region where liquid and solid exist together. Let's follow one alloy as it cools: it starts to solidify when it crosses the upper line, and finishes only when it crosses the lower line.

In other words, an alloy does not solidify at a single temperature like a pure metal, but over a range of temperatures. For many pairs of elements the diagram looks different. Let's look at aluminum and silicon. When silicon is added to aluminum, the melting temperature drops, down to one point where it is lowest: 12.6 percent silicon, and 577 degrees. This point is called the eutectic point. At this composition the alloy solidifies at a single temperature, like a pure metal, and flows very well.

That is why most aluminum casting alloys are close to it. And the same idea is behind soft solders, which were chosen at the composition that melts at the lowest temperature. And now the most important diagram in the course: iron carbon. The horizontal axis is percent carbon, and the vertical axis is temperature. Let's meet the regions. At the top, at high temperatures, is austenite: iron in the FCC structure we met in chapter 1, which can hold up to 2.1 percent carbon.

At low temperatures is ferrite: iron in the BCC structure, which holds almost no carbon, so it is soft and ductile. An important horizontal line runs at 727 degrees: below it, austenite is no longer stable. And at one point on this line, 0.77 percent carbon, the austenite transforms all at once into a new structure. Finally, the boundary at 2.1 percent carbon divides the diagram in two: steels on the left, and cast iron on the right.

Most engineering steels sit in a small corner on the left side, between zero and one percent carbon. What happens when steel cools slowly from austenite? At 0.77 percent carbon, the austenite breaks down into thin alternating layers: soft ferrite and hard cementite. This structure is called pearlite, because under the microscope it shines like mother of pearl.

In a steel with less carbon, for example a steel with 0.45 percent, part of the austenite first turns into ferrite, and only the rest turns into pearlite. The result is a mixture: roughly half ferrite and half pearlite. The more carbon there is, the more pearlite, and the stronger and harder the steel, and the less ductile. And in a steel with more than 0.77 percent carbon, like bearing steels and some tool steels, the extra carbon forms cementite at the grain boundaries.

But what if we don't give the steel time? If it is cooled from austenite very quickly, for example by quenching in water or oil, the carbon has no time to leave the lattice and form cementite. It stays trapped, and the lattice is forced into a distorted, strained structure. This structure is called martensite, and it is very hard and very brittle. Note that martensite does not appear on the phase diagram, because the diagram describes slow cooling, and martensite forms precisely when there is no time.

relatively low, and as carbon is added it rises, up to about 0.6 percent, and from there it adds almost nothing. This explains a choice every designer knows: a steel with 0.45 percent carbon hardens well, and a steel with 0.2 percent hardly at all. What does this mean in the shop? The carbon content is the first number that tells you what can be done with a steel. A low carbon steel will hardly harden in an ordinary hardening treatment. If you need a hard surface, you move to surface hardening, which we will meet in chapter 7.

Heat treatment temperatures are set from the diagram: you heat above the line where the steel becomes fully austenite, and that is where every hardening process starts. And in welding, the more carbon there is, the more the zone next to the weld can cool quickly, turn into brittle martensite, and crack. And what drives the cost? Alloying elements like nickel and molybdenum are far more expensive than iron. A requirement for a chemical composition in a narrow range limits the suppliers and the heats. And every heat treatment that makes use of the diagram adds a step, time, and inspection.

To sum up: an alloy is stronger than the pure metal thanks to solid solution and additional phases. The phase diagram shows what forms on slow cooling. With slow cooling, steel becomes ferrite and pearlite, and with fast cooling, martensite. In the next chapter we will collect all the ways to strengthen a metal: cold work, fine grain, precipitation hardening, and the martensitic transformation.

4. Strengthening Mechanisms

In chapter 1 we saw that a metal deforms when dislocations move through it. From this comes one principle that explains every way of strengthening a metal: put obstacles in the way of the dislocations. There are four main kinds of obstacle. Foreign atoms in solid solution, which we met in chapter 3. Other dislocations that tangle with each other. Grain boundaries. And tiny particles of a second phase. In this chapter we go through each one, and also see what undoes it.

The simplest way to strengthen a metal is to deform it cold. In cold rolling, drawing through a die, or bending, the dislocations multiply and tangle, and the metal becomes stronger and harder. This is work hardening. That is how cold-drawn bars, hard stainless steel sheet, and springs are made. But there is a price: the more cold work, the lower the ductility.

In aluminum, the letter H in the temper designation means work hardening. H14 is half hard, and H18 is full hard. Work hardening has two more sides worth knowing. The first: it can be reversed. Annealing, meaning heating to a suitable temperature, lets new, soft grains form in place of the deformed ones. This is recrystallization, and the metal returns to the soft condition.

The second: cold work leaves residual stresses in the material. When material is removed by machining, the stresses are released unevenly, and the part can warp. That is why a long, slender part machined from cold-drawn bar sometimes needs a stress relief treatment, or machining in stages. The second way is grain refinement. As we saw in chapter 1, every grain boundary stops dislocations, so a finer grain gives higher strength. And unlike the other methods, it improves toughness too.

In modern steels, a fine grain is achieved with tiny additions, hundredths of a percent, of niobium, vanadium, or titanium, together with rolling at a controlled temperature. The result is strong structural steels that are still easy to weld. The third way is precipitation hardening, and it is the secret behind the strong aluminum alloys. The process has three steps. In the first step, solution treatment, the alloy is heated to a high temperature, where the alloying elements dissolve completely in the metal.

In the second step it is cooled quickly, usually in water, so the elements stay trapped in solution. The third step is aging: heating to a moderate temperature for a few hours, and tiny precipitates begin to form. The tiny precipitates are spread throughout the grains, and each one is a barrier to dislocations. Strength rises with aging time, up to a peak.

If heating goes on too long, the precipitates grow, there are fewer of them, and strength falls. This is overaging. In aluminum, the temper T6 means solution treatment followed by artificial aging. T4 means natural aging, at room temperature. The difference is huge: 6061 aluminum in the soft condition, O, starts to yield at around 55 megapascals. In T6 it reaches 276. The same mechanism works in 17-4PH stainless steel, the alloy Inconel 718, and maraging steels.

The fourth way, and the strongest of all, is the martensitic transformation, which we met in chapter 3. Steel is cooled quickly from austenite, and the trapped carbon distorts the lattice so much that dislocations can hardly move. This is how strengths of 2,000 megapascals and more are reached. But fresh martensite is too brittle to use, so it is always tempered: heated again to a moderate temperature, giving up some hardness in exchange for toughness. A familiar example: 4140 prehard steel, which arrives already hardened and tempered, at about 30 Rockwell C, and can be machined directly.

And now the practical question: what undoes each mechanism? Work hardening disappears with annealing, and therefore also with welding. Grain refinement is damaged by long heating at high temperature, which makes the grains grow. Precipitation hardening is damaged by any heating that causes overaging, or dissolves the precipitates again. That is why, when 6061-T6 is welded, the zone next to the weld loses a large part of its strength. And martensite softens with any heating above its tempering temperature.

In other words, every heating a part goes through after heat treatment can change its properties. What does this mean in the shop? Before welding, hot bending, or a coating applied with heat, check how the material got its strength. And what drives the cost? Precipitation hardening, and hardening and tempering treatments, need controlled furnaces and close control of times and temperatures. Distortion after heat treatment requires extra stock for finish machining.

And cold-drawn bars cost more than hot-rolled bars, but they arrive at more precise dimensions with a better finish, so they sometimes save machining. To sum up: four mechanisms, and one principle, stop the dislocations. In the next chapter we start going through the families of materials, beginning with the most important of all: carbon steels.

5. Carbon Steels

Carbon steel is the most basic and most widely used steel: iron, carbon, and a little manganese. The manganese ties up the sulfur and improves strength. It also contains small amounts of phosphorus and sulfur, left over from the production process and limited by the standard. It has no deliberate alloying elements such as chromium, nickel, or molybdenum. That is why it is the cheapest, and its properties are set mainly by the amount of carbon and the heat treatment.

Carbon steels are divided into three families by carbon content. Low carbon: up to about 0.3 percent. Medium carbon: from 0.3 to 0.6 percent. And high carbon: above 0.6 percent, up to about one percent. As we saw in chapter 3, the more carbon there is, the stronger and harder the steel, and the better it hardens, but also the less ductile and the less weldable. Low carbon steel, also called mild steel, is the workhorse of industry.

It is used for sheet, pipe, sections, and beams, and also for simple machined parts. Familiar examples are 1018 and 1020, and the structural steels S235 and S355. It is easy to weld, bend, and machine, but it hardly hardens in an ordinary hardening treatment. When it needs a hard surface, only the surface is hardened, as we will see in chapter 7. Medium carbon steel is the choice for machine parts: pins, shafts, gears, and anchor bolts.

Its best-known representative is 1045, called C45 in the European standard. It hardens well. You can harden and temper the whole part, or induction harden only the area that wears, for example a bearing seat on a shaft. But welding needs care: because of the carbon, the zone next to the weld can harden and crack, so preheating is usually needed. High carbon steel, such as 1060 and 1095, is used for springs, blades, and high-strength wire.

After hardening it is very hard and wear resistant. On the other hand, it is more brittle, harder to machine in the hardened condition, and it is almost never welded. Now for the names. In the American system, AISI and SAE, a steel has a four-digit number. The first two digits tell you the type of steel: 10 is plain carbon steel, 11 and 12 are free-machining steels, with added sulfur.

The last two digits are the carbon content in hundredths of a percent. So 1045 is a plain carbon steel with 0.45 percent carbon. In the European standard, C45 is the same steel, and C45E is the version with controlled sulfur, suited to hardening. In the past, under DIN, it was called Ck45. In Japan it is called S45C. Every steel also has a European material number, and for C45 it is 1.0503.

Structural steels are named differently. In S355J2, the letter S means structural steel, 355 is the minimum yield strength in megapascals, and J2 is the impact requirement we met in chapter 2. Under the old DIN names, structural steels were called St37 and St52, and you still hear these names on the shop floor. There are also carbon steels designed specifically for fast machining, such as 12L14 and 1215. They contain added sulfur, and sometimes lead. The additions break the chip into short pieces and reduce friction, so you can cut faster, with less tool wear and a good finish.

They are very common on automatic lathes. The price is lower strength and ductility, and poor weldability, so they are not suitable for heavily loaded parts. We will come back to this in chapter 19. What does this mean in the shop? Carbon steel rusts, so it almost always needs a coating or paint. The material arrives hot rolled, with an oxide layer, or cold drawn, at precise dimensions.

And what drives the cost? Carbon steel itself is the cheapest. What adds cost is heat treatment, a low- temperature impact requirement with a certificate, and cold-drawn or ground material at a precise size. Sometimes a free-machining steel, slightly more expensive per kilogram, is the one that saves the most machine time. In the next chapter we will learn the heat treatments of steel: annealing, normalizing, hardening, and tempering.

6. Heat Treatment of Steel

In chapter 3 we saw that the internal structure of steel depends on how it is heated and cooled. Heat treatment is built on exactly that. The four basic heat treatments of steel are annealing, normalizing, hardening, and tempering. All of them have three stages: heating to a certain temperature, holding there, and cooling at a certain rate. The main difference between them is the cooling rate. In annealing, the steel is heated above the line where it turns into austenite, held there, and cooled very slowly, inside the furnace.

The result is the softest condition: coarse pearlite and ferrite. Annealing is used to make machining or cold forming easier, and to remove the hardening left by cold work. In high carbon steels, spheroidize annealing is used. It turns the cementite into small spheres, and that is the easiest condition for machining them. And there is also stress relieving: heating below the transformation temperature. It does not change the structure, it only lowers the internal stresses, for example after welding or heavy machining.

Normalizing is similar to annealing, but the cooling is done in still air, outside the furnace. Cooling is faster, so the pearlite is finer, and the grain is small and uniform. The steel comes out somewhat stronger and harder than after annealing. Normalizing is usually done after forging or hot rolling, to even out the structure before machining or before hardening. In hardening, the steel is heated until it has turned completely into austenite, and then cooled very fast, by quenching. As we saw in chapter 3, rapid cooling produces martensite.

It is quenched in water, oil, polymer, or gas. Water cools fastest, so it gives the highest hardness, but also the highest risk of distortion and cracking. Oil cools more slowly, and is safer for complex parts. Martensite forms only where cooling is fast enough. At the surface, cooling is fastest, and in the middle of a thick section it is slower. In a carbon steel like 1045, in a thick bar, only an outer layer hardens, and the core stays soft.

The ability of a steel to harden in depth is called hardenability, and it is improved by alloying elements, as we will see in chapter 8. Fresh martensite is very hard, but too brittle. So after hardening, the steel is tempered: heated again to a temperature below the transformation line, and held there. The higher the tempering temperature, the lower the hardness, and the higher the toughness. Low tempering, around 200 degrees, keeps the hardness high, and suits tools and bearings.

High tempering, around 550 to 650 degrees, gives good strength and high toughness, and suits shafts and bolts. Hardening followed by high-temperature tempering is called quenching and tempering. And it is important to temper right after hardening, before the part cracks from the stresses. Hardening is the most dangerous treatment for a part. Steel expands when it turns into martensite, and the part can distort or crack. Exposed sections cool first, and that creates uneven stresses. The risk is especially high with uneven sections, sharp corners, and holes close to an edge.

Another problem: when heated in air, the surface loses carbon and stays soft. So extra stock is left for grinding after heat treatment, or the part is heated in a protective atmosphere or a vacuum. What does this mean in the shop? On the drawing you write a hardness range, for example 40 to 45 Rockwell C, not just "hardened". Design uniform sections and corner radii, and leave stock for finish machining after heat treatment.

And what drives the cost? Every heat treatment is another step, with furnace time and transport to the subcontractor. A protective atmosphere or vacuum costs more than an ordinary furnace. And straightening, grinding after distortion, and hardness checks add more. In the next chapter we will meet a way to harden only the surface: carburizing, nitriding, and induction hardening.

7. Surface Hardening

Many parts have two requirements that seem to contradict each other. The surface has to be very hard, to resist wear and contact, for example on gear teeth or a bearing seat. But the part as a whole has to be tough, so it does not break under impact. The solution is to harden only the surface, and keep a tough core.

And there is a bonus: the hardened case stays in compression, and that greatly improves the fatigue life of the part. The most common method is carburizing. In carburizing, a low carbon steel is heated in a carbon-rich atmosphere, at around 900 to 950 degrees. At this temperature the steel is austenite, and carbon slowly diffuses into the surface, up to about 0.8 percent.

The longer it is held, the deeper the case, usually half a millimeter to two millimeters. Then it is quenched and tempered at a low temperature. The result: a surface of about 60 Rockwell C, and a soft, tough core. Steels such as 8620 and 16MnCr5 are used. Carburizing is the choice for gears, pins, and bearing races.

When part of the component must stay soft, for example a thread, the areas that must stay soft are masked, for example with copper plating or a special paste. Because there is a quench, there is also distortion. So stock is left for grinding, but not too much. The hardened case is thin, and if too much of it is removed, it is lost. Nitriding works differently.

Nitrogen diffuses into the surface at a low temperature, around 500 to 570 degrees, and forms hard nitrides with elements such as chromium, aluminum, and molybdenum. There is no quench, so there is almost no distortion, and the part comes out almost at final size. The case is thin, a few tenths of a millimeter, but very hard. The part has to be hardened and tempered beforehand, at a tempering temperature higher than the nitriding temperature.

A white layer forms on the surface, hard and brittle. Sometimes it is wanted, and sometimes it is removed. The drawback: the process is long, sometimes tens of hours. In induction hardening, a coil heats only the surface, within seconds, and immediately afterwards water is sprayed on. It suits medium carbon steels, such as 1045 and 4140, which harden on their own without adding carbon.

You can harden only the area that needs it, for example a bearing seat or gear teeth, and the case depth is set by the frequency of the current. Flame hardening works on the same principle, with a burner, and suits large parts. So which one do you choose? Carburizing gives a deep, strong case in a low carbon steel, but with distortion and grinding.

Nitriding gives a thin, very hard case, with almost no distortion, but it is a long process. And induction is fast and local, for medium carbon steel, and pays off mainly in quantity, because each part needs its own coil. VVEIGL UVES LO TPIS GEE TE AIS OLIV. WEE LEI Miawvinty yjvyu vepevlly LEIS VEO VME YEE GI Ha UliNoeo, IVI CAGIIIVILS an effective case depth of 0.8 to 1.2 millimeters, at 58 to 62 Rockwell C, and you mark where hardening is allowed. VVEIGL UVES LO THIS GEE TE REIS OLIN. WEE LEI Miavvinty yjvyu yepevlly EIS VEO VEY GI Pa UliNoeo, IVI CAGIIIVIS an effective case depth of 0.8 to 1.2 millimeters, at 58 to 62 Rockwell C, and you mark where hardening is allowed.

WVEIGL UVES LO TPIS GEE TE REIS OLIV. WEE hI Miarvinty yjvu epevlily LEIS VEO VME YEE GI Ha UliNoeo, IVI CAGIIIVIS an effective case depth of 0.8 to 1.2 millimeters, at 58 to 62 Rockwell C, and you mark where hardening is allowed. Machine before surface hardening, and grind after it. And what drives the cost? Long furnace cycles, masking areas that must stay soft, grinding a hard part, and for induction, a dedicated coil for each part. In the next chapter we will meet the alloy steels, and the elements that give them their properties.

8. Alloy Steels

In chapters 5 to 7 we saw what carbon steel can do: it is cheap, easy to work, and hardens well when it has enough carbon. But it has four limitations. It hardens only to a shallow depth, so in a thick section the core stays soft. To harden it you need very fast cooling, in water, and that increases the risk of distortion and cracking. In the cold it becomes brittle. And at high temperature it softens quickly.

Alloy steels are designed to solve exactly these problems. In this chapter we deal with low alloy steels, where the total of the alloying elements is usually up to about five percent. To understand alloy steels, you have to separate two ideas that are easily confused. Hardness is how hard the martensite is, and as we saw in chapter 3, it is set almost entirely by the carbon. Hardenability is how deep the part hardens, and that is exactly what the alloying elements improve.

Take two thick bars with the same carbon content, one of carbon steel and one of alloy steel, and harden them both. At the surface both will reach roughly the same hardness. But in the carbon steel the hardness drops off a few millimeters in, while in the alloy steel it stays high deep into the section. Hardenability is measured with the Jominy test. You take a bar 25 millimeters in diameter, heat it to austenite, and spray a jet of water on one end only.

That end cools very fast, and the further away you go, the slower the cooling. Then you measure hardness at points along the bar, and you get a curve. In 1045 steel the curve drops after just a few millimeters. In 4140 steel it stays high for much longer. This curve tells the designer what diameter can be hardened right through to the core, and with which quench. Why does this happen? To understand it, we use the continuous cooling diagram, CCT. It shows which structure will form at each cooling rate.

The horizontal axis is time, and the vertical axis is temperature. In the middle of the diagram there is a nose-shaped region: if the cooling curve crosses it, pearlite or other soft structures form. If cooling is fast enough to get around the nose, martensite forms. In a carbon steel the nose is very close to the axis, so very fast cooling is needed. Alloying elements delay the breakdown of austenite and push the nose to the right, to longer times.

So even slower cooling, in oil or even in air, is enough to get martensite, even in the middle of a thick section. Now let's meet the elements, one by one. Manganese improves hardenability, and ties up the sulfur. Chromium improves hardenability, and forms hard, wear-resistant carbides. Above 12 percent it turns the steel into stainless steel, as we will see in chapter 10. Nickel greatly improves toughness, especially in the cold.

Molybdenum greatly improves hardenability, reduces sensitivity to temper embrittlement, and improves strength at high temperature. Vanadium refines the grain and forms very hard carbides. Silicon strengthens the ferrite, and is common in spring steels. And boron, in tiny amounts, thousandths of a percent, greatly improves hardenability, which is why it is used in hardened bolts. In the American system, the first two digits tell you which elements the steel contains.

41 is chromium molybdenum. 43 is nickel chromium molybdenum. 86 is nickel chromium molybdenum in smaller amounts. 51 is chromium. And the last two digits are, as in carbon steel, the carbon content in hundredths of a percent. In the European system, the name starts with the carbon content times a hundred, followed by the element symbols. In the name 42CrMo4, the number 42 means 0.42 percent carbon.

The 4 at the end belongs to the first element, chromium, and is divided by four, meaning one percent chromium. Molybdenum appears in the name, but without a number of its own. The factor for chromium, manganese, and nickel is four, and for molybdenum and vanadium it is ten. The two most common alloy steels in the shop are 4140 and 4340. 4140, called 42CrMo4 in Europe, is the workhorse: pins, shafts, gears, and tool holders.

It also comes prehardened, quenched and tempered to about 30 Rockwell C, as we saw in chapter 4. 4340 adds about 1.8 percent nickel. The nickel gives higher toughness and deeper hardenability. So it is the choice for thick, critical parts, such as crankshafts, heavy shafts, and aerospace parts. In Europe its closest equivalent is 34CrNiMo6. P¥¥Y TEIVES OL Vio Y¥YVECE AS te vvlliy. VUVeYV IVI VaIivul tani, VWV¥EtiVii WYN PEIwe thi Vi aver 7, CIM VLiVY, iit bearing steel, with one percent carbon and one and a half percent chromium, which reaches a hardness above 60 Rockwell C.

P¥¥Y PEIVVES OL Vio YYVECE IN Mh vviliyy. VUVeYV IV VaIivul t2tiy, WVEtiwil VW¥N FEIWe hl Vi Mmapveel 7, CIM VLiVY, iit bearing steel, with one percent carbon and one and a half percent chromium, which reaches a hardness above 60 Rockwell C. Two things to know about tempering and welding alloy steels. The first: in certain temperature ranges, tempering actually lowers the toughness. This is temper embrittlement. So tempering in these ranges is avoided, and steels with molybdenum are chosen when the part may be exposed to them.

The second: the alloying elements that improve hardenability also act during welding. The zone next to the weld cools fast, turns into martensite, and can crack. To estimate the risk, you calculate the carbon equivalent: the carbon content, plus a weighted share of the other elements. The higher the carbon equivalent, the harder the welding, and the more you need preheating and controlled cooling. In 4140 it is high, so welding it needs a proper procedure. We will come back to this in chapter 20.

What does this mean in the shop? In thick sections, never replace 4140 with 1045 just because it is cheaper. After hardening, the core will stay soft. Prehardened steel saves the heat treatment after machining, and with it the distortion and the grinding. But it machines more slowly, and tools wear faster. And what drives the cost? Nickel and molybdenum are expensive, and their prices fluctuate. Alloy steel costs more than carbon steel, and a nickel steel like 4340 costs even more. Welding with preheat, and a certificate requirement with composition and an impact test, add cost.

To sum up: carbon decides how hard the steel can get, and the alloying elements decide how deep, with which quench, and how much toughness remains. In the next chapter we will meet the family where the alloying elements are taken to the extreme: tool steels.

9. Tool Steels

Tool steels are the steels used to machine, form, and cut every other material: dies, punches, die- casting and plastic molds, drills, and taps. They have to combine three properties that are hard to combine: high hardness, wear resistance, and enough toughness not to break. To achieve that, they contain a lot of carbon, and alloying elements such as chromium, molybdenum, tungsten, and vanadium, which form very hard carbides. Each series of tool steels balances these properties differently, according to its use.

In the American system, each series is marked by a letter. W is water hardening. O is oil hardening. A is air hardening. And D is high carbon, high chromium. These four series are cold work steels. S is for shock resistance. H is for hot work. M and T are high speed steels for cutting tools. And P is for plastic molds. The most common steels from these series are 01, A2, D2, S7, H13, M2, and P20.

Let's start with the cold work steels. 01 is the simplest: oil hardening, easy to machine, and suited to simple tools and gauges. A2 contains about five percent chromium, and hardens in air. The slow cooling gives very little distortion, so it suits precision tools. ML VV GMUUL VEIS CEI CE TIGHT PEE UNEER VCHERIUTE CALI LYRIS PEPUSEIR WETEUEEEIUATEE. PEIN WCHERIUEE CAE chromium form a large amount of coarse chromium carbides, and that gives it excellent wear resistance. ML VYING GNU VII GI CE PIGEEE PEE UREIR VCERIUEE CALI YVR IV DR PEPUSEIR WETEUEEEIUAIEE. FEIN WCHERUEE Cts chromium form a large amount of coarse chromium carbides, and that gives it excellent wear resistance.

That is why it is the choice for blanking dies and shear blades. The price is lower toughness, and difficult grinding. S steels, such as S7, were deliberately designed with less carbon. They reach a lower hardness, around 55 Rockwell C, but withstand repeated impact without cracking. That is why punches, chisels, and hammers are made from them. This is exactly the trade-off we saw in chapter 0, between hardness and toughness, only here it is chosen on purpose.

H steels are designed for hot work: die-casting dies, forging dies, and extrusion dies. The part is exposed again and again to heating and cooling, and the surface expands and contracts with every cycle. Eventually a network of fine cracks forms: thermal fatigue. H13, with about five percent chromium, plus molybdenum and vanadium, keeps its strength when hot, and resists these cracks well. It usually works at a hardness of 44 to 50 Rockwell C. And because of these properties, it is also used for demanding plastic molds.

High speed steels, such as M2, contain tungsten, molybdenum, chromium, and vanadium. Their key property is that they keep their hardness when hot, even when the cutting edge gets very hot during cutting. They reach a hardness of 62 to 65 Rockwell C, and for decades they were used for all cutting tools. Today most cutting tools are made of cemented carbide, not steel. Cemented carbide is harder and more heat resistant, but more brittle and more expensive.

High speed steels are still common in drills, taps, special tools, and interrupted cutting. The heat treatment of tool steels is different too. In highly alloyed steels, such as D2, H13, and M2, something interesting happens: when tempered at around 550 degrees, the hardness rises again. This is secondary hardening. It occurs when tiny carbides of vanadium, molybdenum, and tungsten precipitate from the martensite.

In addition, after hardening, some of the austenite has not turned into martensite. This retained austenite is unstable, and can later turn into martensite and change the dimensions. So they are tempered two or three times, to turn the retained austenite into martensite, and to temper that too. And the hardening itself is usually done in a vacuum furnace, so the surface does not lose carbon. In conventional tool steels, the carbides form while the large ingot solidifies. They are coarse, and lined up in rows along the rolling direction.

In powder metallurgy steels, the molten steel is sprayed into tiny droplets that solidify quickly into powder, and the powder is then pressed under heat and pressure into a solid block. In powder steels the carbides are small and evenly distributed. The result: better toughness, easier grinding, and the ability to add much more vanadium and wear resistance without losing toughness. The price: they are far more expensive. In Europe, tool steels are known mainly by their material number. Numbers starting with 1.2 are tool steels, and with 1.3, high speed steels.

D2 is approximately 1.2379. H13 is 1.2344. A2 is 1.2363. 01 is 1.2510. M2 is 1.3343. And P20 is 1.2311. These numbers are worth knowing, because orders and certificates use them as much as the American names. Note that the match is not always exact, so for a critical part, compare the composition.

What does this mean in the shop? Tool steels arrive in the soft, annealed condition, at around 200 to 250 Brinell. They are rough machined, stress relieved, machined a little more, and sent for heat treatment. After heat treatment the part is hard, so finishing is done by grinding, by EDM, or by hard milling. After EDM it is worth tempering again, because the surface that melted and resolidified stays brittle.

And what drives the cost? The material itself is expensive, and powder steels even more so. Vacuum heat treatment, grinding hard material, and EDM are slow and expensive processes. So it is important to choose the simplest steel that does the job, and not always the most advanced one. In the next chapter we will meet stainless steel, the family where chromium no longer just strengthens, but protects against corrosion.

10. Stainless Steels

Stainless steel is steel with at least ten and a half percent chromium. The chromium reacts with the oxygen in the air, and forms a passive layer of chromium oxide on the surface. The layer is very thin, only a few nanometers, and transparent, but it seals the metal off from the environment. And its great advantage: when it is scratched, the passive layer forms again by itself, as long as there is oxygen.

But it is important to remember: stainless steel is not immune to corrosion. It is resistant, and how resistant depends on its composition and the environment. There are five families of stainless steel, according to their crystal structure. Austenitic, with nickel, in the FCC structure. Ferritic, with chromium only, in the BCC structure. Martensitic, with more carbon, which can be hardened. Duplex, which is half austenite and half ferrite. And precipitation hardening stainless, P H.

Each family has completely different properties, so "stainless" on a drawing is an incomplete specification. The most common family is the austenitic one, the 300 series. 304 contains about eighteen percent chromium and eight percent nickel. It is the basic stainless steel: food equipment, kitchens, and general structures. 316 adds two to three percent molybdenum, which greatly improves resistance to chlorides, such as seawater, salt, and cleaning agents.

The nickel keeps the austenitic structure even at room temperature, so this stainless steel is not attracted to a magnet in the soft condition. It also does not harden by heat treatment, only by cold work, and it keeps excellent toughness even at cryogenic temperatures. It has two important variants. The first is the L grade, such as 304L and 316L, with less than 0.03 percent carbon. PEIN PEGOVEE. VUEISEE WEIGH Y CLG HNO VIS U! fo PISCE PIR REIS PCE UE PY BY OY MEY Sy, [VE CAGIITIVIN next to a weld, the carbon combines with the chromium and forms chromium carbides at the grain boundaries.

The zone next to the boundary loses chromium, and starts to corrode. This is sensitization to intergranular corrosion. In the L grade there is not enough carbon for this to happen, so it is the one used for welding. The second variant is 303, with added sulfur for fast machining. The price: lower corrosion resistance, and poor weldability. Machining austenitic stainless steel needs the right habits. It work hardens strongly, especially under the cutting tool.

If the tool rubs instead of cutting, the surface layer hardens, and the next cut becomes even harder. So you work with sharp tools, a steady and sufficient feed, and without letting the tool dwell in place. It also conducts heat poorly, so the heat concentrates at the cutting edge. And in stainless-on-stainless threads there is a risk of galling: the bolt and the nut seize and lock together. A suitable lubricant is used, or a combination of different stainless grades.

Ferritic stainless steel, such as 430, contains chromium only, without nickel. So it is cheaper, attracted to a magnet, and highly resistant to stress corrosion cracking, but its corrosion resistance is lower than that of 304. Martensitic stainless steel, such as 410, 420, and 440C, contains more carbon, so it can be hardened. 420 is used for knives and surgical instruments, and 440C, with about one percent carbon, reaches a hardness of about 58 Rockwell C, and is used for bearings and valves.

The price: its corrosion resistance is the lowest of all the families. Duplex stainless steel, such as 2205, combines roughly half austenite and half ferrite. The combination gives about twice the yield strength of 316, and very good resistance to pitting and stress corrosion cracking. That is why it is used in the chemical industry, in piping, and in offshore installations.

The drawback: it is harder to machine and weld, and it needs a carefully followed welding procedure. The last family is precipitation hardening, such as 17-4PH. It contains chromium, nickel, and copper, and hardens by aging, as we saw in chapter 4. Its big advantage for the shop: you machine it in condition A, the soft condition, and then age it at a relatively low temperature, without quenching, with very little dimensional change. The condition is set by the aging temperature. H900 is the strongest and hardest, about 40 to 44 Rockwell C. H1150 is softer and tougher.

That is why it is very common in precision parts for the defense and aerospace industries. Stainless steel also has its own forms of corrosion. The first is pitting. In an environment with chlorides, the passive layer breaks down at one point, and a small pit forms that penetrates deep. The second is crevice corrosion: in places with no oxygen, for example under a gasket or in a tight joint, the passive layer does not rebuild.

The third is chloride stress corrosion cracking, in austenitic stainless steel, mainly above about 60 degrees. Pitting resistance is estimated by the PREN number: the chromium percentage, plus three and a third times the molybdenum percentage, plus sixteen times the nitrogen percentage. For 304 it is about eighteen, for 316 about twenty-five, and for 2205 about thirty-five. In Europe, stainless steels have numbers starting with 1.4.

The material number of 304 is 1.4301, and of 304L it is 1.4307. 316 is 1.4401, and 316L is 1.4404. 303 is 1.4305, 430 is 1.4016, 2205 is 1.4462, and 17-4PH is 1.4542. What does this mean in the shop? Stainless steel must be kept separate from carbon steel. Chips, brushes, and tools from carbon steel leave iron on it, and in those spots it will rust.

After machining, it is passivated: an acid treatment that cleans the iron off the surface and rebuilds the passive layer. And on the drawing you write the exact grade, not just "stainless". And what drives the cost? Nickel and molybdenum, so 316 costs more than 304, and both cost more than 430. Duplex and 17-4PH cost even more. Machining is slower than carbon steel, and passivation is another step in the process.

To sum up: stainless steel is not one material but a whole family, and the choice of grade decides the corrosion resistance, the strength, and the cost. In the next chapter we will meet cast iron.

11. Cast Iron

Cast iron is iron with a lot of carbon, usually two and a half to four percent, plus one to three percent silicon. On the iron carbon diagram we met in chapter 3, it sits to the right of the 2.1 percent boundary, close to the eutectic point. So it melts at around 1,150 to 1,250 degrees, a few hundred degrees lower than steel, and it flows well in the mold. This makes it possible to cast complex parts from it, with internal cavities, cheaply.

Engine blocks, pump housings, machine tables, and brake discs are all usually made of cast iron. The difference between the types of cast iron lies not mainly in the composition, but in the form the carbon takes. In most types, the carbon separates out as graphite. And whether the graphite appears as flakes or as spheres changes the properties completely. In gray iron the graphite is in flakes. In ductile iron it is in spheres. In CGI it takes an intermediate form, like short worms.

And in white iron there is no graphite at all, and the carbon is bound up in cementite. Gray cast iron is the most common. The name comes from the color of its fracture surface. The graphite flakes act like tiny notches inside the material. In tension they concentrate stress, so gray iron is brittle, with an elongation of less than one percent, and a relatively low tensile strength. But in compression it is three to four times stronger. And it has advantages that steel does not.

Gray iron absorbs vibration, which is why machine tables are made from it. It conducts heat well, which makes it suitable for brake discs. And the graphite lubricates the tool and breaks the chip, so it is very easy to machine. If a small amount of magnesium is added to the molten iron, just before casting, the graphite rounds up and forms spheres instead of flakes. A sphere does not concentrate stress the way a flake does, so the material becomes ductile. This is ductile cast iron, also called nodular iron.

Its strength is roughly twice that of gray iron, and its elongation can reach fifteen percent or more. That is why it replaces forged steel in many parts: crankshafts, gears, suspension parts, and water pipes. Ductile iron can be strengthened even further with a special heat treatment called austempering. The result is ADI. ADI goes through austempering, and its strength rises a lot, to the level of quenched and tempered steels.

It is about ten percent lighter than steel, and absorbs vibration better. It is used in gears and in parts for agricultural and heavy machinery. In white cast iron, the carbon does not separate into graphite, but stays bound in cementite. This happens with fast cooling, or when there is little silicon. Its fracture is white and shiny. It is very hard and very wear resistant, but very brittle, and it can hardly be machined with cutting tools, only ground. It is used for parts that wear, such as mill liners and slurry pumps.

If white iron is held at high temperature for a long time, the cementite breaks down into graphite in the form of clumps, and the material becomes ductile. This is malleable iron. It used to be common in pipe fittings, and today ductile iron has almost completely replaced it. CGI sits in the middle, between gray and ductile. It is much stronger than gray iron, and still Keeps good thermal conductivity and vibration damping.

That is why it has become the material of choice for the blocks and heads of heavy diesel engines, where the pressures are high. In the European standard, the name of a cast iron tells you the type of graphite. GJL is gray, from lamellar graphite. GJS is ductile, from spheroidal graphite. GJV is CGI, and GJM is malleable. Then come the numbers. In gray iron, GJL-250 means a minimum tensile strength of 250 megapascals.

Ductile iron has two numbers: GJS-400-15 means 400 megapascals, and an elongation of 15 percent. The American standard uses a similar system, but in different units. Machining cast iron is different from machining steel. The casting skin is hard and abrasive, with sand and oxides, so the first pass should cut beneath it, not skate over it.

Gray iron produces graphite dust instead of chips, so it is usually machined dry, with extraction. In thin walls, fast cooling in the casting sometimes creates areas of white iron, hard spots that break tools. And welding cast iron is difficult: it is brittle, cracks easily, and needs preheating and nickel electrodes. What does this mean in the shop? A cast part arrives with machining allowances and casting deviations, so it should be checked before machining.

And watch for holes and porosity that are exposed during machining. And what drives the cost? The casting itself is cheap per kilogram, but it needs a mold and a pattern, and it only pays off in quantity. A requirement for nondestructive testing, for checking the nodularity of ductile iron, or for a heat treatment like ADI, adds cost. And sometimes, in small quantities, a part machined from steel bar is cheaper than a casting.

To sum up: the form of the graphite decides all the properties of cast iron. In the next chapter we move on to the non-ferrous metals, starting with aluminum.

12. Aluminum

Aluminum weighs about a third as much as steel, conducts heat and electricity well, and does not rust, because a thin, hard oxide layer forms on it by itself. As we Saw in chapter 1, its structure is FCC, so it does not become brittle in the cold. But it also has weaknesses. Its modulus of elasticity is about a third of steel's, so an aluminum part bends more. It melts at 660 degrees, and loses strength at temperatures of just a few hundred degrees. And it has no fatigue limit, as we saw in chapter 2.

Aluminum alloys fall into two families, according to how they are strengthened. The first family is heat treatable. It is strengthened by precipitation hardening, as we saw in chapter 4: solution treatment, quenching, and aging. These are the 2000, 6000, and 7000 series. The second family does not respond to heat treatment, and is strengthened only by cold work. These are the 1000, 3000, and 5000 series.

This difference decides everything: the temper, the behavior in welding, and what happens to the part when it is heated. Every alloy has a four-digit number, and the first digit tells you the main alloying element. The 1000 series is pure aluminum, over ninety-nine percent: an excellent conductor and corrosion resistant, but soft. The 2000 series is with copper: strong, fatigue resistant, but sensitive to corrosion.

The 3000 series is with manganese, for sheet and heat exchangers. The 4000 series is with silicon, mainly for welding wire. The 5000 series is with magnesium: very corrosion resistant, and easy to weld. The 6000 series is with magnesium and silicon, the most common series for extrusions and machining. And the 7000 series is with zinc, the strongest of them all. The most common alloy in the shop is 6061 in the T6 condition.

It machines well, resists corrosion, can be welded and anodized, and reaches a yield strength of about 276 megapascals. In Europe the common equivalent is 6082, used for the same purposes, with similar or even slightly higher strength. 6063 is softer, and is used mainly for architectural extrusions. When very high strength is needed, you move to the 2000 and 7000 series.

7075 in the T6 condition reaches a yield strength of about 500 megapascals, close to quenched and tempered steel, at a third of the weight. That is why it is the base material of the aircraft industry, and also common in molds and heavily loaded mechanical parts. 2024 is very fatigue resistant, and is used in aircraft fuselages. But both have weaknesses. Corrosion resistance is low, so 2024 sheet sometimes comes clad with pure aluminum, Alclad. They can hardly be fusion welded. And 7075 in the T6 condition is sensitive to stress corrosion cracking.

So the T73 condition is sometimes used, which has been deliberately overaged: it is slightly less strong, but much more resistant to cracking. The 5000 series is the choice for marine environments and welded structures. 5083 and 5754 are very resistant to seawater, and stay strong even after welding, because they do not depend on heat treatment. Their temper is marked with the letter H. For example, H32 means cold worked and then stabilized, to the quarter hard level.

The temper is part of the name, and without it the specification is incomplete. F is as fabricated, with no control. O is soft, annealed. H is strain hardened by cold work. And T is heat treated. In T tempers, the digit tells you exactly what was done. T3 is solution treated, cold worked, and naturally aged. T4 is solution treated and naturally aged. T5 is cooled from hot working and artificially aged, common in extrusions. T6 is solution treated and artificially aged. And T7 is deliberately overaged.

And now the important part for the shop: the digits 51 that follow, as in T651 or T7351. They mean the material was stretched a few percent after quenching, to relieve internal stresses. Plate or block in the T651 condition distorts much less in machining, so for machined parts you almost always order T651 and not just T6. Alongside the wrought alloys there are casting alloys, which contain a lot of silicon, close to the eutectic point we saw in chapter 3.

A356 is a common casting alloy that can be heat treated to T6, and is used for wheels and equipment housings. In die casting, for instrument housings and automotive parts, alloys such as A380 are used. Aluminum is usually easy to machine, and it can be cut at very high speeds. But in soft aluminum the material sticks to the cutting edge, and the finish suffers. So hard tempers like T6 are preferred, with sharp, polished tools, and good coolant.

For especially fast machining there is 2011, with additions that break the chip. In thin walls aluminum distorts easily, and that is where T651 makes the difference. In welding, as we saw in chapter 4, 6061 loses a large part of its strength next to the weld, and 4043 or 5356 filler wire is used. PMINMA TEE UMEILGAUE VVELEE OLE UE VE CLG OD OLS E, TEE Cl PIMATEEINE WLEVEEUJETEEIWEER, EIR CHEMAEE EEE IEEE UME IEVUIN SO. FX VEE come back to this in chapter 17. The surface is protected by anodizing, as we saw in the coatings course. What does this mean in the shop? On the drawing you write the alloy and the temper, for example 7075 T7351, not just "aluminum".

Order T651 for machined parts, and keep in mind that any heating, such as welding, oven curing of paint, or hard anodizing, can change the properties. And what drives the cost? Aluminum costs more than steel per kilogram, and 7075 costs more than 6061. Aerospace grade material with certificates costs even more. On the other hand, it machines very fast, so for machined parts the total cost is not always higher. To sum up: in aluminum, the series tells you what is inside, and the temper tells you what was done to the material. In the next chapter we will meet copper and its alloys.

13. Copper and Its Alloys

Copper is the best conductor after silver: of electricity and of heat. It conducts electricity about six times better than steel, and one and a half times better than aluminum. That is why almost every electrical conductor, busbar, heat exchanger, and induction coil is made from it. It is corrosion resistant, has an FCC structure, is very ductile, and is easy to form.

But it is heavy, with a density of almost nine grams per cubic centimeter, more than steel, and it is expensive. There are several types of pure copper. The most common is ETP, with a small amount of oxygen. It is cheap and an excellent conductor, but if it is heated in an atmosphere containing hydrogen, for example in brazing, it can crack. For such uses, and for vacuum, oxygen-free copper is used, OFHC.

And water and gas pipes are usually made of DHP copper, which has been deoxidized with phosphorus. Conductivity is measured on the IACS scale, and pure copper sits at around one hundred percent. Almost any addition to copper strengthens it, but lowers its conductivity. So a copper alloy is chosen by the balance between the two properties. An example every plant knows is copper chromium zirconium, which is used to make spot welding electrodes.

It keeps about eighty percent of the conductivity of pure copper, and stays hard even in the heat of welding. Brass, bronze, and beryllium copper sit at the other end of the scale: much stronger, but with a quarter of the conductivity, or even less. In copper and brass too, strength depends on the amount of cold work, so they have tempers as well. There are four main levels: soft, quarter hard, half hard, and hard. In the American standard they are marked with a letter and a number, such as 060 for soft and HO2 for half hard.

In the European standard, the temper is marked with the letter R and the minimum tensile strength, such as R290. It is worth stating the temper in the order, because a soft bar and a hard bar of the same alloy behave completely differently in machining and bending. Brass is copper and zinc. With up to about thirty-five percent zinc, the whole structure is FCC, and the brass is very ductile. 70/30 brass, seventy percent copper and thirty percent zinc, suits deep drawing, such as cartridge cases.

With more zinc, as in 60/40 brass, a second phase forms. The brass is stronger, and good for hot forging, but less ductile in the cold. Zinc is cheaper than copper, so brass is cheaper than pure copper. In the shop, the most common brass is free-cutting brass, C36000, called CW614N in Europe. It contains about three percent lead. The lead breaks the chip into small pieces and acts as a lubricant, so it can be cut at very high speeds with an excellent finish.

On the machinability index for copper alloys, it is the reference, rated one hundred. For comparison, bearing bronzes are around seventy, and pure copper only around twenty. That is why it is the classic material for taps, pipe fittings, and small parts on automatic lathes. Important to know: in contact with drinking water there are now restrictions on lead, and lead-free brass is used.

Brass has two weaknesses. The first: in certain waters brass loses zinc, dezincification. The zinc dissolves, leaving a weak, porous copper skeleton. So for water fittings there is a brass resistant to this, marked DZR. The second: cold-worked brass with internal stresses can crack in the presence of ammonia. This is stress corrosion cracking, once called season cracking.

Stress relief by gentle heating prevents it. Bronze was originally copper and tin, and today the word covers a broad family. Phosphor bronze, with tin and a little phosphorus, is strong and fatigue resistant, and is used for springs, electrical contacts, and bushings. Cast bronzes with tin and lead are the classic material for bearing bushings, because they slide well against steel.

The most common is C93200, also called SAE 660. Its lead is dispersed in tiny droplets, and acts as a lubricant when the oil briefly runs out, so a bronze bearing survives a short dry run much better than a steel one. And aluminum bronze, with about ten percent aluminum and a little iron and nickel, is very strong and resistant to wear and seawater. It is used for ship propellers, pump parts, and heavily loaded bushings.

Cast aluminum bronze, such as C95400, reaches a tensile strength of more than five hundred megapascals. Copper nickel, with ten or thirty percent nickel, is especially resistant to seawater, and also prevents the buildup of marine organisms. That is why it is the standard material for seawater piping and heat exchangers on ships and in desalination plants. And the same alloy is used for coins. Beryllium copper reaches the strength of steel.

It contains less than two percent beryllium, and is strengthened by precipitation hardening, as we saw in chapter 4. It is machined in the soft condition, then aged at a moderate temperature, and it reaches a hardness of about 40 Rockwell C, and a strength of more than 1,200 megapascals. It is used for springs and electrical contacts, and also for non-sparking tools, for places with an explosion hazard. But beryllium dust is dangerous to breathe, so it is ground with extraction, and according to safety procedures.

Ordinary machining, with coolant, is considered safer, but the procedures must still be followed. Copper joins easily by soldering and brazing. In soft soldering, with tin, at around two hundred degrees, water pipes and electrical components are joined. In brazing, with silver alloys or copper phosphorus, at six hundred to nine hundred degrees, the joint is much stronger.

Copper phosphorus alloys clean the surface themselves when brazing copper to copper, but they are not used on steel, because the joint becomes brittle. What does this mean in the shop? Pure copper is soft and gummy, and sticks to the tool, so you work with very sharp tools. Brass, by contrast, is one of the easiest materials to machine. Because of its high conductivity, welding copper needs a lot of heat, so it is often joined by brazing instead. And in contact with aluminum, in electrical connections, galvanic corrosion occurs.

And what drives the cost? Copper itself is expensive, and its price moves with the market. Beryllium copper is far more expensive. On the other hand, copper and brass chips have high scrap value, so it pays to collect them separately, and not mix them with steel chips. To sum up: copper for conductivity, brass for machining, bronze for sliding and seawater, and beryllium copper for strength. In the next chapter we will meet titanium.

14. Titanium

Titanium weighs about 4.5 grams per cubic centimeter, roughly sixty percent of the weight of steel, but its alloys are as strong as quenched and tempered steels. So it has one of the best strength-to-weight ratios there is, and it keeps it up to about 400 degrees. Its passive layer, titanium oxide, is very stable, so it is almost completely resistant to seawater, chlorides, and the human body. But it also has properties that make it hard to work with. Its modulus of elasticity is about half that of steel. Its thermal conductivity is very low, about a seventh of steel's. And it is very expensive, because producing it is complicated and takes a lot of energy.

To produce it, the ore is turned into a chloride, reduced with magnesium, and the resulting sponge is then melted two or three times in a vacuum. That is the main reason for the price. Like iron, titanium has two crystal forms. Below about 880 degrees it has the HCP structure, called the alpha phase. Above that it changes to the BCC structure, called the beta phase. Elements such as aluminum, oxygen, and nitrogen stabilize the alpha phase, and elements such as vanadium and molybdenum stabilize the beta phase. According to the balance between them, the alloys are divided into three families: alpha, alpha beta, and beta.

Commercially pure titanium comes in four grades, from Grade 1 to Grade 4. The difference between them is mainly the amount of oxygen: more oxygen, more strength, and less ductility. PEIN PEIVOL UVEDTEEIVEE Oo WIGS 2, VVEREE Ce EEEEE EEE LETTE yiviu vtbeiigttt Vi aQayvyVuUt 2iv PIV YyoePoevoveatlo. HlLivo Vavy ‘Vv form and weld, and is used for heat exchangers, chemical equipment, desalination plants, and marine parts. PEIN PEIMPOL UVETTEEIVEE IO WIGS 2, VVEREE Ce EEEEE EEE EGET yiviu vtreiigttit Vi avyuuUt 2iv PIV YyoePoevovedtlo. HlLivo Vavy tv form and weld, and is used for heat exchangers, chemical equipment, desalination plants, and marine parts. PEIN PEIVOL UVETTEEIVEE Oo WIGS 2, VVEREE Ce PETER EERE EMaE EE yiviu vtreiigtit Vi anvyvuUt 2iv PIV YyoePoevovadio. tlio Vavy ‘Vv form and weld, and is used for heat exchangers, chemical equipment, desalination plants, and marine parts.

When even higher corrosion resistance is needed, there are grades with added palladium, such as Grade Uke The most important alloy is Grade 5, titanium with six percent aluminum and four percent vanadium. It is an alpha beta alloy, and on its own it accounts for about half of all the titanium in use. Its yield strength is about 880 megapascals, three times that of Grade 2. It can be strengthened a little more by solution treatment and aging. medical implants a purer version is used, Grade 23, in the ELI grade, with less oxygen and more toughness.

In the shop it usually arrives in the annealed condition, at a hardness of about 30 to 36 Rockwell C, and in that condition it is already as strong as a quenched and tempered steel. There are also beta alloys, such as Ti-10-2-3, which reach even higher strength after aging, and are used in heavy parts such as landing gear and springs. And there are alpha alloys, which work at higher temperatures in jet engines. at the cutting edge. In steel, most of the heat leaves with the chip. In titanium, much of it stays at the edge, and the tool wears quickly.

At that heat, titanium also reacts chemically with the tool material and sticks to it. And because of the low modulus, thin walls push away from the tool and chatter. So the rules are fixed: low cutting speed, about a third to a quarter of steel, a generous feed per tooth, sharp tools, high-pressure coolant, and continuous contact, without letting the tool dwell in place. The machine and the workholding also need to be especially rigid. In milling you use climb milling, and toolpaths that keep the tool in constant contact, so the heat does not rise and fall with every revolution. In drilling, chip evacuation is the problem, so a drill with through-tool coolant is preferred. And in tapping, titanium grips the tap, so special taps and a suitable lubricant are used.

Machining too aggressively does not just destroy tools, it also leaves stresses and a damaged layer on the surface, which lowers the fatigue life of the part. Titanium also has a safety hazard. Fine chips and titanium powder can ignite, and the fire burns at a very high temperature. They are not put out with water, but with a class D extinguisher or dry sand. So fine chips are not allowed to build up near the machine, and they are kept separate from other chips. And in grinding, titanium sparks are white and brilliant, which is also a quick way to identify it. When heated in air, above about 500 degrees, oxygen diffuses into the surface and forms a hard, brittle layer, alpha case.

fitio tayvl VEE CLG VIGURo, oY Ih io PeiiVyeUu vy thiawvl ete Vi Viitiiiival Seerieeriy, Vi iit re! Liv PIV GQIeU Tia vacuum or in argon in the first place. In welding too, the whole hot zone needs inert gas shielding, on the front and the back. Pitio fayvl VG CLG VIGURo, VY Ih io TeriiVyeu vy thiawvl etl Vi Viitiiiival fetberiy, Vi tii re! Liv PIV GQIeU Tia vacuum or in argon in the first place. In welding too, the whole hot zone needs inert gas shielding, on the front and the back. The color of the weld tells the story: silver or light straw is fine, while blue, gray, or white indicate contamination. PMI CVV TEIVEIt ofliacati tt Hilye- PERCATEIMATEE CEREUS LeU LY yer, Gv TH OLGHHwOoo VIS’, OY GIGI OVicw compound is used. And in contact with aluminum, titanium is the noble one, and it is the aluminum that corrodes.

common today in aerospace and implants, because it saves most of the material that would otherwise go to chips. Printed parts usually go through hot isostatic pressing, which closes internal porosity and improves fatigue life. Viiwih OPIVUulU yjvyu MEIVVO™S CELCAPEIUATEE, CALINE VWVEIW EE TIN R. EF ERCHEEERAEEE FEkwO V¥EICEE WY EES to ViItiIVGl Git otitl yee ' high, or when the environment is very corrosive. When weight matters but the strength needed is moderate, aluminum is much cheaper. When stiffness matters, steel is better, because its modulus is twice as high. And when the temperature rises above about five hundred degrees, you move to nickel alloys.

WVEIGL UVES LO THING TE REIS OLIV. WEE LEI Miavvinty yjvu W¥YEEANS LEIS VCAGYEL yiever GM CLG, IU CAGIIIVIN Grade 5 to AMS 4928, and plan the machining in advance, including workholding and coolant. And what drives the cost? The material itself costs tens of times more than steel per kilogram, machining is several times slower, and tools wear quickly. In aerospace, most of the material is sometimes removed by machining, so it pays to start from a forging close to the final shape, or from 3D printing. And clean titanium chips are worth money, so they are collected separately.

resistance, but it demands respect in machining, safety, and heating. In the next chapter we will meet the nickel alloys and other special metals.

15. Nickel Alloys and Special Metals

Some parts work where steel can no longer hold up: inside a jet engine, in a gas turbine, or in a chemical reactor with boiling acid. That is where nickel-based alloys come in, the superalloys. They keep their strength and creep resistance at temperatures of six hundred to a thousand degrees, along with resistance to oxidation and corrosion. Nickel stays FCC all the way to its melting point, so it has no phase change to damage its properties as it heats up. In a modern jet engine, about half the weight of the engine is made of these alloys. The secret of their strength is, once again, precipitation hardening, as we saw in chapter 4. The strength comes from tiny precipitates called gamma prime, a compound of nickel with aluminum and titanium.

PEIVOe PIECVIVPEGLL YS GIS CIGNMIS VVUTE GL TUTE CERI IGU, OY LEIS REY VV YP PEY MIVIVMUGIIViloe VWilell Utiitl alloys have already softened. In some alloys niobium is added, which forms a similar precipitate, and that is the case in Inconel 718. PEIVOe PIECVIVPEGLL YS GIS CLGNMIS VVOETE GL TUTE CPIM LGU, OY LEISY RESP VV YP PEY MIVIVMUGIIVilo YWiitll UtiItl alloys have already softened. In some alloys niobium is added, which forms a similar precipitate, and that is the case in Inconel 718. BVLvV PIS LEIS PEPCATEE PCAPEEEEIM OD. EVIVZEIDSE SPUY fo TTHUIAY Glu VVYRPRS', Gti to Veopeviany PeVIolaiit (YY VOVaYYatvul and certain acids. Its precipitation hardened version, Monel K-500, is used for pump shafts and marine fasteners. VMI VL fo THURS E, VETTE, Gabi hiwiyvusvi Uitt. th tO OLLIE yt mMmiiwu vy VV OVIMEIVEL, to very VVEIITVOIN resistant, and is relatively easy to weld, so it is common in piping, and in the marine and chemical industries.

Inconel 718 is the most widely used of them all. It is precipitation hardened, reaches a yield strength of more than a thousand megapascals, and keeps it up to about six hundred and fifty degrees. It is used in engine discs and shafts, in fasteners, and in oil and gas production tools. And Hastelloy C- 276, with a lot of molybdenum, is the choice for the harshest chemical environments. In the hottest parts of the engine, such as the turbine blades, special casting alloys are used. Today blades are cast as a single crystal, with no grain boundaries at all, because grain boundaries are where creep begins.

As we saw in chapter 1, grain boundaries strengthen at room temperature, but at high temperature they become the weak point. Machining nickel alloys is among the hardest there is, even harder than titanium. They work harden strongly under the tool, conduct heat poorly, and keep their strength even in the heat at the cutting edge, so the tool meets strong material all the time. They also contain abrasive carbides. So you work at very low cutting speeds, with sharp tools, a steady feed, and never let the tool rub.

At the depth-of-cut line, notch wear forms on the tool, so the depth of cut is varied slightly from pass to pass. In the engine industry, ceramic tools are also used, at high speeds, for roughing. And anyone machining Inconel 718 needs to know what condition the material arrives in: annealed, or already aged and hard. In finishing with carbide tools, the cutting speed in Inconel 718 is around twenty to fifty meters per minute, about half that of titanium. High-pressure coolant, rigid workholding, and close monitoring of tool wear are a must, because a worn tool hardens the surface even more.

Welding has its own rules too. Inconel 625 and 718 are considered relatively weldable. But alloys with a lot of aluminum and titanium can crack in the heat treatment after welding. PS TEE CALE PEIN GU Yuo, VIVES fo VEILIUG. PeOIMUSS VE OUI, ISG, UI US VEU VIGURe HELIS YEU. | you clean thoroughly before welding, and don't use tools or brushes that have been used on other materials. Nickel also has another surprising use. Invar, iron with thirty-six percent nickel, hardly expands with heat, about a tenth as much as ordinary steel. So it is used in measuring instruments, optical structures, and molds for composites. Kovar, with nickel and cobalt, expands exactly like glass, so it is joined to glass in hermetic electronic parts.

For Invar to hold its dimensions over time, it is stabilized by heat treatment after rough machining, and finish machined afterwards. Cobalt alloys, such as Stellite, are highly resistant to wear, corrosion, and heat. Stellite is a cobalt alloy for hardfacing, welded onto wearing surfaces such as valve seats, cutting edges, and pump surfaces. iw Mehhauvtwwiy PIV alo, VUVIT Geo LU tyre ', fiviyvusvi PRATER, CCALERCALQAEEE, CALINS PEINJAZEUAE EE, PEER GAL very iyi temperatures. Tungsten melts at over three thousand four hundred degrees, and is two and a half times as heavy as steel.

iim Mehhauwtwiy PIV talo, VUVIT Geo LU ysl ', Hiiyvuvt PRAEEE, CCAPERCAERALEE, CALINA PEENJAZEMAE EE, PEER Gah voy five temperatures. Tungsten melts at over three thousand four hundred degrees, and is two and a half times as heavy as steel. rim Mehhawtwwiy PIV alo, VUVIT Geo LUI t'yvsel ' fiiyvusi PRAEEE, CCALERCAERALEE, CALINS PEIN EUAEEE, PEIN Cal very rye temperatures. Tungsten melts at over three thousand four hundred degrees, and is two and a half times as heavy as steel. tue tyvuwll wavy eivy-, WVEREE PEEUINWE CAEEME TEUEE, WVCHEE WR PICU TEI, GI GINS UO IVE Va Wergitto ait radiation shielding. Molybdenum is used in furnace parts, and tantalum in chemical equipment and implants. tu tyvwwrl ttwavy envy, WVEREE PEIWINNWE CHEEME TEUZEE, WHEE WS PICA EEIS, GI GAINS UO IVE VOIGIUO Veigitito aint radiation shielding. Molybdenum is used in furnace parts, and tantalum in chemical equipment and implants.

Pure tungsten is hard and brittle, so it is machined mainly by grinding and EDM. Pits GAL Lit eryeee Vita Vi tiie Lavin. fayiwolutii, trite HYIeot VEEUVLUI GE PEI tG, GNU CVV Lilli iit Worry! of aluminum. The alloys AZ31 and AZ91 are used for instrument housings, automotive parts, and aerospace. Magnesium is very easy to machine, but as with titanium, its fine chips and powder can ignite, and they are not put out with water. And zinc castings, such as Zamak, melt at a low temperature and are used for small, cheap die castings, such as handles and locks.

What does this mean in the shop? With nickel alloys the material itself is very expensive, and lead times are sometimes long. On the drawing you write the alloy, the heat treatment condition, and the standard. PMID ER ED VUE REE INDE UVEE EL SETCAR PEER UE MEIN OU GHNUYO GES PIE CARLIN SE PIG, GI PUN GI Oh PERN stainless steel. So to identify material that got mixed up in stock, a handheld XRF analyzer is used, which shows the composition in seconds. And what drives the cost? PMID ER ED VUZE REE INE DU UVEE EL SETCAR PEEWIOR UE LEIS GHUYO GES PIE CRAIG OY SPEIER A, GEE PUN GI Ok PERS stainless steel. So to identify material that got mixed up in stock, a handheld XRF analyzer is used, which shows the composition in seconds. And what drives the cost? PMID ER ED VUZE REE INE EU UVEE EL SEGAL PEER UE MEIN GHUYO GES FIVE CARLIE OU SPEIER, GI PUN GEE IVOh PER stainless steel. So to identify material that got mixed up in stock, a handheld XRF analyzer is used, which shows the composition in seconds. And what drives the cost?

The material, the slow machining, tool wear, and aerospace-level certificates and testing. On the other hand, nickel and cobalt chips are worth a lot of money, so they are collected separately. To sum up: nickel alloys are the material for the harshest heat and corrosion, and around them there is a family of special metals, each for a particular problem. In the next chapter we take a closer look at one special metal: tungsten.

16. Tungsten

Tungsten is a metal of records. Its melting point is the highest of all metals, over three thousand four hundred degrees. Its density is like that of gold, almost two and a half times that of steel. Its modulus of elasticity is twice that of steel, so it is very stiff. And it absorbs radiation very well. But it also has two big weaknesses. ductile. This transition temperature depends on the purity and on the processing the material has been through, so two tungsten bars can behave completely differently. And in air, above about five hundred degrees, it oxidizes quickly. So at high temperatures it works only in a vacuum or in a protective gas.

Tungsten is almost never cast, because it is very hard to reach and control such a temperature. Instead, it is made from powder. Tungsten oxide is reduced to a fine metal powder, pressed into shape, and sintered at a very high temperature, until the particles bond together. Then the block is worked hot, by rotary swaging, rolling, and drawing, into bars, sheet, and wire. That is how lamp filaments, TIG welding electrodes, heating elements for vacuum furnaces, and anodes in X-ray tubes are made.

In TIG welding electrodes, the color at the tip marks the addition: green for pure tungsten, gray for ceria, and gold or blue for lanthana. The red electrodes, with thorium, are being replaced today because of their radioactivity. In industry, pure tungsten is often not used, but rather tungsten heavy alloys. They contain between ninety and ninety-seven percent tungsten, plus a little nickel and iron, or nickel and copper. During sintering, the nickel and iron melt and wrap around the tungsten grains, producing a structure of hard grains in a ductile matrix.

The density drops a little, to seventeen to eighteen and a half, but the material becomes strong and ductile enough to be machined. they are used near sensitive instruments. In the ASTM B777 standard they are divided into four classes, by tungsten content. hh LWlioiil vtbeiigttit fv GaIVUuliu Sergi PIMPIUISU bY Ga LIVUOoGaiiUu PIV YoePoeoveGtly, Giiuse tiitwii Viv tyetmr fanye from a few percent to tens of percent, depending on the tungsten content and on how they were processed. Ahh (wlio vtreiigtit fv GaIVuUuliu erygit PIMPIUISU bY a LIVUOoGaiiu PIV YyoePoevovdty, Giius tiimii Viv yet fanye from a few percent to tens of percent, depending on the tungsten content and on how they were processed.

helicopters, and engines. When you need to stop radiation: shielding and collimators in medical and industrial equipment. And when you need stiffness: long anti-vibration boring and drilling bars, because the high modulus and the weight damp the vibration. Machining heavy alloys is similar to machining hard cast iron. Sharp carbide tools from the K group are used, with a positive rake, and at moderate speeds. The chip is short and brittle, sometimes powdery. The edges of the part tend to crumble, so chamfers and radii are designed into the edges, and the last pass is taken gently. Drilling and tapping are possible, but thread milling is safer than a tap.

Pure tungsten, on the other hand, is hardly ever machined with cutting tools: it cracks and crumbles, so it is ground, or machined by EDM. The most important use of tungsten in the shop is in the cutting tools themselves. Cemented carbide is grains of tungsten carbide, bonded with cobalt, and sintered together. Its hardness is around ninety Rockwell A, its modulus of elasticity is three times that of steel, and it withstands enormous compression, but it is brittle in tension and under impact. The more cobalt, and the larger the grains, the tougher the carbide, but the less hard and wear resistant. Very fine grains, below one micron, give both hardness and good toughness.

PES IO VV CLG UV IM VUE YI GU HY OIA YIU, VY SEIS PEICRWEICeE WTEI V. FOEME OLEH, IVE IU stainless steel, K for cast iron, N for non-ferrous metals, S for heat resistant alloys and titanium, and H for hardened materials. Within each group, a low number is a hard, wear-resistant grade, and a high number is a tough grade, for impact and interrupted cutting. And most inserts carry a coating, CVD or PVD, which extends their life. The same material is also used for wear parts: punches, drawing rings, spray nozzles, and gauges. Molybdenum is a close relative of tungsten. It melts at about two thousand six hundred degrees, is half as heavy, and is easier to machine.

It is used in furnace parts, glass melting electrodes, and fixtures for parts in heat treatment. The alloy TZM, with a little titanium and zirconium, is stronger when hot. And as we saw in chapter 8, molybdenum is also an important alloying element in steels. Here too there is a safety issue. Fine tungsten powder, and especially dust from grinding cemented carbide, which contains cobalt, is very harmful to the lungs. So carbide is ground wet, with extraction, and the dust is never blown off with compressed air. And with heavy alloys, even a small part weighs a lot, so lifting and clamping need to be planned.

What does this mean in the shop? When ordering a heavy alloy, you specify the class according to the standard, for example ASTM B777 class 3, and whether the part needs to be non-magnetic. At receiving, the density can be checked by the Archimedes method, weighing in air and in water, and this is a quick and reliable check of the tungsten content. And what drives the cost? The material itself is very expensive, and sold by weight, so every unnecessary gram costs money. Producing from powder to near-net shape saves material and machining. And tungsten chips and offcuts are worth a lot of money, and worn carbide inserts can be recycled too, so they are collected separately.

To sum up: tungsten is the metal for extreme heat, for weight in a small volume, and for shielding, and it is also the basis of the cutting tools we use every day. In the next chapter we start talking about how materials behave in service, beginning with corrosion.

17. Corrosion

Corrosion is the metal returning to the form in which it was found in nature. Iron was extracted from an oxide, and it tends to go back to it. The process is electrochemical, like a small battery. the anode the metal loses electrons and dissolves. The electrons flow through the metal to the cathode, and there they react with oxygen and water. And the electrolyte, usually water with salts, closes the circuit. Remove any one of the four components, and the corrosion stops. That is the basis of every method of protection. The simplest form is uniform corrosion: the whole surface is eaten away at a similar rate. It is the most predictable, and it is measured in millimeters per year.

In steel structures, a corrosion allowance is added to the thickness, so that even after years of service enough material remains. pushes joints apart. The corrosion rate depends heavily on the environment: humidity, salts, temperature, and acidity. The ISO 9223 standard divides environments into categories, from C1, a dry indoor space, up to C5 and CX, a harsh marine or industrial environment. Along the coastal strip of Israel, the environment is usually in the higher categories, and an unprotected steel part will rust there much faster than inland.

WVEICTE CVV UPPED PEI tio CULE Tt Lilt ed VVLerieue Vi Gi Vievetl Viyty, a vMatlwly IVilito, Gti VIS VE Littl corrodes quickly. This is galvanic corrosion. The galvanic series ranks metals by their tendency to corrode. At the active end are magnesium, zinc, and aluminum. Then steel and cast iron. Then copper and brass. And at the noble end, passive stainless steel, titanium, and graphite. PEI PURINE VG LEISY GI FE REIS OUEIMU, SEIS POLS LEIS VVEIVOIUEE. PINE CEI CEU GE PCI PICARD LU. G OlliCa steel bolt in a large copper sheet will corrode very fast. A small stainless steel bolt in a large aluminum sheet will do almost no harm. PEI PUPREISE QVC LEIS Y GI FE REIS OUEIMU, SEIS POLST LEIS VUVEIUVOIUEE. PINE REIS CHEE PCURIL PEI LUM. & OlliCa steel bolt in a large copper sheet will corrode very fast. A small stainless steel bolt in a large aluminum sheet will do almost no harm.

Familiar examples: an aluminum part held with steel bolts outdoors, or a copper pipe connected to a galvanized pipe. In both cases, without insulation, the active metal corrodes next to the joint. The same principle can be put to work for us. Sacrificial zinc anodes protect a ship's hull: zinc is more active, so it corrodes instead of the steel, and it is replaced from time to time. Prat iv GivY Very yetvar Hetty PIViLvtyo VILL Ve ¥Y¥EICI iit VVvatiny fv VVUIGViitlu. FPulru iV PIPVIive aihtu large structures, impressed current cathodic protection is used, where a power supply drives a current that turns the whole structure into a cathode. Prat iv Gio Very yelvar Heatly PIViLvtyo VILLE UCVOT V¥iEiweitl tii VVatiny fo VVIGWViitluU. Puru iV PIP Hiveo aim large structures, impressed current cathodic protection is used, where a power supply drives a current that turns the whole structure into a cathode.

To prevent galvanic corrosion between parts, they are separated with insulating washers and sleeves, or coated, and preferably it is the cathode that gets coated. As we Saw in chapter 10, there are localized forms of corrosion, more dangerous than the uniform kind, because they penetrate deep without much showing on the outside. Pitting creates narrow, deep pits. Crevice corrosion develops in places without oxygen, under gaskets and in tight joints. And intergranular corrosion advances along the grain boundaries. In aluminum of the 2000 and 7000 series there is exfoliation corrosion: thin layers of the material swell and separate from each other, along the rolling direction.

MIC voes VVIIVolVil Ui GUNniity Hayyetito viny WVEISEE CHEERS UVEITIS GIS PISO Vy! IV. 4 VUVOUE YIN material, tensile stress, and a specific environment. In austenitic stainless steel, that is hot chlorides. In brass, ammonia. MLIVOCeS VVITVorVil vi GUNiiy Hayvyetilo viny WYEISTE CHEER VUVEIMELIV IO GIS VIS OUTt Vy! IV. & VUVOUE YIN material, tensile stress, and a specific environment. In austenitic stainless steel, that is hot chlorides. In brass, ammonia. In carbon steel, strong hot alkalis. And in 7075 aluminum in the T6 condition, moisture and salt. The stress can also be internal, left over from machining, bending, or welding. So cracking is prevented by choosing a resistant material, by stress relief, or by changing the environment.

atoms diffuse into the steel, collect at points of stress, and cause brittle fracture, sometimes hours or days after assembly. PIS VME VE EEF SEE GED VISE IGALEEI, QUI VIO CHETEEI, GAL INE VV NCEMATE EN VVEREE CHEER CIS ULI. VY GEM electroplating, the parts are baked to drive out the hydrogen, within a few hours, according to the standard. PIS VMUEPUES SD VE EEF SEE GED VIS ULE VIGIL, QAI VIO CHETEEI, CAE INE VV EMETE ES VVEREE CHEER CIS ULIVU. VY GEM electroplating, the parts are baked to drive out the hydrogen, within a few hours, according to the standard. They are usually heated to around two hundred degrees for several hours, starting no more than a few hours after plating ends. The stronger the steel, the longer the bake.

And for very high-strength bolts, a coating that does not introduce hydrogen is sometimes chosen. And there are more combinations. Corrosion fatigue: a corrosive environment lowers the fatigue resistance of a part, so even steel no longer has a fatigue limit in seawater. Erosion corrosion: fast-flowing liquid, especially with particles, strips away the passive layer again and again. And fretting between mating surfaces: tiny movements between two tight-fitting parts, such as a bearing on a shaft, create oxide powder and reduce fatigue life.

avoid crevices and pockets where water collects, provide drainage, and never join metals far apart in the galvanic series without insulation. And then coatings and paint, as we learned in the coatings course, cathodic protection, and corrosion inhibitors. What does this mean in the shop? Machined steel parts rust quickly, especially where they were touched by hand, because sweat contains salt. So you work with gloves, protect the parts with oil, and pack them in VCI bags, which release a corrosion inhibitor.

The machine's coolant matters too: too low a concentration causes rust on the parts and on the machine, so it is checked regularly. In the store, parts are not kept near acids or cleaning agents, and the same racks and tools are not used for stainless steel and carbon steel. And the salt spray test, ASTM B117, is measured in hours, and is used to compare coatings, but it does not predict exactly how long the part will last in the field. And what drives the cost? WVYMGIIYo, 4 PRIMES PEVIOLGIIL THIGH IG OUI GO CLG OD OLE E, HIOUIGLIVEE MU RYVE UH PIV halo, GI Leotliity. Uk a corrosion failure in the field costs far more than all of these. In the next chapter we will look at wear and friction.

18. Wear and Friction

PIS VIG VGA UY PIU MIC GR. ELIS Y WY VU. VIGO HE MUG, Yea! eetil, GHUUYWWayu, vveciono, « cutting tools all lose material gradually, until their dimensions or finish no longer meet the requirements. The field that studies friction, lubrication, and wear is called tribology, and in this chapter we will understand why things wear, and how to choose materials and surface treatments that extend their life. When two surfaces touch, they touch only at the tips of the peaks, and the real contact area is very small, sometimes less than one percent of the apparent area. At these points the pressure is enormous, and the peaks weld together and tear apart again and again. That is why the friction force depends on the load, but hardly at all on the apparent area.

PEIN UMVOTEIVIOCTIL VE TEIUVLIVEE VOLYYOUCT Mary VALLES GI CLE fo GEUVUIE WEIS TiCall, CGALIMs VVEREE yyyvey PUMP IUVGALIVVEE TL drops to a tenth or even less. And there is a phenomenon everyone who works with machines knows: stick-slip. PEIN UVOTEIVICTIL VE TEIUVLIVEE VOLVO Mary VEEL GI CLE fo GEUVUIU WEIS TiCail, CALIMs VVEREE yyve IUMPIVGALIVEE EL drops to a tenth or even less. And there is a phenomenon everyone who works with machines knows: stick-slip. At low speed, static friction is higher than sliding friction, and the part moves in small jerks instead of smoothly. That is why the slideways of machine tools use PTFE-based sliding liners, whose static and dynamic friction are almost equal.

Good lubrication separates the surfaces. The Stribeck curve shows the three lubrication regimes, as a function of speed, viscosity, and load. In boundary lubrication, at low speed or high load, the peaks still touch, and the oil protects only through a thin layer of additives. In mixed lubrication, part of the load is already carried by the oil. wear happens at start-up and shutdown, when the speed is low and lubrication falls back into the boundary regime. sealing out dirt. And under high loads, oils with extreme pressure additives are used, which form a protective layer on the metal surface.

There are four main wear mechanisms. The first is adhesive wear: the peaks stick to each other, tear, and transfer material from one surface to the other. extreme case the parts seize. The second is abrasive wear: a hard particle, or a hard peak, ploughs grooves in the softer surface. This is the wear caused by dust, sand, and chips. The third is surface fatigue, in rolling contact, as in bearings and gears. In rolling bearings, pitting and spalling occur: the highest stress lies just below the surface, a crack starts there, and eventually a piece breaks away.

And the fourth is corrosive wear, in which oxide layers form and are removed again and again. And there is also flow-related wear: erosion by a fluid carrying particles, and cavitation, in which vapor bubbles collapse next to the surface and hammer it, as in pumps and propellers. hardness. The volume of material worn away increases in direct proportion to the load and the sliding distance, and decreases in inverse proportion to the hardness of the surface being worn. 2 PGAPUINM oo io LIS fiicaiit wu Gyettiot WUC. PUI TE GMI GOVE VEG REISE FO CHEE TEEERUE CHEER ERIS WE REET EER. surface about 1.2 times harder than the abrasive particles hardly wears at all. Sand, for example, is harder than most hardened steels.

2 PGPUIV oo to LIS Piicaiit wy EyGttiot WUC. PUI TEE AMI GOIVR VEG LEIS TS FO CHEE TEEERUERCHEER PRINS WE REEMA E ER. surface about 1.2 times harder than the abrasive particles hardly wears at all. Sand, for example, is harder than most hardened steels. The basic rule for choosing a material pair: never put two identical, soft metals against each other. Dry steel on steel, or stainless on stainless, tend to stick and seize. The classic pair is a hardened steel shaft against a bronze bushing, as we saw in chapter 13. Sintered bronze bushings, with oil soaked into them, lubricate themselves. Plain bearings of white metal, Babbitt, are very soft, and are used in large hydrodynamic bearings.

And polymers are an excellent choice for light and medium loads: bushings of PTFE, POM, and PEEK slide well even without oil. When a more resistant surface is needed, there are two ways: harden the material itself, or add a layer. Hardening can be done by carburizing, nitriding, or induction, which we met in chapter 7. Adding a layer can be done with hard chrome, PVD coatings such as TiN and DLC, thermal spraying of tungsten carbide, or welding on a hard layer such as Stellite. And the choice of method depends on the load, the required layer thickness, and the process temperature the part can tolerate.

The thickness also varies widely between methods: a PVD coating is only a few microns, hard chrome tens to hundreds of microns, and nitriding and carburizing tenths of a millimeter to millimeters. And watch the temperature: some PVD processes run at several hundred degrees, and can soften a steel that was tempered at a low temperature. The finish matters too. Surface roughness, Ra, has a big effect on wear. A surface that is too rough wears its mating part and breaks the oil film. A surface that is too polished sometimes does not hold oil, so in engine cylinders a pattern of fine grooves is created on purpose, to hold oil.

And in rolling contact, steel cleanliness is critical: small inclusions just below the surface are the starting points of spalling. That is why bearing steels such as 52100 are produced to especially high cleanliness, and hardened to more than 58 Rockwell C. What does this mean in the shop? On the drawing of a wearing part you specify the hardness of both surfaces, the roughness, and the type of lubrication, not just the material. Keep things clean: dirt and chips that get into a bearing or a slideway are ready-made abrasive wear. New parts sometimes go through a run-in, at reduced load, so the peaks are smoothed gently.

“Eta yjvyu Mawiiaely LEIS PEI VIIGTOre vy iho erprpLo! Give. iio Gil yivvYee HIMAIVGLLS GNMIGOIVYES VYUG!, OITiIVaier and torn metal indicates adhesion, and pits and spalls indicate surface fatigue. And what drives the cost? “wii yjvu Mawrialy REIS PIS VIIGTOTit vy ilo erprpre! Give. iio Gil yivvYSe HIMAIVGLLS GNMIGOIVE FVYUG!, OItiIVaier and torn metal indicates adhesion, and pits and spalls indicate surface fatigue. And what drives the cost? Hardening and hard coatings, grinding to a precise finish, and lubrication and sealing systems. But machine downtime because of a worn part is usually far more expensive. In the next chapter we move on to the material in manufacturing, starting with machinability: why one material machines easily, and another breaks tools.

19. Machinability

iviaavl etary fo bivVve meauny GF PHIGALW TE IGEE IWS ho PLOW VO PPIGAUITICU. POUL Th PO TI Ge viligic vr VPY! Ly, Hihiv several together: how long the tool lasts, what finish you get, what chip forms, and how much force and power it takes. iviavl teary tv tive mreauny GO PHIGALW IGE IW ho PLOW WS PPIGAUITISU. POUL Tk to TIVE eG viligic el VPpY! Ly, hiv several together: how long the tool lasts, what finish you get, what chip forms, and how much force and power it takes. A material with good machinability lets you cut fast, with a tool that lasts, a short chip, and a good finish. And a material with poor machinability does the opposite, on every one of these measures. To compare, the machinability rating is used. The machinability rating is measured against 1212 steel, which is rated one hundred percent.

1018 steel sits around seventy percent, 1045 around fifty-five, annealed 4140 around sixty-five, 304 stainless around forty-five, Grade 5 titanium around twenty, and Inconel 718 around ten. Aluminum machines faster than any steel, and gets ratings of hundreds of percent. It is not a precise number, and tool makers use their own tables, but it gives a good order of magnitude, and in cost estimating it is one of the biggest factors in machining time. Another measure, useful for calculating power and forces, is specific cutting force: the force needed to cut one square millimeter of chip.

In aluminum it is around seven hundred newtons per square millimeter, in carbon steels around fifteen hundred to two thousand, in titanium similar to steel, and in nickel alloys around three thousand. V¥IIC’At UML iiiiitVso tiiavl HterCaMEniky . PROe UE GH, PIGEUEIN OO. PY EPICALWEIGHE LEIGAL PO LY PIGIU WY GO LEIS CUE, WUE material that is too soft is also a problem: it smears, sticks to the tool, and produces a long chip and a torn finish. In steels, the comfortable hardness range for machining is roughly 180 to 250 Brinell. Next, work hardening: materials such as austenitic stainless steel and nickel alloys harden under the tool. Thermal conductivity: when it is low, as in titanium, the heat stays at the cutting edge. Hard particles, such as carbides, wear the tool. And the chemical tendency of the material to stick to the tool material.

The microstructure matters too. In medium carbon steel, a structure of ferrite and pearlite machines well. In high carbon steel, spheroidize annealing, which we saw in chapter 6, turns the carbides into spheres and greatly improves machinability. And hardened martensite is already hard machining. PEIN PHIMVIMOIVilo Tit Lit PhiGatweiiaagh Gi very ii cacrik. thianiyasr VV VUIIULS THIViUOIVilo Hei, VMEVaUoL ti Ty break the chip and act as a lubricant at the edge. And aluminum oxides are abrasive, because they are very hard, almost as hard as the tool itself. PEIN PHIMVIMOIVilo Tit Lit PHiGatweiiaagh Gi vywry mii SCALE. Ehicae ye VO VUIIULS PHIViUoIiVilto Hei, MEVGaGUSL tl Vy break the chip and act as a lubricant at the edge. And aluminum oxides are abrasive, because they are very hard, almost as hard as the tool itself.

Free-machining steels, which we met in chapter 5, are built on this principle. The 11 series, such as 1144, contains sulfur. The 12 series contains sulfur and phosphorus. And 12L14 also contains lead, and machines almost twice as fast as 1212. The same idea exists in 303 stainless, in leaded brass, and in 2011 aluminum. But these additions lower fatigue resistance, ductility, and weldability. And because of restrictions on lead, such as RoHS, lead is now being replaced by bismuth or silicon. rim vl ave Vi tin Vi Ll od Lwlilo yjvu a met. PAYive 'y VVEILIIUYUS Vi Lt od IMTOO TE UU LEIS PHICGALW IG, GI TL fo dangerous, wrapping around the part and the tool, and getting in the way of automatic work. A discontinuous chip forms in brittle materials, such as cast iron.

And a segmented chip, shaped like saw teeth, forms in titanium and hardened materials. A chip breaker on the insert bends the chip until it breaks. And there is one more phenomenon everyone who works a lathe knows: built-up edge. In soft materials and at low speed, material sticks to the edge, builds up on it, and breaks off again and again. The result is a poor finish and unstable dimensions, and the cure is usually a higher speed, a sharper tool, or a suitable coating. Pim CVV TEIGALW iG to VilVvovli euvviuiny t\V iit Piiaawwiiaa Meity PriavViliiitlMu. iyi Sd ed Vitel, VVTIIWViitwu carbide, cermet, ceramic, CBN, and PCD. CBN, cubic boron nitride, is the choice for hardened steel and hard cast iron.

POM, MPMVIEF VE J OSGEEEEIN CHICHE EEE INE, FO REIRD VEIUIUR EE CHEMAT EERE IREE EE, CUVEE, CEI VUITIPUVONwo, MUL TIVE IU Ole, because at high temperature the carbon in the diamond diffuses into the iron, and the tool wears very fast. POM, MPMVIEF VEY OSGEEEIN CHICHE EEE INE, FO REID EIEN EE CHPMAEE EEE IRAE EE, CUVEE, CEI UUITIPUVO wo, MUL TIVE IU OLE, because at high temperature the carbon in the diamond diffuses into the iron, and the tool wears very fast. Taylor's equation relates cutting speed to tool life. With carbide tools, raising the cutting speed by twenty percent can cut tool life in half, or even more. vw LEIS fo GH CUViiVilii Sd taut muy tv ASSP Hhiawvl blll | REEEINS OFIVVEL, GI OIVYY I muy Lilaake (VeVi are not constantly being changed. And in difficult-to-machine materials, the economic speed is much lower.

The coolant also plays a role in machinability. It cools, lubricates the contact between the chip and the tool, and flushes away the chips. At low speeds lubrication matters more, and at high speeds cooling. Gray cast iron is usually machined dry. In titanium and nickel alloys, high-pressure coolant through the tool breaks the chip and greatly extends tool life. And there is also minimum quantity lubrication, an oil mist instead of flooding, which suits some materials and saves fluid and cleaning. IVICQUEETETEEIY GIOU VHA LEIS VEEL TOUTE. WUE PME GI PISO ICG OLIVES HELIS OULIGUS lay. tt hard turning, a white layer sometimes forms, a thin, very hard layer of re-formed martensite, which can crack.

PEE MAMA EEIS PEICALW ICO, WUE EUEEEE. PEIN W¥EID EE CIMA UE PEICALW IGE PO PW TEIVVO TEE Ge re! & WVEkil Geek iicas stresses, it distorts. So for precision parts, you rough machine, stress relieve, and only then finish machine. PEE MAUEULEIS PEICALW IG O, WUE EUEEEE. P\LINE WEI EE Ch IME UE PEICALW IGE PO PTC TEIVVO TEE Ge re! & VVECil GekNiiica stresses, it distorts. So for precision parts, you rough machine, stress relieve, and only then finish machine. PEE MAME EEIS PEICALW TIC O, WUE TUEEEE. PEIN W¥EICEE Ch OIMA UE PEICALW IGE PO TWIP TIVE Ge re! & VWVEtil Geek iica stresses, it distorts. So for precision parts, you rough machine, stress relieve, and only then finish machine. What does this mean in the shop?

WYVitii wi IVeVANIY TEE GAUVGADIVS VPI LEICE GE TEIVEID PEICAWTIIICAMID PEICGALW IG! VE UVEIUEELEIVEE UGE WO UIIVOUl. & prehardened steel versus an annealed one with heat treatment afterwards, or a free-machining steel, where it is allowed. WYVitii wl IVVUANIIY TEE GAUVGADIVS VPI CEICE CE PEIVID PEICAWTIIIGAMZID PEICALW IG! VE UVEIUEELIVEE UGE WO UIIVOUl. & prehardened steel versus an annealed one with heat treatment afterwards, or a free-machining steel, where it is allowed. pressure coolant, and extra passes for finishing and stress relief. In the next chapter we will look at welding, and at what the heat of welding does to the material.

20. Welding and Its Effect on the Material

Welding is really a small casting and a rapid heat treatment, at the same moment. In the middle, in the weld, the metal melts and solidifies again, with a cast structure. Around it is the heat-affected zone, which did not melt, but went through very fast heating and cooling. And beyond it, the base metal, which did not change. The fusion zone, the heat-affected zone, and the base metal are three different materials, within one part. And most welding problems are found in the heat-affected zone. How much heat goes into the material is set by the heat input: the voltage, the current, and the travel speed of the welder. More heat gives a wider affected zone, slower cooling, and coarser grains.

Less heat gives faster cooling, and therefore a harder structure. In steel, the heat-affected zone goes through all the heat treatments we have met, within seconds. Next to the fusion line the grains grow a lot. A little further away they become finer. And if the steel contains enough carbon and alloying elements, the fast cooling turns this zone into hard, brittle martensite. This is where the most common problem arises: cold cracking. It needs three things together: a hard structure, hydrogen, and stress.

And it sometimes appears only hours or days after welding, so nondestructive testing of critical welds is usually done only after some time has passed. As we saw in chapter 8, the risk is estimated from the carbon equivalent. Below about 0.4, the steel welds easily. Between 0.4 and 0.6, preheating is needed. PMI GMS UU, MULTE PECTIC CELI GEE VUE FYI PIS GEL CIC CHRIEIS EER GIS PISS. FP PePIVeatiniyy, Wouany va hundred to three hundred degrees, slows the cooling, prevents hard martensite, and gives the hydrogen time to escape. The interpass temperature is also maintained. And low-hydrogen electrodes are used, such as E7018, which are stored in a drying oven, because moisture is a source of hydrogen.

For illustration: an ordinary structural steel, such as S235, is around 0.35, and it is welded without preheating. 4140 steel is around 0.8, and it has to be preheated to two hundred fifty to three hundred degrees, and heat treated after welding. Post-weld heat treatment, PWHT, does two things: it tempers the heat-affected zone, and it relieves the internal stresses the welding left behind. In carbon manganese steels it is usually done at around six hundred degrees, and welding codes require it above a certain thickness.

But with quenched and tempered steel you have to be careful: if the treatment temperature exceeds the original tempering temperature, the steel softens. IS Vevuviiu vr VMISTID Oo TIVE Ul GUNiity, WVEEIUEE IVETE ¥W¥EEEINN LEIS VY¥UIUE IO VU yiliyy. wUliUl aGitu el Voy miu. form liquid films at the grain boundaries, and a deep, narrow weld under high restraint tears apart before it has even solidified. That is why free-machining steels, with sulfur or lead, can hardly be welded, as we saw in chapter 19. Stainless steel has its own rules. In austenitic stainless steel you have to watch out for sensitization to intergranular corrosion, so L grades are welded, as we saw in chapter 10.

It also distorts more than steel, because it expands more and conducts heat less. And to prevent hot cracking, filler wires such as 308L and 309L are designed to leave a little delta ferrite in the weld. J7t. 0 GIVOY UVOSU LY FYCIU OLIGO OI LY VGN! OLE. PWVIGEI LU TIOILIY OLGITHUOD OLE PIS U US PIViiweatiiiy and post-weld heat treatment, and duplex needs a controlled heat input, to preserve the balance between its two phases. J7tW./§ 0 GIVY UVES U LY FYOCIU OLGA OL CY VGN! OLE. VIGIL TIOILILY OLGIHUCOD OLE PSU US PIViieatiiiy and post-weld heat treatment, and duplex needs a controlled heat input, to preserve the balance between its two phases. In aluminum, the oxide layer melts at a temperature three times higher than the metal itself, so it is cleaned off before welding, and processes that break it up are used.

Hydrogen from moisture creates porosity, so cleanliness and dry wire are critical. And in alloys strengthened by heat treatment, such as 6061 in the T6 condition, the heat-affected zone loses a large part of its strength, as we saw in chapters 4 and 12. The 2000 and 7000 series alloys can hardly be fusion welded, so friction stir welding is used: a rotating tool heats and stirs the metal in the solid state, without melting it, and the joint is much stronger. Welding also distorts the part. The weld shrinks as it cools, and pulls the part with it.

So a balanced welding sequence is planned, tack welds are made, fixtures are used, and sometimes the parts are pre-set at an angle, so they straighten out as they cool. And the residual stresses that remain can reach the yield strength of the material. When a welded part is machined, these stresses are released, and the part moves. So for precision parts, you weld, stress relieve, and only then finish machine. The other materials we have met also have their rules, and we saw them in their own chapters. Titanium needs full inert gas shielding over the whole hot zone. Nickel alloys need meticulous cleanliness.

Copper conducts the heat away from the weld, so it needs a lot of heat. And cast iron needs preheating and nickel electrodes. And when two different metals are welded together, you also have to think about the difference in thermal expansion, galvanic corrosion between them, and sometimes a buffer layer. W¥iiGat UV oO Lillo TEIW GE Tit LEI oF VP: VYVIiiiy at mii Caan re! LiiweeUso ati errRIVYSU WwuIMity ed VUCuUUrL, a WPS, and a procedure qualification record, a PQR, to a standard such as ISO 15614 or AWS D1.1, and a qualified welder. V¥itat UV Lillo TEI GE Tit Lilt oF VP: VeVeIiiiy ate Hite caret re! LriweeUeso ait err IiVYLSU WeIrminy vr VUCuUUL, a WPS, and a procedure qualification record, a PQR, to a standard such as ISO 15614 or AWS D1.1, and a qualified welder.

how critical the part is. And the acceptance level for weld defects is defined by a standard such as ISO 5817, with quality levels B, C, and D. And what drives the cost? Preheating, post-weld heat treatment, nondestructive testing, qualifications, and correcting distortion. So it is worth asking already at the design stage: does this need to be welded at all, or can it be bolted, brazed, or machined from one piece? In the next chapter we will look at the forms in which material is supplied, and how they affect properties and cost.

21. Supply Forms

rite variiu amy WOE GALTTEVE TE UPPED Piik IVtiio. WO, piety, mMrygitty, Vevtiit>y; VAL UOIVET, LUNE, VI a re! Lirrauen from powder. And the form changes not only the price and the amount of machining, but the properties themselves. Forged 4140 and cast 4140 are, in effect, two different materials. In this chapter we will look at the supply forms, and how to choose between them. Most bars and plates are made by rolling. Hot-rolled steel arrives with a black oxide layer, mill scale, and sometimes also with a decarburized layer. Its tolerances are wide, but it has few internal stresses. Cold-drawn bars arrive bright, with tight tolerances, and with higher strength thanks to the cold work.

But they contain internal stresses, so an asymmetric part machined from them can distort. And there are also peeled bars and ground bars, at tolerance h9 or better, with the outer skin removed, along with the decarburized layer. To give an order of magnitude: in a hot-rolled round bar, the diameter tolerance is usually tenths of a millimeter to a millimeter or more, depending on size. In a cold-drawn bar it is around h11, and a ground bar can reach h8 or even h7.

For plate, flatness matters too. Hot-rolled steel plate arrives with an oxide layer and with waviness, and cold-rolled plate is smoother and flatter. In aluminum, as we saw in chapter 12, plate in the T651 condition has been stretched to relieve stresses. And when maximum flatness and stability are needed, there is cast aluminum tooling plate, such as MIC-6: it is weaker than rolled plate, but almost stress free, and stays flat even after heavy machining. edge. But flame, plasma, and laser cutting create a heat-affected zone at the edge, and in hardenable steels that edge can be very hard.

And there are also plates and blocks that arrive pre-machined, ground and flat, which save the first machining operation. Rolling also gives the material a direction. The grains and inclusions are stretched along the rolling direction, like the grain in wood. So the properties differ in different directions, and that is anisotropy. Strength, ductility, and impact resistance are better along the rolling direction, and worse across it. And the weakest is the through-thickness direction. When a thick plate is pulled in the thickness direction, for example beneath a weld, it can tear in layers. This is lamellar tearing.

So for loaded parts, the rolling direction is marked on the drawing, and the part is positioned in the plate accordingly. re Miygitty™, tri yet TiIVV¥Y IVEIVY¥ oO Cit vilaVe Vi tii re! . hia miytv VIGHINOIIGIE, IVE VAG, rit yierr flow curves around the shoulders and radii, instead of being cut through them, as happens in a part machined from bar. ue Miygitt™, tit yiete IVY IVEIVY¥ oO Clit vilaVe Wi tii re! &. hia MiyysY VIGEINOITICIA, IVE VAGHIIVIL, tii yierr flow curves around the shoulders and radii, instead of being cut through them, as happens in a part machined from bar. So a forged part is more resistant to fatigue and impact. There is open-die forging, for large, simple parts, and closed-die forging, for complex parts in quantity, very close to the final shape.

The drawback: the die is expensive, and the lead time is long. Casting allows the most complex shapes, as we saw in chapters 11 and 12, and in the manufacturing processes course. Cast material is uniform in every direction, but it has porosity, coarser grains, and lower ductility than forged material. So critical parts get stricter inspection, and sometimes hot isostatic pressing, to close the porosity.

In aluminum, extruded profiles are a very common supply form. You can get almost any cross-section, with internal cavities, in the T5 or T6 condition. Tubes come in two main types: seamless tubes, made from a solid billet, and welded tubes, rolled from strip and welded along their length. Seamless tube withstands pressure better, and welded tube is cheaper and more precise in wall thickness. And for hollow parts there are also hollow bars, which save long drilling. Small parts in large quantities are sometimes made from powder. The powder is pressed in a die and sintered, giving an almost finished part, with controlled porosity, as in oil-impregnated bushings.

Metal injection molding, MIM, allows very small and complex parts, at high density. And 3D printing, which we met in the chapters on titanium and tungsten, suits small quantities and shapes that cannot be made any other way. Here too, the properties can differ in different directions, depending on the build direction. Each supply form needs a different machining allowance. With hot-rolled steel you have to remove the oxide layer and the decarburized layer, usually a millimeter or more per side, depending on size.

With drawn or ground bar you can leave less, and sometimes not machine the outer diameter at all. And with forgings and castings, the allowance is set on the casting or forging drawing. In aerospace, the ratio between the material bought and the material that stays in the part is measured: buy-to-fly. When the ratio is ten to one, ninety percent of the expensive material goes into chips. What does this mean in the shop? Order standard sizes that are easy to get, and watch whether the material comes in metric or inch sizes, because mixing the two causes waste. And check whether a close size in stock saves cutting and machining. For precision, asymmetric parts, prefer material with few stresses, or stress relieve after roughing.

For parts loaded in fatigue, consider forging. And what drives the cost? Ground bar costs more than hot- rolled bar, forging needs a die, and casting needs a pattern and inspection. But sometimes a supply form that costs more per kilogram saves far more in machining. In the next chapter we will learn how to order material correctly: standards, certificates, and receiving inspection.

22. Standards Certificates and Receiving Inspection

iw Miarvinty vaye “Tier. LAUL LEIS PICA ica veUPpPiS! PIV EUS CY PIV VY PEE TEI. CW VVEEIUEE OLCEIUIGHE MU, Tht which condition, at what size and tolerance, with which certificate, and whether there are special requirements. HIUVTIVINIS VU! Minnie PEIPQALW IG LEGAL IVR prygeee MUL To TIVE, GH Th to UlOVUVVYO OU viny GAIkwl TA tia VO machined, or worse, in the field. In this chapter we will learn how to order material, how to read a certificate, and what to check when material arrives. PEIUMEEIMIS RS VES METIY TEICARWEICHE REICH, PUUIRND PIETER MU Fo PIU, GIMENO UIOUU YIU VINNY Gh I tide Ve machined, or worse, in the field. In this chapter we will learn how to order material, how to read a certificate, and what to check when material arrives. *. ws NE NN EPR ENE NNN SE OF TOPE Ne Ee EPP EPR OF Ue FER E OPE a SSA RENEE SEEN 8 OE Eee dd J owe eRe Be ew Des gL to ASTM A29, or 42CrMo4 to EN 10083. The condition: annealed, quenched and tempered to a certain hardness, or a temper such as T651.

ian) yyvye MIMS PIVIMULSS 4 IVY PAWU CITI iito. PIS PhiGatweiiaa: Mevlyi tative Gti otaiiuaiu, iV CAGHIIVIS eee, to ASTM A29, or 42CrMo4 to EN 10083. The condition: annealed, quenched and tempered to a certain hardness, or a temper such as T651. *. ws NE NN EPR ENE NNN SE OF TOPE Ne Ee EPP EPR OF Ue FER E OPE a SSA RENEE SEEN 8 OE Eee dd J owe eRe Be ew Des gL to ASTM A29, or 42CrMo4 to EN 10083. The condition: annealed, quenched and tempered to a certain hardness, or a temper such as T651. PEIN EVEEEE, VIL, GH CYIVI GIVE, IV VAGHIIIVIS GAVVIN UlGaYVibriVUliUs Wal Gat ii’. Le meee aim Marcy. ruil special requirements, such as ultrasonic testing, grain size, cleanliness, or a low-temperature impact test. And finally, the type of certificate required, and the requirement for marking and traceability.

PE MIMS TIS UYU IVY TEINS Cilio. PUI Wal, eteay PETEEEEEEIU REO UCI RUE, OP EP LM PAW PIE PN, GUEST ITER and tempered to 28 to 32 Rockwell C, cold drawn, four meters, with ultrasonic testing and a 3.1 certificate. FEMMES TIS VV IVI TEINS Citi. PUI Wa, freaky PEPEEEEEEI AWE UCI RUE, OPE OU PNW DIVE PN, GUEST IER and tempered to 28 to 32 Rockwell C, cold drawn, four meters, with ultrasonic testing and a 3.1 certificate. FAO WHS TGV OV SEE CEU EIU BEI VMOU, MEVIQG TIGRIS TEU TEER PRE OLGIUG GI TU GV ayo tI OG material. 4140 and 42CrMo4 are very close, but their composition and property ranges are not exactly identical. FAO WYN TGV OV STE CEE UU EIU BEI VMOU, MEV TIGRIS TEUEEE MEEEIRPEE OLGIUIG GIw TIVE Cia yo LHI OG material. 4140 and 42CrMo4 are very close, but their composition and property ranges are not exactly identical.

FAO WHS TGV OV SEE CEU EIU BEI VMOU, MEVIQG TIGRIS TEU TEER PRE OLGIUG GI TU GV ayo tI OG material. 4140 and 42CrMo4 are very close, but their composition and property ranges are not exactly identical. So if substitution is allowed, you write it on the drawing or in the order, and do not leave it to the supplier to decide. And for critical parts, compare the actual composition and properties. manufacturer's declaration that the material conforms to the order, with no test results. A 2.2 certificate includes general test results, from routine production. A 3.1 certificate includes the results of tests on the specific production lot, and is signed by the manufacturer's inspection representative, who is independent of the production department.

And a 3.2 certificate is like a 3.1, but a representative of the customer or an independent inspection body has also approved it. The defense and aerospace industries usually require at least a 3.1. On old certificates you will sometimes see the previous name of the standard, DIN 50049, and a type 3.1B certificate. That is the same certificate that is called 3.1 today. And in the United States the certificate is usually called an MTR, a mill test report. pivyvy uv yjvyu PeGaUM GA VVEPLIIVGte. FiO UE GAH, LEIS PIN CAL PIU E EOE. ER TEEEIND LEIS PEECALN EE TCae Ete yvu PiGaiiteo CY Lit results on the paper. Then the chemical composition: compare each element with the limits in the standard.

and sometimes an impact test. Then the heat treatment condition, and the standard it was tested to. And finally, the signature and stamp. Any line that does not match the order is a reason to stop. Let's take an example: a certificate for a quenched and tempered 4140 bar. The heat number appears at the top of the certificate, and is also stamped on the end of the bar. Carbon is 0.41, within the range of 0.38 to 0.43. Chromium and molybdenum are within range. Yield strength, hardness, and elongation meet the requirement.

And the tests were done to ASTM A370. What raises a red flag? required test that is missing, a different standard from the one ordered, or a certificate without a signature. WVEITE LEIS PEICALW IG GlIiVNo, eveivinty Hiopevienvil iv UV. VViliVvVale LEIS VRP LibiVaace FV¥Etil LEI Viel. Inspect visually: cracks, laps, rust, and defects. Measure dimensions. Check that the marking on the material matches the heat number on the certificate. And for important materials, verify the material grade itself: with a portable hardness test, and with PMI, in which a handheld XRF analyzer shows the composition.

Material that does not conform goes into quarantine, and does not enter production until it is decided what to do with it. And be careful with the quick tests sometimes used in the shop: a magnet distinguishes austenitic stainless steel from steel, but not one steel from another. And a spark test on a grinder gives a hint about the carbon content, but it does not replace a composition test. Traceability must be maintained inside the plant too. When a bar is cut, the marking is transferred to every piece. Offcuts are marked, not thrown onto a common rack.

PE REICAILY PICO SEIS ME VIS GIS VAN DY TICE YIU, OY 4 MG UGE MS PMC TIREEE VV ERE IU PCIE IY GEES label. And buy only from approved suppliers, because counterfeit material and fake certificates are a real risk. PE REICAULY PIGHIILO LEIS MG VIS GIS VAR DY PIC YIU, OY 4 ME UGE MS PC TIREEI VVERE IU PRCT IY GEES label. And buy only from approved suppliers, because counterfeit material and fake certificates are a real risk. PE REICAILY PIGHIILO SEIS MG VIS GIS VEN MY PIC YIU, OY 4 ME UGE MS PMCTIREEIC UE VVEREIUUER PRCHUEEIY GEES label. And buy only from approved suppliers, because counterfeit material and fake certificates are a real risk. And what do you do when something does not conform? If the certificate is missing, a sample can be sent to an accredited lab for composition and property testing, but it is expensive and takes time.

SE SEEN FRENTE NEE EN FEN OE Ee Ne PABA AJAY DP “Mere sw wewws Se NN FERRE NEE PEN OR Ee Ne etetant al betne “Pt SEN OSE EEN Ey in writing, before work starts. And if there is no approval, the material is rejected. A verbal decision, next to the machine, is the source of most problems. ee eee ee eek ee ee 2 eee AE ee Nel NAT ES 7 J “Mere Sway wwws Se Ne FERRE NEE PEN OREN Ne ey “et SEEN OSE EEN Ey in writing, before work starts. And if there is no approval, the material is rejected. A verbal decision, next to the machine, is the source of most problems. Se SEEN FR ENA EERE NEE ENE FEE OE Ee NAAT PPh ec! “Mere sey Vewws Se NN FERRE NEE PEN OREN a “Pr SEEN OSE EE Eg in writing, before work starts. And if there is no approval, the material is rejected. A verbal decision, next to the machine, is the source of most problems. The defense and aerospace industries have additional requirements. AMS and MIL standards define not only the composition, but also the production method, the testing, and the cleanliness level.

The customer's requirements flow down to every subcontractor, and full traceability, from the melt to the finished part, is part of the product itself. VViiMat UV Lillo THIWGii tli tii vilvP: rw PeYUINVIIVIIO Tiwi iit Miawvinty GAM LEIS VUOLVETIOE Pitot yVv Tt the material order, word for word. And what drives the cost? A 3.2 certificate costs more than a 3.1, because it needs an outside inspector. Special tests, such as ultrasonic or low-temperature impact, add cost and time. Aerospace grade material costs more, and there is sometimes a minimum order quantity. more than all the certificates put together. In the next chapter, the last in the course, we will bring it all together: how to choose a material.

23. Material Selection

Throughout the course we have met dozens of materials. In this last chapter we will bring it all together, and answer the question behind every drawing: how do you choose a material? A good choice is not the strongest material, and not the cheapest, but the material that does the job at the lowest total cost, with reasonable risk. PRIS PIV VY VIGO TEUETEE LEIS PUIURIUIEE UE REID PPCHE RL, PER PEMEEE REIS PEPCARW ET ical. FTE Oe yjvu MAVTHEIS Chit requirements. Then you go through the candidates, and filter out any that fail even one essential requirement. Those that remain are ranked by cost and advantages. And finally the choice is verified: by calculation, by a prototype, or by previous experience.

VVC UY YUU PISS OY NIU CAME REIS VG. PEI IU. OIG, VY, VE OEIVUNR VG. PUY Otllt thin Vat must be, and how much it can weigh. What temperature it works at, and in what environment: humidity, seawater, chemicals. VVC UY YUU PISS OU NEI CAM REIS VG. PEI IU. OLE, VY, VE OEIVUR VG. PUY Otlit thin Pat must be, and how much it can weigh. What temperature it works at, and in what environment: humidity, seawater, chemicals. WVEIVCEISE LEIS ETS fo YYUCH. FIV ¥¥ Th V¥iti Bt PhiGaue,. PETQQUEH iy, Wweriilty, PIV GAL LIV GALI TE, VVatiiiy-. V¥Vitcat tii tolerances are, and how stable it needs to be over time. And how many units, at what price, and with which customer and standard requirements. WVEIVCLEISE LEIS IS fo YYUCGH. PIV ¥¥ TL ¥¥iti WS Pict. PEIQQWEETE TI, Wweriity, PIV GAL CIWS TEL, VVatiiiy-. V¥itcat iit tolerances are, and how stable it needs to be over time. And how many units, at what price, and with which customer and standard requirements.

And it is important to separate essential requirements from desirable ones. An essential requirement, such as seawater resistance or a temperature of six hundred degrees, rules a material out immediately. A desirable requirement, such as low weight or easy machining, is a consideration in the ranking. An important tool is the Ashby materials charts. Ashby charts plot strength against density, or stiffness against density, for all the material families together. And they are used with a performance index: when you want a stiff, light beam, for example, what matters is not the modulus alone, but the square root of the modulus divided by the density.

By this index, aluminum beats steel for a light beam, even though its modulus is three times lower. And for a tension rod, where the modulus divided by density is what matters, they are almost equal. When there are several good candidates, a decision matrix is used. VU Tot LEIS VUPiteiia, yit Vavii Vii. G@ VWergitt, wv CAGHIIIVIS vic yee t¥¥writy tive rY' Veil, VViiVolVi resistance twenty, cost twenty-five, machinability twenty, and availability ten, and score each material from one to five on each criterion. For a marine bracket, for example, 316L will score high on cost and machinability, duplex on strength and corrosion, and titanium on corrosion and weight, but low on cost.

The total score does not decide for us, but it shows clearly what we are giving up with each choice. LVlvo yVv atl vVuy't! VOVETIS FCiiitiqaar VAGHIIVICS. ian} ye! MYA Oliatit. yYuELr MiiteuU ail twelliivel CU riety, Gti IV a thick section or a high load, 4340. The gears themselves: carburizing steels such as 8620 and 16MnCr5, which we saw in chapter 7. LVtvo yVv tile vVuy' VOVETIS PCriiitiaat VAGHIIVICS. ian) ye! WYA oliait. yuEL Miu Gail twllivel VU Sr iery, Gti IV a thick section or a high load, 4340. The gears themselves: carburizing steels such as 8620 and 16MnCr5, which we saw in chapter 7. An aircraft bracket: 7075 in the T7351 condition, and where it is hot or corrosive, Grade 5 titanium. A marine fitting: 316L, duplex, aluminum bronze, or Grade 2 titanium.

or MIC-6 when flatness is critical. A blanking punch: D2, A2, or a powder steel when the load is especially high. A part in a hot exhaust system: 321 and 310 stainless for the heat, and above that, Inconel 625 or 718. And a bushing sliding on a shaft: bronze, or a polymer such as POM or PEEK. And let's go through one example from start to finish: a seawater pump shaft. The essential requirements: seawater resistance, and enough strength in torsion and fatigue. The candidates: coated 4140, 316 stainless, duplex, and 17-4PH. 4140 is ruled out, because a single scratch in the coating starts corrosion. 316 is resistant, but too weak for this load.

more sensitive to crevice corrosion in stagnant water, so it matters whether the pump stands idle for long periods. And between the two, the decision comes down to cost, availability, and how easily they can be machined on the machines we have. paying for it in machining and heat treatment. Requiring high hardness without thinking about toughness. apart in the galvanic series. Choosing an exotic material that is not in stock. And not knowing what welding or heat treatment will do to the material.

PMID TEIUOR TEEERUERCHEER,. REIS CURE UVOL VE LEIS PGE, PIU LEIS VEINS Vl Nivylaitt. PEI PHIGARwHIG VMIIVS Io VI uy Vii part. Added to it are machining time, which depends on machinability, heat treatment, coating, testing, and the risk of rejects. FPMIM TEIVOE’ Hii RGrik. LEIS CVLGEE UVot VE LEIS re! &, PIV’ CEI vr Ve rY' Nitya. Pit thiaawweiiaar el VY tivo Vi uy Vii part. Added to it are machining time, which depends on machinability, heat treatment, coating, testing, and the risk of rejects. Sometimes 303 instead of 304, 12L14 instead of 1018, or aluminum instead of steel, saves more in machining than it costs in material. And sometimes a material that costs more per kilogram, and arrives prehardened, saves heat treatment, distortion, and a grinding operation.

about one and a half to two, aluminum and 304 stainless about four to six, titanium twenty or more, and nickel alloys thirty or more. But remember that aluminum weighs a third as much as steel, so by volume it is much cheaper than it looks per kilogram. And a good choice also takes availability into account. A common material, in standard sizes, arrives fast and cheap. A rare material, or a non-standard size, can add weeks and cost. So it is worth knowing what material suppliers keep in stock, and asking them before you decide.

In this course we went through the fundamentals of metallurgy, steels and cast iron, the non-ferrous metals, behavior in service, the material in manufacturing, and ordering. And if there is one thing to take from all of this, it is three questions worth asking about every material: what does the part need, what does the manufacturing process need, and what does it cost in the end. Whoever asks these three questions chooses right in most cases. Thank you for joining us in the Materials and Metallurgy course from METAL HUB.