Metallurgy Fundamentals
The science behind the engineering: phase diagrams (Fe-C, Al-Cu), crystal structures (BCC/FCC/HCP), grain size and the Hall-Petch relationship, TTT/CCT diagrams, solidification and casting metallurgy, and fractography โ understanding why materials behave the way they do.
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Metallurgy Fundamentals
Phase Diagrams ยท Crystal Structure ยท Grain Size ยท TTT/CCT ยท Solidification ยท Fractography ยท Failure Analysis
๐ Phase Diagrams โ The Foundation of Metallurgy
A phase diagram maps the stable phases of a material at each temperature and composition. Every heat treatment decision starts here.
Fe-C (Iron-Carbon) Phase Diagram
The single most important diagram in metallurgy. Maps the relationship between temperature (up to ~1600ยฐC) and carbon content (0-6.67%) for steel and cast iron.
Key phases:
โข Ferrite (ฮฑ) โ BCC, soft, magnetic, max 0.022%C at 727ยฐC
โข Austenite (ฮณ) โ FCC, non-magnetic, max 2.14%C at 1147ยฐC
โข Cementite (FeโC) โ 6.67%C, hard and brittle
โข Pearlite โ lamellar mix of ferrite + cementite (0.76%C)
Critical temperatures:
โข A1 = 727ยฐC โ eutectoid (pearlite โ austenite)
โข A3 = 727-912ยฐC โ ferrite disappears (depends on %C)
โข Acm = austenite + cementite boundary
โข 1147ยฐC โ eutectic (liquid โ austenite + cementite)
โถ YouTube
Key phases:
โข Ferrite (ฮฑ) โ BCC, soft, magnetic, max 0.022%C at 727ยฐC
โข Austenite (ฮณ) โ FCC, non-magnetic, max 2.14%C at 1147ยฐC
โข Cementite (FeโC) โ 6.67%C, hard and brittle
โข Pearlite โ lamellar mix of ferrite + cementite (0.76%C)
Critical temperatures:
โข A1 = 727ยฐC โ eutectoid (pearlite โ austenite)
โข A3 = 727-912ยฐC โ ferrite disappears (depends on %C)
โข Acm = austenite + cementite boundary
โข 1147ยฐC โ eutectic (liquid โ austenite + cementite)
Steel vs Cast Iron โ The 2.14% Line
<2.14%C = Steel โ forgeable, weldable, heat-treatable
>2.14%C = Cast Iron โ castable, brittle (gray) or ductile (SG)
Hypo-eutectoid (<0.76%C): ferrite + pearlite. Softer, more ductile. Most structural steels (1018, 4130).
Eutectoid (0.76%C): 100% pearlite. Balanced strength/hardness.
Hyper-eutectoid (>0.76%C): pearlite + cementite network. Harder but brittle. Tool steels, bearings (52100).
Rule of thumb: more carbon โ harder but less ductile and harder to weld.
>2.14%C = Cast Iron โ castable, brittle (gray) or ductile (SG)
Hypo-eutectoid (<0.76%C): ferrite + pearlite. Softer, more ductile. Most structural steels (1018, 4130).
Eutectoid (0.76%C): 100% pearlite. Balanced strength/hardness.
Hyper-eutectoid (>0.76%C): pearlite + cementite network. Harder but brittle. Tool steels, bearings (52100).
Rule of thumb: more carbon โ harder but less ductile and harder to weld.
Al-Cu Phase Diagram (Aluminum Alloys)
Explains precipitation hardening (aging) in 2xxx and 7xxx aluminum alloys.
Solvus line: the solubility limit of Cu in Al drops sharply with temperature โ this is the key to age hardening:
1. Solution treat (~500ยฐC) โ dissolve all Cu into solid solution
2. Quench โ trap Cu atoms in supersaturated solution
3. Age (natural or artificial at 120-190ยฐC) โ Cu precipitates form, blocking dislocations โ strength increases
T6 temper: solution treated + artificially aged = peak strength
Over-aging: precipitates coarsen โ strength decreases
โถ YouTube
Solvus line: the solubility limit of Cu in Al drops sharply with temperature โ this is the key to age hardening:
1. Solution treat (~500ยฐC) โ dissolve all Cu into solid solution
2. Quench โ trap Cu atoms in supersaturated solution
3. Age (natural or artificial at 120-190ยฐC) โ Cu precipitates form, blocking dislocations โ strength increases
T6 temper: solution treated + artificially aged = peak strength
Over-aging: precipitates coarsen โ strength decreases
Stainless Steel Phase Relationships
Schaeffler/DeLong diagram: predicts microstructure from Cr/Ni equivalents.
โข Austenitic (304, 316): FCC, non-magnetic, cannot be hardened by heat treatment
โข Ferritic (430): BCC, magnetic, no phase transformation
โข Martensitic (410, 440C): hardenable, magnetic
โข Duplex (2205): 50/50 austenite + ferrite โ best of both worlds
โข PH (17-4PH): martensitic + precipitation hardening
Sensitization (danger!): 450-850ยฐC โ Cr carbides form at grain boundaries โ Cr-depleted zone โ intergranular corrosion. Solution anneal at 1050ยฐC+ to fix.
โถ YouTube
โข Austenitic (304, 316): FCC, non-magnetic, cannot be hardened by heat treatment
โข Ferritic (430): BCC, magnetic, no phase transformation
โข Martensitic (410, 440C): hardenable, magnetic
โข Duplex (2205): 50/50 austenite + ferrite โ best of both worlds
โข PH (17-4PH): martensitic + precipitation hardening
Sensitization (danger!): 450-850ยฐC โ Cr carbides form at grain boundaries โ Cr-depleted zone โ intergranular corrosion. Solution anneal at 1050ยฐC+ to fix.
๐ Crystal Structure โ Why Metals Behave Differently
The atomic arrangement determines everything: ductility, strength, slip systems, and why some metals are easy to form while others crack.
BCC โ Body-Centered Cubic
Ferrite (ฮฑ-Fe) Cr Mo W V
Atoms per unit cell: 2
Packing factor: 68%
Slip systems: 48 (but only 12 active at room temp)
Behavior: strong, less ductile than FCC. Has a ductile-to-brittle transition temperature (DBTT) โ a critical concern for cold-weather structures (ships, bridges, pipelines).
Key fact: ferritic steels become brittle below DBTT. This caused the Liberty Ships fractures in WWII.
โถ YouTube
Atoms per unit cell: 2
Packing factor: 68%
Slip systems: 48 (but only 12 active at room temp)
Behavior: strong, less ductile than FCC. Has a ductile-to-brittle transition temperature (DBTT) โ a critical concern for cold-weather structures (ships, bridges, pipelines).
Key fact: ferritic steels become brittle below DBTT. This caused the Liberty Ships fractures in WWII.
FCC โ Face-Centered Cubic
Austenite (ฮณ-Fe) Al Cu Ni Au 304 SS
Atoms per unit cell: 4
Packing factor: 74%
Slip systems: 12 (all active at room temp)
Behavior: excellent ductility and formability. No DBTT โ stays ductile even at cryogenic temperatures.
Why austenitic stainless steel is used for cryogenic: FCC structure doesn't become brittle at -196ยฐC (LNโ). Ferritic/martensitic steels would shatter.
Atoms per unit cell: 4
Packing factor: 74%
Slip systems: 12 (all active at room temp)
Behavior: excellent ductility and formability. No DBTT โ stays ductile even at cryogenic temperatures.
Why austenitic stainless steel is used for cryogenic: FCC structure doesn't become brittle at -196ยฐC (LNโ). Ferritic/martensitic steels would shatter.
HCP โ Hexagonal Close-Packed
Ti (ฮฑ) Mg Zn Zr Co
Atoms per unit cell: 6
Packing factor: 74% (same as FCC)
Slip systems: 3 basal (very few!)
Behavior: limited ductility, strong texture dependence (anisotropic). Harder to form than FCC or BCC metals.
Why Mg is hard to form at room temp: only 3 slip systems. Needs 200-300ยฐC to activate pyramidal slip โ warm forming required. Same issue (less severe) for titanium.
Atoms per unit cell: 6
Packing factor: 74% (same as FCC)
Slip systems: 3 basal (very few!)
Behavior: limited ductility, strong texture dependence (anisotropic). Harder to form than FCC or BCC metals.
Why Mg is hard to form at room temp: only 3 slip systems. Needs 200-300ยฐC to activate pyramidal slip โ warm forming required. Same issue (less severe) for titanium.
Allotropic Transformations
Some metals change crystal structure with temperature. This is the entire basis of steel heat treatment:
Iron:
โข Room temp โ 912ยฐC: BCC (ferrite)
โข 912ยฐC โ 1394ยฐC: FCC (austenite)
โข 1394ยฐC โ 1538ยฐC: BCC (delta ferrite)
Titanium:
โข <882ยฐC: HCP (alpha)
โข >882ยฐC: BCC (beta)
Why this matters: when austenite (FCC) is quenched fast enough, it can't transform back to ferrite โ instead it forms martensite (BCT), which is the hardest phase in steel.
Iron:
โข Room temp โ 912ยฐC: BCC (ferrite)
โข 912ยฐC โ 1394ยฐC: FCC (austenite)
โข 1394ยฐC โ 1538ยฐC: BCC (delta ferrite)
Titanium:
โข <882ยฐC: HCP (alpha)
โข >882ยฐC: BCC (beta)
Why this matters: when austenite (FCC) is quenched fast enough, it can't transform back to ferrite โ instead it forms martensite (BCT), which is the hardest phase in steel.
๐ Grain Size โ The Hall-Petch Relationship
Smaller grains = higher strength AND higher toughness. One of the few strengthening mechanisms that improves both simultaneously.
ASTM E112 โ Grain Size Number
Formula: N = 2(G-1) grains per square inch at 100ร
Higher G = finer grain = stronger. Defense/aerospace specs often require G โฅ 5.
โถ YouTube
| ASTM G | Avg. diameter (ฮผm) | Typical use |
|---|---|---|
| 1-3 | 200-100 | As-cast, annealed slowly |
| 4-6 | 90-45 | Normalized steel, general |
| 7-9 | 32-16 | Q&T steel, fine-grained |
| 10-12 | 11-6 | Micro-alloyed, HSLA |
| 13-14 | 4-3 | SPD, nanocrystalline |
Hall-Petch Equation
ฯ_y = ฯโ + k / โd
Where:โข ฯ_y = yield strength
โข ฯโ = friction stress (lattice resistance)
โข k = Hall-Petch slope (material constant)
โข d = average grain diameter
Implication: halving the grain diameter increases yield strength by ~41% (โ2 factor). This is why normalizing (air cool from austenite) produces finer grain and higher strength than slow furnace cooling (annealing).
Grain Refinement Methods
1. Micro-alloying: small additions of Nb, V, Ti pin grain boundaries โ prevent grain growth during hot rolling. This is how HSLA steels achieve high strength without high carbon.
2. Controlled rolling: finish rolling below recrystallization temperature โ pancaked austenite โ more nucleation sites โ finer ferrite grain.
3. Normalizing: heat to 30-50ยฐC above A3, air cool โ finer and more uniform grain than as-rolled.
4. Severe plastic deformation (SPD): ECAP, HPT โ nanocrystalline grain (G > 14). Research/specialty.
2. Controlled rolling: finish rolling below recrystallization temperature โ pancaked austenite โ more nucleation sites โ finer ferrite grain.
3. Normalizing: heat to 30-50ยฐC above A3, air cool โ finer and more uniform grain than as-rolled.
4. Severe plastic deformation (SPD): ECAP, HPT โ nanocrystalline grain (G > 14). Research/specialty.
Grain Growth โ The Enemy
Problem: holding too long at high temperature โ grains grow โ strength drops.
When it happens:
โข Welding HAZ (heat-affected zone) โ the #1 concern
โข Over-soaking during heat treatment
โข Hot forging at excessive temperature
Prevention: micro-alloying (Nb, V, Ti), controlled soaking time, correct austenitizing temperature (not higher than needed).
When it happens:
โข Welding HAZ (heat-affected zone) โ the #1 concern
โข Over-soaking during heat treatment
โข Hot forging at excessive temperature
Prevention: micro-alloying (Nb, V, Ti), controlled soaking time, correct austenitizing temperature (not higher than needed).
โ ๏ธ This is why welding thick sections of fine-grained steel (like HSLA) requires careful heat input control โ you can undo the grain refinement in the HAZ.
๐ TTT / CCT Diagrams โ Transformation Kinetics
Phase diagrams tell you what phases are stable. TTT/CCT diagrams tell you what phases you actually get at a given cooling rate.
TTT โ Time-Temperature-Transformation
Also called "isothermal transformation diagram" or "C-curve" (because of its shape).
How to read it: start from austenite temperature, drop vertically to a holding temperature, read the time to start/finish transformation.
Regions (from top to bottom):
โข Pearlite nose: ~540ยฐC โ fastest diffusion-based transformation
โข Bainite bay: 250-540ยฐC โ needle-like, strong
โข Ms line: ~300ยฐC (for 0.4%C) โ martensite start
โข Mf line: ~200ยฐC โ martensite finish
To get martensite: must cool fast enough to miss the pearlite nose entirely.
โถ YouTube
How to read it: start from austenite temperature, drop vertically to a holding temperature, read the time to start/finish transformation.
Regions (from top to bottom):
โข Pearlite nose: ~540ยฐC โ fastest diffusion-based transformation
โข Bainite bay: 250-540ยฐC โ needle-like, strong
โข Ms line: ~300ยฐC (for 0.4%C) โ martensite start
โข Mf line: ~200ยฐC โ martensite finish
To get martensite: must cool fast enough to miss the pearlite nose entirely.
CCT โ Continuous Cooling Transformation
More practical than TTT because real cooling is continuous, not isothermal.
Reading CCT: overlay a cooling curve on the diagram. Where it crosses the transformation boundaries โ that's the microstructure you get.
Slow cool (furnace): crosses pearlite region โ soft, annealed
Medium cool (air/oil): may produce pearlite + bainite mix
Fast cool (water): misses nose โ martensite
Critical cooling rate: the minimum cooling rate to get 100% martensite โ depends heavily on alloy content. Plain carbon steel needs very fast water quench; 4340 can be oil-quenched (alloys shift the nose right).
Reading CCT: overlay a cooling curve on the diagram. Where it crosses the transformation boundaries โ that's the microstructure you get.
Slow cool (furnace): crosses pearlite region โ soft, annealed
Medium cool (air/oil): may produce pearlite + bainite mix
Fast cool (water): misses nose โ martensite
Critical cooling rate: the minimum cooling rate to get 100% martensite โ depends heavily on alloy content. Plain carbon steel needs very fast water quench; 4340 can be oil-quenched (alloys shift the nose right).
Martensite โ The Hardest Phase
What it is: a supersaturated solid solution of carbon in BCT (body-centered tetragonal) iron. Forms by diffusionless (shear) transformation from austenite.
Properties:
โข Extremely hard (up to 65+ HRC for high-carbon steel)
โข Very brittle in as-quenched state
โข Must be tempered to gain usable toughness
Ms temperature depends on: carbon content and alloying. More C/alloy โ lower Ms โ risk of retained austenite (untransformed austenite trapped at room temperature).
โถ YouTube
Properties:
โข Extremely hard (up to 65+ HRC for high-carbon steel)
โข Very brittle in as-quenched state
โข Must be tempered to gain usable toughness
Ms temperature depends on: carbon content and alloying. More C/alloy โ lower Ms โ risk of retained austenite (untransformed austenite trapped at room temperature).
๐ก This is exactly why high-carbon tool steels (D2, M2) need sub-zero treatment (-80ยฐC) โ their Mf is below room temperature, so some austenite is retained after normal quenching.
Bainite โ The Underappreciated Phase
Forms between pearlite and martensite temperature ranges.
Upper bainite (350-540ยฐC): feathery, moderate strength, limited toughness
Lower bainite (250-350ยฐC): acicular, high strength + good toughness โ often better than tempered martensite!
Austempering: quench to bainite range and hold โ all austenite transforms to bainite. Advantages: no quench cracks, less distortion, good strength+toughness without tempering.
ADI (Austempered Ductile Iron): same principle applied to ductile cast iron โ transforms to "ausferrite" โ exceptional properties for castings.
Upper bainite (350-540ยฐC): feathery, moderate strength, limited toughness
Lower bainite (250-350ยฐC): acicular, high strength + good toughness โ often better than tempered martensite!
Austempering: quench to bainite range and hold โ all austenite transforms to bainite. Advantages: no quench cracks, less distortion, good strength+toughness without tempering.
ADI (Austempered Ductile Iron): same principle applied to ductile cast iron โ transforms to "ausferrite" โ exceptional properties for castings.
๐ก๏ธ Solidification โ From Liquid to Solid
How metal freezes determines casting quality: porosity, segregation, dendrite arm spacing, and grain structure.
Dendritic Solidification
Most metals solidify as dendrites (tree-like crystals) growing into the liquid.
SDAS (Secondary Dendrite Arm Spacing): the key quality metric.
โข Smaller SDAS โ faster cooling โ better mechanical properties
โข Typical: sand cast 50-200ฮผm, permanent mold 20-50ฮผm, die cast 5-15ฮผm
Between the dendrite arms: last liquid to solidify โ enriched in solute โ micro-segregation โ potential porosity, hot tearing, and property variation.
โถ YouTube
SDAS (Secondary Dendrite Arm Spacing): the key quality metric.
โข Smaller SDAS โ faster cooling โ better mechanical properties
โข Typical: sand cast 50-200ฮผm, permanent mold 20-50ฮผm, die cast 5-15ฮผm
Between the dendrite arms: last liquid to solidify โ enriched in solute โ micro-segregation โ potential porosity, hot tearing, and property variation.
Casting Defects โ Metallurgical Root Causes
Shrinkage porosity: liquid metal contracts ~3-6% on solidifying. If feed metal can't reach the last-to-solidify zone โ cavity. Solution: risers, directional solidification.
Gas porosity: dissolved gas (Hโ in Al, Nโ in steel) comes out of solution during solidification โ round pores. Solution: degassing, vacuum casting.
Hot tearing: semi-solid metal pulled apart by thermal contraction before fully solid. Solution: avoid sharp corners, add fillets, adjust alloy composition.
Segregation: macro (center of ingot enriched) and micro (between dendrites). Homogenization anneal can fix micro, not macro.
Gas porosity: dissolved gas (Hโ in Al, Nโ in steel) comes out of solution during solidification โ round pores. Solution: degassing, vacuum casting.
Hot tearing: semi-solid metal pulled apart by thermal contraction before fully solid. Solution: avoid sharp corners, add fillets, adjust alloy composition.
Segregation: macro (center of ingot enriched) and micro (between dendrites). Homogenization anneal can fix micro, not macro.
๐ Fractography โ Reading the Fracture Surface
A fracture surface tells the story of how a part failed โ the loading type, the material condition, and where the crack started. This is the detective work of metallurgy.
Ductile Fracture
Desirable in most cases
Visual clues:
โข Gray, fibrous, "torn" appearance
โข Significant plastic deformation (necking) before fracture
โข Cup-and-cone fracture in tension specimens
โข 45ยฐ shear lips at edges
Under SEM: dimples (micro-void coalescence) โ the classic sign. Dimple shape indicates loading: equiaxed = tension, elongated = shear.
Meaning: the material absorbed energy before failing. The part was not operating far above its design capacity โ it gave warning before breaking.
โถ YouTube
Visual clues:
โข Gray, fibrous, "torn" appearance
โข Significant plastic deformation (necking) before fracture
โข Cup-and-cone fracture in tension specimens
โข 45ยฐ shear lips at edges
Under SEM: dimples (micro-void coalescence) โ the classic sign. Dimple shape indicates loading: equiaxed = tension, elongated = shear.
Meaning: the material absorbed energy before failing. The part was not operating far above its design capacity โ it gave warning before breaking.
Brittle Fracture
Dangerous โ no warning!
Visual clues:
โข Bright, shiny, crystalline, flat surface
โข Chevron marks (V-patterns) pointing back to origin
โข Little or no plastic deformation
โข No necking, no shear lips
Under SEM: cleavage facets (flat, smooth, with river marks) โ trans-granular brittle fracture. Or smooth grain-boundary facets โ inter-granular brittle fracture (corrosion, temper embrittlement, hydrogen).
Meaning: catastrophic. The material had no capacity to absorb energy. Causes: wrong material, operating below DBTT, hydrogen embrittlement, temper embrittlement.
Visual clues:
โข Bright, shiny, crystalline, flat surface
โข Chevron marks (V-patterns) pointing back to origin
โข Little or no plastic deformation
โข No necking, no shear lips
Under SEM: cleavage facets (flat, smooth, with river marks) โ trans-granular brittle fracture. Or smooth grain-boundary facets โ inter-granular brittle fracture (corrosion, temper embrittlement, hydrogen).
Meaning: catastrophic. The material had no capacity to absorb energy. Causes: wrong material, operating below DBTT, hydrogen embrittlement, temper embrittlement.
Fatigue Fracture
The #1 cause of in-service failure
Visual clues:
โข Beach marks (clamshell marks) โ concentric arcs radiating from origin. Each mark represents a period of crack growth.
โข Smooth initiation zone + rough final fracture zone
โข Origin almost always at a stress concentrator: fillet, keyway, corrosion pit, machining mark
Under SEM: striations โ microscopic lines, each one = one load cycle.
Key insight: beach marks tell you the crack was growing for a long time before final failure. NDT inspection at the right interval could have caught it.
Visual clues:
โข Beach marks (clamshell marks) โ concentric arcs radiating from origin. Each mark represents a period of crack growth.
โข Smooth initiation zone + rough final fracture zone
โข Origin almost always at a stress concentrator: fillet, keyway, corrosion pit, machining mark
Under SEM: striations โ microscopic lines, each one = one load cycle.
Key insight: beach marks tell you the crack was growing for a long time before final failure. NDT inspection at the right interval could have caught it.
Creep & Stress Rupture
High temperature service
Visual clues:
โข Intergranular (along grain boundaries) cracks and voids
โข Grain boundary sliding visible under microscope
โข Elongated, necked appearance in creep rupture
โข Oxide scale on fracture surface (exposed at high temp for a long time)
Meaning: the part was operating above ~0.4รTm (absolute melting point) under sustained stress. Turbine blades, boiler tubes, furnace components.
Visual clues:
โข Intergranular (along grain boundaries) cracks and voids
โข Grain boundary sliding visible under microscope
โข Elongated, necked appearance in creep rupture
โข Oxide scale on fracture surface (exposed at high temp for a long time)
Meaning: the part was operating above ~0.4รTm (absolute melting point) under sustained stress. Turbine blades, boiler tubes, furnace components.
โ ๏ธ Failure Analysis โ Systematic Investigation
When a part fails in service, the metallurgical investigation follows a structured process to find the root cause and prevent recurrence.
The 8-Step Failure Analysis Process
1. Preserve evidence โ DO NOT clean or touch the fracture surface! Match fracture halves, photograph everything in-situ.
2. Collect history โ service conditions, loading, time in service, maintenance records, similar failures.
3. Visual/macro examination โ fracture origin, crack path, deformation, corrosion products.
4. Non-destructive testing โ MPI/PT for other cracks, UT for internal defects.
5. Fractography (SEM) โ ductile vs brittle, fatigue striations, intergranular/transgranular.
6. Metallography โ cross-section, microstructure, grain size, inclusions, decarburization.
7. Chemical analysis โ OES/XRF verify correct alloy. EDS on fracture surface for contaminants (Cl, S, P).
8. Mechanical testing โ hardness profile, tensile test from adjacent material if available.
2. Collect history โ service conditions, loading, time in service, maintenance records, similar failures.
3. Visual/macro examination โ fracture origin, crack path, deformation, corrosion products.
4. Non-destructive testing โ MPI/PT for other cracks, UT for internal defects.
5. Fractography (SEM) โ ductile vs brittle, fatigue striations, intergranular/transgranular.
6. Metallography โ cross-section, microstructure, grain size, inclusions, decarburization.
7. Chemical analysis โ OES/XRF verify correct alloy. EDS on fracture surface for contaminants (Cl, S, P).
8. Mechanical testing โ hardness profile, tensile test from adjacent material if available.
Common Root Causes โ Quick Reference
| Symptom | Likely cause |
|---|---|
| Intergranular + Cl on surface | SCC (stress corrosion cracking) |
| Intergranular + recent plating | Hydrogen embrittlement |
| Beach marks from keyway | Fatigue โ stress concentration |
| Cleavage at low temp | DBTT โ wrong material selection |
| Soft core, hard case, crack at interface | Case hardening too deep |
| Decarburized surface + fatigue | Heat treatment atmosphere |
| Large grain + low hardness | Over-soaking / wrong temperature |
Hydrogen Embrittlement โ A Special Danger
Delayed fracture โ can happen hours/days after load!
Susceptible materials: high-strength steel (>1000 MPa / >32 HRC), especially martensitic.
Sources of hydrogen:
โข Electroplating (especially Cd, Zn, hard Cr) โ MOST COMMON
โข Welding (moisture in electrode coating)
โข Pickling / acid cleaning
โข Service environment (sour gas / HโS)
Prevention: bake out within 4 hours after plating (190ยฐC, 8-24h per ASTM B849). Pre-plate stress relief. Material substitution below 32 HRC for critical parts.
Susceptible materials: high-strength steel (>1000 MPa / >32 HRC), especially martensitic.
Sources of hydrogen:
โข Electroplating (especially Cd, Zn, hard Cr) โ MOST COMMON
โข Welding (moisture in electrode coating)
โข Pickling / acid cleaning
โข Service environment (sour gas / HโS)
Prevention: bake out within 4 hours after plating (190ยฐC, 8-24h per ASTM B849). Pre-plate stress relief. Material substitution below 32 HRC for critical parts.
Metallographic Examination โ What to Look For
Etchant: 2-5% Nital (for carbon/alloy steel), Kalling's (for stainless), Keller's (for aluminum).
What the microstructure tells you:
โข Pearlite spacing: fine = faster cooling (normalized), coarse = slow (annealed)
โข Prior austenite grain size: reveals heat treatment temperature
โข Decarburization: ferrite layer at surface = furnace atmosphere problem
โข Banding: alternating layers of ferrite/pearlite = segregation from casting
โข Inclusions: MnS (elongated, gray), AlโOโ (angular, dark) โ stress risers for fatigue
Standard: ASTM E3 (preparation), ASTM E407 (etching), ASTM E112 (grain size).
What the microstructure tells you:
โข Pearlite spacing: fine = faster cooling (normalized), coarse = slow (annealed)
โข Prior austenite grain size: reveals heat treatment temperature
โข Decarburization: ferrite layer at surface = furnace atmosphere problem
โข Banding: alternating layers of ferrite/pearlite = segregation from casting
โข Inclusions: MnS (elongated, gray), AlโOโ (angular, dark) โ stress risers for fatigue
Standard: ASTM E3 (preparation), ASTM E407 (etching), ASTM E112 (grain size).