Gray Cast Iron

Cast iron with flake graphite

150–400 MPaTensile strength

98–250 MPaYield

160–300 HBHardness

7.15 g/cm³Density

100–170 GPaModulus E

55%Machinability

Typical values for the temper in the title

Gray cast iron contains about 3–3.5% carbon and about 2% silicon. Most of the carbon is present as graphite flakes, which give the fracture its gray color, hence the name. The flakes weaken it in tension (it is brittle, with under 1% elongation), but give special advantages: excellent vibration damping, good thermal conductivity, easy machining, and low-cost casting of complex shapes.

In compression it is much stronger than in tension (roughly 3–4 times). So it is the classic material for machine-tool bases, engine blocks, brake discs and drums, pump bodies and covers.

Mechanical and physical properties

Property	Value	Note
Tensile strength Rm	150–400 MPa
Yield strength Rp0.2	98–250 MPa	no defined yield (brittle material); database value
Elongation A	<1%
Hardness	160–300 HB
Modulus of elasticity E	100–170 GPa	varies with grade: higher in stronger grades
Density ρ	7.15 g/cm³
Machinability	55%	relative rating in the site database

Chemical composition (wt %)

Element	Range	Role
C carbon	about 2.5–4.0 (typical)	mostly as flake graphite
Si silicon	about 1–3 (typical)	promotes graphite
Mn manganese	about 0.5–1 (typical)
Fe iron	balance

Typical composition. For gray iron the standard specifies strength, not composition: the foundry adjusts composition to the grade and section size.

Grades and conditions

In cast iron, the grade is chosen by minimum strength, tested on a cast test bar. In a thick section properties are lower than the test bar, because it cools more slowly.

Grade / condition	Properties	When to choose it
A48 Class 20 / EN-GJL-150	min Rm about 138 / 150 MPa	simple parts, maximum damping
A48 Class 30 / EN-GJL-200	min Rm about 207 / 200 MPa	general machine parts
A48 Class 40 / EN-GJL-250 to 300	min Rm about 276 / 250–300 MPa	machine bases, engine blocks
A48 Class 50 / EN-GJL-350	min Rm about 345 / 350 MPa	highly loaded parts

Applications

Machine-tool bases and beds
Engine blocks and cylinder heads
Brake discs and drums
Pump and valve bodies
Covers and gearbox housings

Fabrication

Machining

Very good: graphite breaks the chip and acts as a lubricant. Usually machined dry. The black dust needs extraction.

Welding

Difficult: the high carbon forms hard, brittle zones near the weld. Repairs use nickel electrodes, preheat and slow cooling, in short runs.

Damping

Among the highest of engineering metals, so it suits precision machine bases.

Compared with similar materials

Material	Rm / Rp0.2	Welding	When it is the better choice
Ductile Iron	414–825 / 276–621 MPa	Medium — Ni Rod	spheroidal graphite: much stronger and ductile · material page
CGI (Compacted Graphite Iron)	300–450 / 250–350 MPa	Difficult	compacted graphite: in between, for diesel engines · material page
1045	570–700 / 310–450 MPa	Medium	steel: strong and ductile, does not damp vibration · material page

Common mistakes

Using gray iron in a part under impact or tensile load

It is brittle. For such a part, ductile iron or steel.

Designing to a "yield strength"

Gray iron has no defined yield strength: it breaks with almost no plastic deformation. Design to tensile strength with a suitable safety factor.

Using test-bar properties in a thick section

Strength is lower in a thick section.

Equivalents, standards and supply

EN	EN-GJL-250
DIN	GG-25 (old)
JIS	FC250
ASTM	A48 Class 35/40 (close)
Standards	ASTM A48 / EN 1561
Forms	casting only
Producers	Waupaca Foundry · Georg Fischer · Grede

Learn more

Values are typical and for comparison only. In cast iron, properties depend strongly on wall thickness and cooling rate in casting. For design, rely on the standard and test-bar results. Sources: ASTM A48, A536, A897, A532, A439 and EN 1561/1563/1564, producer data sheets, and the METAL HUB materials database.
Updated: October 31, 2026