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Gray Cast Iron (Class 30)

Gray Cast Iron Class 30 is named for its roughly 207 MPa (30 ksi) tensile strength. Its graphite-flake structure gives two standout traits: excellent machinability (4.5/5) and outstanding vibration damping. The same flakes make it brittle, with near-zero elongation (0.5%) and no true yield point — it is a compression-loaded, vibration-damping material, not a ductile one.

How Gray Cast Iron (Class 30) machines

At 4.5/5 it is one of the most machinable metals here; the graphite flakes act as built-in chip breakers and lubricant, giving short chips, low cutting forces, and good finishes with long tool life. This easy machining, combined with low cost, is a key reason it remains the default for large machined castings.

Manufacturing & processing

Gray iron is supplied as castings and machined to final features. It casts fluidly into complex shapes with thin and thick sections, and its low shrinkage aids dimensional control. Corrosion resistance is modest (2.0/5). Being brittle, it must not be designed for tensile or impact loading — keep stresses compressive.

Typical applications

Used for engine blocks, cylinder heads, machine tool bases, brake rotors, pump and valve bodies, and gear housings — parts that benefit from vibration damping, rigidity in compression, and cheap, easy machining. Best where mass and stiffness dampen vibration and loads stay compressive rather than tensile.

When to choose it

Choose gray iron Class 30 for large, rigid, vibration-damping castings under compressive load where cost and machinability matter — machine bases and engine blocks especially. If the part sees tensile or impact loads, switch to ductile iron, which is far tougher. For high strength, use cast or forged steel.

Suitable surface finishes

Common finishes for Gray Cast Iron (Class 30): powder coating, black oxide. Use the finish selector →

FAQ

Why does gray iron have no true yield point?
Its graphite flakes interrupt the metal matrix, so it fractures with almost no plastic deformation (0.5% elongation) — it behaves brittly in tension and never shows the ductile yielding of steel. Designers treat the listed strength as an ultimate value and keep the material in compression.
What makes gray iron so good at damping vibration?
The graphite flakes break up and absorb vibrational energy within the structure, giving gray iron far better damping than steel. That is why machine tool bases, engine blocks, and large frames use it — the casting itself quiets vibration, improving accuracy and reducing noise.
When should I use ductile iron instead of gray iron?
Choose ductile iron when the part sees tensile or impact loads. Gray iron is brittle (0.5% elongation), while ductile iron's nodular graphite gives around 12% elongation and a real yield point. Gray iron stays the pick for compression-loaded, vibration-damping, easy-to-machine castings.

Property values are typical/nominal for early guidance and vary by temper, grade, supplier and heat treatment. Confirm critical specs against a certified datasheet or with an mfgiq engineer.