Equivalent Carbon Content - Cast Iron

Cast Iron

For cast iron the equivalent carbon content (CE) concept is used to understand how alloying elements will affect the heat treatment and casting behavior. It is used as a predictor of strength in cast irons because it gives an approximate balance of austenite and graphite in final structure. The following formulas are used to determine the CE in cast irons:

This CE is then used to determine if the alloy is hypoeutectic, eutectic, or hypereutectic; for cast irons the eutectic is 4.3% carbon. When casting cast iron this is useful for determining the final grain structure; for example, a hypereutectic cast iron usually has a coarse grain structure and large kish graphite flakes are formed. Also, there is less shrinkage as the CE increases. When heat treating cast iron, various CE samples are tested to empirically determine the correlation between CE and hardness. The following is an example for induction hardened gray irons:

Composition † Carbon equivalent‡ Hardness (convert from hardness test)
C Si HRC HR 30 N Microhardness
3.13 1.50 3.63 50 50 61
3.14 1.68 3.70 49 50 57
3.19 1.64 3.74 48 50 61
3.34 1.59 3.87 47 49 58
3.42 1.80 4.02 46 47 61
3.46 2.00 4.13 43 45 59
3.52 2.14 4.23 36 38 61
†Each sample also contained 0.5–0.9 Mn, 0.35–0.55 Ni, 0.08–0.15 Cr, and 0.15–0.30 Mo.
‡Using the CE second equation.

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