Carbon equivalent formula for grey cast iron
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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.[citation needed]The following formulas are used to determine the CE in cast irons:[8]
{\displaystyle CE=\%C+0.33\left(\%Si\right)+0.33\left(\%P\right)-0.027\left(\%Mn\right)+0.4\left(\%S\right)}{\displaystyle CE=\%C+0.33\left(\%Si\right)}[9]{\displaystyle CE=\%C+0.33\left(\%Si+\%P\right)}[10]
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.[11] Also, there is less shrinkage as the CE increases.[10] 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:[9]
Composition [%]†Carbon equivalent‡Hardness [HRC] (convert from hardness test)CSiHRCHR 30 NMicrohardness3.131.503.635050613.141.683.704950573.191.643.744850613.341.593.874749583.421.804.024647613.462.004.134345593.522.144.23363861†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.
{\displaystyle CE=\%C+0.33\left(\%Si\right)+0.33\left(\%P\right)-0.027\left(\%Mn\right)+0.4\left(\%S\right)}{\displaystyle CE=\%C+0.33\left(\%Si\right)}[9]{\displaystyle CE=\%C+0.33\left(\%Si+\%P\right)}[10]
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.[11] Also, there is less shrinkage as the CE increases.[10] 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:[9]
Composition [%]†Carbon equivalent‡Hardness [HRC] (convert from hardness test)CSiHRCHR 30 NMicrohardness3.131.503.635050613.141.683.704950573.191.643.744850613.341.593.874749583.421.804.024647613.462.004.134345593.522.144.23363861†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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