Heat Treat Tips: Induction Hardening Cast Iron

Induction hardening of cast irons has many similarities with hardening of steels; at the same time, there are specific features that should be addressed.

Unlike steels, different types of cast irons may have similar chemical composition but substantially different response to induction hardening. In steels, the carbon content is fixed by chemistry and, upon austenitization, cannot exceed this fixed value. In contrast, in cast irons, there is a “reserve” of carbon in the primary (eutectic) graphite particles. The presence of those graphite particles and the ability of carbon to diffuse into the matrix at temperatures of austenite phase can potentially cause the process variability, because it may produce a localized deviation in an amount of carbon dissolved in the austenitic matrix. This could affect the obtained hardness level and pattern upon quenching.

Thus, among other factors, the success in induction hardening of cast irons and its repeatability is greatly affected by a potential variation of matrix carbon content in terms of prior microstructure.

If, for some reason, cast iron does not respond to induction hardening in an expected way, then one of the first steps in determining the root cause for such behavior is to make sure that the cast iron has not only the proper chemical composition but matrix as well.

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