A common problem experienced in the local hardening of ductile (spheroidal graphite) iron is incorrect surface temperature. Many heat treaters experienced in the hardening of steels, do not take into consideration the basic differences between iron and steel. Because iron contains discrete particles of graphite, the composition of the matrix will change with heating temperature and time. This change includes the diffusion of carbon from the graphite into the matrix, which can cause the matrix to become saturated with carbon at higher temperature. In contrast, the matrix composition of steel does not change with heating temperature. Poor temperature control can result in incomplete hardening, excessive levels of austenite, liquation and cracking. This paper reviews the basic difference between iron and steel, and shows examples of problems caused by underheating or overheating.
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