Decomposition behavior of retained austenite (γR) due to heat treatment under critical temperature (A1) of 17 mass%Cr cast iron containing 2 mass%Mo was investigated. Three kinds of specimens, of which treating conditions before heat treatment were as-cast, as-hardened from as-cast state and as-hardened from annealed state, were held at temperatures from 813 K to 923 K for 100 s to 1.2×106s, and then volume fraction of retained austenite and hardness were measured.
Regardless of the pre-treating conditions, the relationship between the decomposition ratio of retained austenite and logarithmic of holding time showed a sigmoidal curve, and decomposition accelerated as the temperature increased. The holding time also increased. The decomposition of retained austenite mainly causes transformation of pearlite or precipitation of carbides. The decomposition rate was smallest with heat-treatment from as cast state.
Hardness increased with increasing decomposition ratio of retained austenite and reached maximum value obtained at 5 to 10%γR.
