Effects of the prior deformation on dispersion of carbide precipitates during isothermal transformation in 12%Cr-0.1%C steel

Effect of prior deformation on the microstructure formed through isothermal transformation was investigated for the purpose of controlling the dispersion of carbide particles in 12%Cr-0.1%C steel. Prior deformation promoted isothermal transformation and softening of the steels. In particular, the isothermal transformation above the nose temperature of TTT diagram (1000 K) was found to be very effective for shortening the treating time and obtaining a soft material which has equiaxed ferritic structure with homogeneously dispersed carbide particles. However, the behavior of microstructural evolution significantly depends on the prior austenite grain size as well as transformation temperature. In the case of transformation of coarse-grained austenite (>40 μm), cellular eutectoid is formed along austenite grain boundaries and this results in the hardening of the material. It was also confirmed that the carbon is enriched into the untransformed austenite during isothermal transformation especially in the coarse-grained materials, and this leads to the formation of the cellular eutectoid structure. In this paper, two ways were proposed for suppressing the formation of cellular eutectoid structure and obtaining homogeneous carbide dispersion: One is the refining of austenite grains by the deformation at recrystallization temperature (1213 K) and the other is the promotion of intragranular nucleation of ferrite by the deformation at unrecrystallization temperature (1133 K).