Isothermal transformation behaviour in 12%Cr-0.3%C steel with partial solution treatment

12mass%Cr-0.3mass%C steel was subjected to partial solution (PS) treatment in the two-phase region of austenite (γ)+M₂₃C₆ carbide, and then isothermally transformed to ferrite (α)+carbide at around 1000K. The isothermal decomposition behaviour of γ has been investigated by means of hardness testing and microstructural observation. Time-Temperature-Transformation diagram obtained by hardness testing showed that the decomposition of γ starts much faster in the PS-treated specimens than the specimens which have been subjected to the full solution treatment in the γ single phase region, because a lot of insoluble carbide particles dispersed within γ grains provide the precipitation sites for the carbide in the subsequent isothermal decomposition treatment. Upon the decomposition of γ in the PS-treated specimens, carbide precipitates on the insoluble carbide in the made of "in-situ" type, and α nucleates simultaneously at the carbon depleted γ region adjacent to the carbide particles by the eutoctoid transformation mechanism. In this transformation, insoluble carbide particles always work as precipitation sites for carbide, hence, typical lamellar structure is not formed but equiaxed α structure is formed around spherical carbide particles which have previously dispersed in the γ matrix at a PS-treatment temperature.