Mechanism of isothermal eutectoid transformation in high carbon stainless steels

Morphology and growth mechanism of eutectoid structure were investigated in 12mass% Cr steels with various carbon content, by means of microstructural examination with optical and electron microscopes. The alloys were once subjected to full solution treatment at a high temperature, and then held at a lower temperature to perform eutectoid transformation isothermally. 12%Cr-0.7%C steel can complete eutectoid transformation, and carbide particles are dispersed homogeneously. On the other hand, in the low carbon steels (the 0.3% C and 0.15% C), eutectoid transformation occurs only at the first stage of isothermal aging, and then γ-α transformation follows at the later stage. This results in the formation of an inhomogeneous structure composed of a cellular eutectoid structure and ferrite in the low carbon steels. The growth rate of eutectoid structure in the 0.7 mass% C steel is controlled by the lattice diffusion of carbon in austenite, and it can be evaluated theoretically as a function of the lattice diffusion coefficient of carbon in austenite and the thickness of diffusion layer.