12%Cr-0.3%C鋼における恒温変態挙動

抄録

In order to get fine grain of large martensitic stainless steels products, it is available to use the isothermal transformation before the austenitizing. When 12%Cr-0.3%C steel is subjected to the full solution treatment and following isothermal transformation at 900-1020K, it is observed that the microstructures are affected by the isothermal ageing temperature. The microstructure transformed isothermally at 1020K is the full eutectoid structure where carbides precipitate homogeneously. On the other hand, the microstructure transformed at 900K is the mixed heterogeneous structure: the eutectoid structure where carbides precipitate densely around the edge of prior austenite grains and the ferrite structure where carbides hardly precipitate in the center of prior austenite grains. These phenomena are concerned with the decreasing in carbon content in the untransformed austenite as the eutectoid transformation progresses. The amount of decreasing in carbon at 900K is larger than at 1020K, which causes the shortage of carbon in austenite in the last period of isothermal transformation. In the case of the isothermal transformation at 900K, the reason of why there are the ferrite structures with no carbide in the center of prior austenite grains is that the massive transformation is induced by the shortage of carbon in untransformed austenite.