Isothermal transformation behavior in 12%Cr-0.3%C steel

In order to get fine grain of large martensitic stainless steels products, it is available to use the isothermal transformation before the austenitizi ng. 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 m icrostructures are affected by the isothermal ageing temperature. The micro structure transformed isothermally at 1020K is the full eutectoid structure where carbides precipitate homogeneously. On the other hand, the microstru cture transformed at 900K is the mixed heterogeneous structure: the eutecto id structure where carbides precipitate densely around the edge of prior au stenite 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 eut ectoid transformation progresses. The amount of decreasing in carbon at 900 K 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 isothe rmal transformation at 900K, the reason of why there are the ferrite struct ures with no carbide in the center of prior austenite grains is that the ma ssive transformation is induced by the shortage of carbon in untransformed austenite.