The role of heating temperature in the martensitic transformation of stainless steels

The study of the martensitic transformation of two stainless steels with 13% chromium and different carbon contents performed in this paper, allows the influence of the heating temperature on the development of this transformation to be evaluated. This parameter has a decisive influence on the martensitic transformation starting temperature and development of this transformation. The martensitic transformation starting temperature is conditioned by the chemical composition of the austenitic phase that undergoes the transformation. In the steels studied, the carbide dissolution processes that occur during heating depend directly on the maximun temperature reached and may modify the chemical composition of the austenite. The Variations in the composition of this phase caused by carbide precipitation during cooling have been avoided by applying a rate of 50 K.s(-1). The results obtained in these conditions show that, in the two steels studied, the martensitic transformation starting temperature decreases as the heating temperature rises, reaching a constant Value in each steel when the maximum temperature is higher than the total carbide dissolution temperature. The dilatometric techniques used in this paper have made it possible to demonstrate different behaviours in the development of martensitic transformation. In these stainless steels, the non-isothermal transformation of austenite to martensite occurs continuously over a range of temperatures or in successive different stages. Till now, the splitting of the martensitic transformation has been related to carbide precipitation during cooling, but splitting stages without such a precipitation have been detected experimentally in this study. Under these conditions, the splitting stages of the martensitic transformation are the result of the chemical heterogeneity of the austenite produced as a consequence of the carbide dissolution processes that take place at different heating temperatures. To evaluate the influence of the heating temperature on the development of the martensitic transformation, specimens of each type of steel were heated to three different temperatures, which give rise to different degrees of austenization. In all the cases, rates of 50 K.s(-1) have also been applied to avoid Variations in the composition of the austenitic phase resulting from carbide precipitation. Thus, the heterogeneity of the austenite prior to ifs transformation into martensite - which gives rise to the phenomenon of splitting - will be the same as the one occurring in this phase in its initial state. In the two steels studied, the martensitic transformation occurs in a single stage when a structure of homogeneous austenite is reached during heating. However, when the concentration of carbon and carbide-forming element in the austenitic phase is heterogeneous, i.e., structure composed of non-homogeneous austenite or austenite + residual carbides, the martensitic transformation of these stainless steels shows splitting in two different stages.