The kinetic of martensite formation in ASTM A 213 grade 91 high chromium heat resistant steel

Mechanical properties on high chromium heat resistant steel and their industrial applications are closely related to the their micro-structure. Therefore the study of the all aspects of the heat treatments which can modify the microstructure are of great importance. To understand the process of microstructure formation the martensite transformation features have been analyzed by DSC. We carried out experiments at different cooling rates. The transformation temperatures Mstart and Mfinish were determined by the onset and the end of the DSC peak, and in particular Mstart= 417 +/- 4 degrees C. The results from these experiments indicate that Mstart is not related to the cooling rate. The transformation course is obtained from the DSC plot (fig. 1). The transformed martensite volume fraction fi results from fi=Ai/Atot, where Ai is the area of the DSC peak for the temperature Ti and Atot is the total area of the peak. The progress of the transformation is not dependent by the cooling rate and can be well described by the equation 1-fi = exp-c(Tkm-Ti) similar to Koinstinen-Marburger empirical relationship (fig. 2). According to Van Bohemen's derivations c is a rate parameter and Tkm is the start temperature of the exponential relationship. The values of Tkm and c obtained from the least square fit of the semi-log plot (fig. 3) are reported in table 2. The progress of the transformation observed for high chromium steel can be well explained with the rationalization of the K-M empirical model proposed by Van Bohemen for plain carbon steel. The martensite formation is accompanied by a change in volume and shape which leads to plastic deformation, increasing the strength of the surrounding austenite. The interaction of chemical and mechanical force defines the course of the transformation and the reaction progresses over a range of temperature. The formation of a specific volume of martensite increases the strength of the surrounding austenite and inhibits the transformation, so that a supplementary driving force is required for the transformation to resume So the extent of the transformation increases with decreasing temperature because the strain energy required is balanced by the increased chemical driving force. The rate parameter c is linked to the strain energy stored in austenite.