Thermodynamik der Martensitumwandlung in Stählen

For calculating martensite start temperatures of carbon or nitrogen containing steels respectively, only empirical formulas were available in the past. Thermodynamic calculations of T o temperatures and driving forces were performed only for the transformation of fcc into bcc structures. In this work, a model will be described which has been developed by the authors for Fe-C and Fe-N alloys, respectively, where martensite is described thermodynamically taking into consideration its tetragonality. It will be shown that this model can be applied also to high-alloyed Fe-Cr-Mn-M steels. The difference of Gibbs free energies between the martensitic and the austenitic phases has been calculated thermodynamically for M s -temperatures which were reported for experimental investigations. For high alloy contents, it is about 2100 J/mol corresponding to the driving force of twin martensite. With respect to lath martensite, a proportionality between driving force and alloy contents was obtained which agrees with factors measured for nitrogen and chromium on the binary systems. The martensite start temperature extrapolated to pure iron amounts to 913 K. It is higher by about 100 K than the value directly measured on pure iron at high cooling rate. Correspondingly the driving force is only 711 J/mol instead of 1200 J/mol. Probably the very low cooling rate of 10 0 ... 10 K/ which was chosen for the experiments on Fe-Cr-Mn-N alloys had caused this difference.