Non-isothermal tempering kinetics in a Cr-Mo-V medium-carbon low-alloy steel: a modeling proposal

The tempering kinetics in a Cr-Mo-V medium-carbon low-alloy martensitic steel was determined by an analytical kinetic model of isochronous conversion and differential dilatometry under non-isothermal conditions. The tempering zone associated with stages II and III, and the degree of tempering was determined by dilatometric analysis and the lever rule on the transformation zone. In a first approximation, the effective activation energy was determined by a Kissinger-type method considering the criterion of peak temperatures at the points of the maximum rate. The Kissinger-type method demonstrated the presence of two overlapping stages. The isochronous conversion analytical kinetic model was proposed to separate and determine the tempering parameters, effective activation energy, and growth exponent by stages and as a function of temperature and tempering degree. The kinetic parameters calculated with the Kissinger-type method were consistent with those obtained with the isochronous conversion method. However, the isochronous conversion method allowed identifying the behavior of the parameters as a function of temperature and transformation degree during tempering. In contrast, with the Kissinger-type method, the parameters were constant throughout the transformation range. Finally, the isothermal conversion diagrams were constructed for each stage, emulating fast tempering conditions, and demonstrating the congruence between the kinetic parameters from continuous to isothermal conditions.