Modeling of the austenite decomposition kinetics in a low-alloyed steel enriched in carbon and nitrogen

A model predicting the effect of carbon (0.1-0.7%C-m) and nitrogen (0.3-0.4%(m) N) enrichment of a low-alloyed steel (23MnCrMo5) on its austenite decomposition kinetics during cooling is introduced here. Its practical purpose concerns the simulation of microstructure evolutions after surface treatments of carburizing, nitriding or carbonitriding. Effects of nitrogen in solid solution in austenite are considered for the first time. Regarding the carbon, previous approaches from literature which estimate IT C curves displacements along time and temperature axes are adapted. Phase transformation kinetics are predicted from phenomenological rules (JMAK, Koistinen-Marburger). However, the number of empirical model parameters is reduced to a minimum thanks to thermodynamic calculations. JMAK kinetic parameters have to be established in one single reference steel composition. The model gathers, in one comprehensive framework, several phenomena which are generally considered separately, such as, e.g., the austenite stabilization after the proeutectoid ferrite transformation (by carbon enrichment) or after the bainite transformation. The kinetics simulations and the predicted final microstructures are critically compared to a large set of experiments carried out on dilatometry samples with different homogeneous C/N composition and samples with composition gradients. After carbonitriding, microstructure and hardness profiles differ strongly from the classical case of carburizing. This highlights the consequences of the previously reported, unexpected acceleration of the austenite decomposition when it has been enriched in nitrogen.