Recrystallization prediction and microstructural evolution in carburized M50NiL steel

Recrystallization in the carburized case of M50NiL steel was achieved without external deformation through optimized carburizing heat treatment, utilizing the particle-stimulated nucleation (PSN) mechanism of the large size M7C3 carbides. This process refined the grain structure to the sub-micron scale. A cellular automaton (CA) model was developed to predict recrystallization during carburizing and quenching, incorporating PSN and thermal deformation effects to quantify the recrystallized fraction in the carburized layer. The influence of thermal deformation on the microstructure of the sub-micron crystalline region (SMCR) was systematically investigated. Focused ion beam-transmission electron microscopy (FIB-TEM) revealed that the SMCR experienced significantly less deformation than the non-sub-micron crystalline region (NSMCR). Thermal deformation also promoted austenite nucleation at carbide interfaces, establishing a cube-on-cube orientation relationship between carbides and adjacent austenite. This semi-coherent interface reduced interfacial energy, thereby suppressing the stress-induced austenite-to-martensite transformation during stress relaxation.