Thermodynamics of Phase Transformation and Microstructure Evolution of 18Cr2Ni4WA Carburized Steel during High‐Temperature Tempering

Abstract

The microstructural evolution and phase transformation mechanism in 18Cr2Ni4WA carburized steel during high‐temperature tempering is investigated using a thermodynamic model. The experimental results demonstrate that the retained austenite in the matrix decreases from the surface to the core, and martensite transform from acicular to lath‐type. Furthermore, the hardness distribution of the carburized layer increase initially, and then decrease. Based on thermodynamic calculations, the driving forces for the decomposition of martensite decomposition and nucleation of cementite in martensite are larger than those for the decomposition of austenite during tempering. Furthermore, cementite preferentially precipitate in martensite. The driving force for the overall phase transformation of the decomposition of the retained austenite is negative, and the driving force for the nucleation of cementite in the low‐carbon retained austenite is positive, indicating that cementite can nucleate only in high‐carbon retained austenite. The low‐carbon retained austenite transform into secondary martensite on cooling after the tempering process while the driving force gradually increase. The decomposition of the initial quenched martensite during the tempering process leads to a decrease in the overall hardness of the carburized layer. The secondary martensite obtained from the retained austenite contribute to the increase in the surface hardness.