Microstructure Distribution and Grain Coarsening Model of GCr15 Steel in the Laser Surface Treatment

The GCr15 steel was subjected to laser surface treatment using a diode laser with rectangular spot. Based on the simulated temperature field, the microstructure distribution of the laser hardened layer was analyzed, and an empirical equation was proposed to predict the peak temperature at different depths. The results indicated that with the decrease of depth, the peak temperature increased exponentially, and the peak temperature at different depths was approximately linear with the power density and the reciprocal of square root of scanning speed. High peak temperature led to the coarsening of grain and the significant dissolution of cementite near the surface of hardened layer. At the deeper position of hardened layer, the prior austenite grain size decreased slightly due to high nucleation rate. The cementite dissolution and coarsening occurred simultaneously near the surface, and the cementite dissolution process was postponed due to the significant enrichment of Cr near the interface of austenite and cementite. Additionally, an empirical model about grain coarsening was established, which can be used to predict the prior austenite grain size near the surface during the laser surface treatment.