Characterizing the Mechanical Properties of the Hardened Layer Induced by Grinding-Hardening

A grinding-hardening process uses the grinding heat to initiate the phase transformation in a workpiece made of quenchable steel and hence to generate its surface hardening. This paper investigates this phase transformation process and the mechanical properties of such hardened layers with the aid of experiments and the finite element analysis. It was found that the phase transformation followed a diffusionless martensitic kinetics in which the external stresses caused by the mechanical loading of the grinding wheel on the workpiece played an important role. The top hardened layer appeared to be a plastically deformed zone of uniform thickness. A metallurgical examination revealed that this layer contained refined martensitic laths, as compared with the ordinary martensite obtained by a conventional quenching method. The microstructural evolution has led to a hardness increase by 1.4 times the ordinary martensite. In addition, the grinding-hardening can promote compressive residual stresses in the ground surface, about 17% higher than the yield strength of the parent ferrite-pearlite material.