Prediction of Surface Microstructure, Grain Size, Martensite Content, and Microhardness of 316L Austenitic Stainless Steel in Surface Mechanical Grinding Treatment Process

Surface mechanical grinding treatment (SMGT) is a promising surface strengthening technique by exerting gradient microstructure, phase transformation, and work hardening on the surface layer. However, due to the large strain and high strain rate, it is difficult to evaluate the microstructure attributes of the SMGT-processed surface. In the present work, an analytical model is developed to correlate the SMGT processing parameters to the surface microstructure, grain size, martensite content, and microhardness. Firstly, the processing force and distributions of stress and strain are identified by developing a multi-physics framework for SMGT process. Afterward, the surface grain refinement induced by dynamic recrystallization (DRX) is modeled by cellular automata (CA) method. The grain size along with the depth of cross section is predicted by Johnson-Mehl-Avrami-Kolmogorov (JMAK) DRX model. The martensite content is evaluated according to the strain-induced martensitic transformation kinetics. Ultimately, accounting for both microstructure alteration and phase transformation, the microhardness variation is predicted. To verify the results derived by analytical model, series of confirmatory experiments were carried out on 316L austenitic stainless steel. The SEM was utilized to observe the surface microstructure of the processed samples. The martensitic transformation was characterized by EBSD and XRD methods. The microhardness measurements were taken on an automatic Vickers hardness tester. It was obtained from the results that the surface microstructure, gradient grain size, phase content, and microhardness distribution derived from predictive model were consistent well with the experimental measurements. The confirmed simulation model is further utilized to understand how the process factors, i.e., ball size and penetration depth, influence the evolution of microstructure. It is found that the finer grain structure, higher martensite content, and microhardness can be obtained by adopting smaller ball and larger penetration depth.