Effect of ball size and impact velocity on the microstructure and hardness of surface mechanical attrition–treated 304L steel: experiment and finite element simulations

Abstract

In surface mechanical attrition treatment (SMAT), the processing parameters like the diameter of hardened steel balls/shots (D) and their impact velocity (V) are crucial in affecting the mechanical properties of the materials. This study is focused on understanding the effect of D and V on microstructural and hardness variation during the SMAT process. The SMAT has been performed on AISI 304L steel using 3-mm and 8-mm diameter balls with a combination of 1 m/s and 10 m/s impact velocity. Microstructure of the SMATed material shows twin distribution near the top surface changeover from coarser to finer when V increases from a low to significantly high value, whereas it changes marginally with an increase in D. Nanoindentation experiments performed along the depth of SMATed material reveal that the near-surface hardness is mainly governed by V and weakly influenced by D. However, the hardened layer thickness is enhanced by increasing either of these parameters. The complementary finite element (FE) simulations of the single impact SMAT process are performed using the rate-dependent Johnson–Cook plasticity model to provide the mechanistic reasons for the behavior observed from the experiments. A strategy to determine the hardness-depth profile of SMATed steel through FE simulations is developed. The hardness behavior of the SMATed steel is linked to the effects of D and V on the residual equivalent stress and equivalent plastic strain at the surface. The hardness away from the surface is influenced by the shot size and shot velocity. The empirical relations that show the dependence of hardness on the SMAT parameters are determined. © 2022, The Author(s), under exclusive licence to Springer-Verlag London Ltd., part of Springer Nature.