Influence of low plasticity burnishing on the formation of strain induced martensite in the surface layer

In this research, AISI 304 austenitic stainless steel was used as work material and the influence of low plasticity burnishing (LPB) parameters (speed, number of passes and pressure) was investigated in terms of equivalent strain, LPB force, strain induced α′-martensite formation, deformation wave morphology and microhardness. Additionally, a correlation between the formation of α′-martensite and surface topography was investigated. The results showed that increasing the number of passes and pressure intensifies strain, while higher speed reduces friction and results in lower and more evenly distributed strain. The formation of LPB-induced α′-martensite was analyzed in a surface layer from 0.255 μm to a maximum depth of 2 μm. The highest content of α′-martensite was identified near the surface and decreased with depth. Mössbauer analysis (depth of ≈0.1 μm) supported this result. This behavior deviates from predictive models. Increasing the LPB speed reduces the α′-martensite content due to the generated adiabatic heat and, in some cases, its formation is nearly inhibited (< 5 %). The increase in the number of passes elevates the strain and, consequently, the α′-martensite content, with a maximum value approximately 90 %. Furthermore, the number of LPB passes was the only parameter to produces a refined grain layer. The increase in pressure elevates the strain, raising the content of α′-martensite. The trend of increasing α′-martensite content with higher numbers of passes and pressure was also observed in the context of waviness, i.e., the increase in deformation that is responsible for the higher α′-martensite content also increases the surface waviness.