Investigation of dynamic recrystallization induced surface integrity in cryogenic machining of AISI 316LVM stainless steel

This study addresses the effects of cryogenic cooling-induced severe plastic deformation (SPD) on microstructure alteration and dynamic recrystallization in the machining of 316LVM alloy. In particular, surface integrity properties such as nanocrystallization, phase transformation, and deformation hardening in the subsurface layer were experimentally investigated. Machining was carried out under liquid nitrogen cooling with cutting parameters consisting of a cutting speed of 300 m/min, a depth of cut of 1.2 mm, and two different feedrates (0.1 and 0.5 mm/rev). An SPD layer with a thickness of approximately 27 to 38 μm and a featureless layer with a thickness of 6 μm were detected in the subsurface of the sample machined at the high feedrate. Furthermore, nanostructures with grain features averaging 93 nm in size were observed in the featureless layer using an atomic force microscope. As a result of X-ray diffraction analysis, while no deformation-induced martensitic phase transformation was observed in the nanocrystal surface layer, a 338% peak broadening was detected. Due to work-hardening, microhardness increased by 115%, reaching 516 HV at a depth of 10 μm in the subsurface. It is thought that the nanocrystallization occurring in the 316LVM alloy with high stacking fault energy results from continuous dynamic recrystallization induced by SPD.