Effect of machining processes on the residual stress distribution heterogeneities and their consequences on the stress corrosion cracking resistance of AISI 316L SS in chloride medium

The effects of machining such as grinding and turning on the microstructural and mechanical changes of the machined surfaces of AISI 316L stainless steel (SS) have been studied. Surface aspects and surface defects have been examined by scanning electron microscopy (SEM). Machining-induced nanocrystallization has been investigated by transmission electron microscopy (TEM). Surface and subsurface residual stress distribution and plastic deformation induced by the machining processes have been assessed by X-ray diffraction (XRD) and micro-hardness measurements, respectively. The susceptibility to stress corrosion cracking (SCC) has been assessed by SEM examination of micro-crack networks which are characteristics of a machined surface immersed in boiling (140 +/- 2 degrees C) solution of MgCl2 (40%) during a 48 h-period. The machined surface properties have been correlated to severe plastic deformation (SPD) resulting from specific cutting state of each process. High cutting temperature and plastic rate are considered to be at the origin of near-surface austenitic grain refinement that leads to equiaxed nanograins with a size ranging from 50 to 200 nm. Ground surface residual stress distribution heterogeneities at the micrometric scale are attributed to the random distribution of the density and the geometry of abrasive grains that represent micro-cutting tools in the grinding process. The relationship between residual stress distribution and susceptibility of the AISI 316L SS to SCC has been demonstrated, and an experimental criterion for crack initiation has been established.