Determination of surface mechanical property and residual stress stability for shot-peened SAF2507 duplex stainless steel by in situ X-ray diffraction stress analysis

Abstract

In this study, SAF2507 duplex (austenite–ferrite) stainless steel was processed by shot peening. The depth–profile variations of the stress state and work hardening in the shot-peened layer were investigated by X-ray diffraction and microhardness tests. The evolutions of surface morphology and microstructure were investigated using scanning and transmission electron microscopy. The stress–strain relations of the shot-peened surface and surface residual stress relaxation caused by cyclic stresses were experimentally determined by in situ X-ray diffraction stress analysis combined with tensile tests. Results showed that shot peening (SP) has induced severe plastic deformation in the surface layer, with the attendant work hardening and high compressive residual stresses (CRS). Nevertheless, there were incompatibilities in plastic deformation between the phases, and more property improvements represented by higher CRS, peak breadth and microhardness were conferred into austenitic phases. SP notably enhanced the surface yield strength of this two-phase alloy. The surface yield strength σ0.2 of ferrite and austenite were around 780 and 1100MPa after SP, which were 59% and 104% respectively higher than that of the untreated surface. The surface stress relaxation under cyclic tensile loadings consisted of a two-stage process. The surface residual stresses relaxed dramatically within the first cycle (N=1), then decreased linearly with the logarithm of loading cycles (N>1). The more severely deformed microstructures in austenite resulted in higher surface yield strength and contributed to higher cyclic stability of residual stress.