Non-destructive characterization of subsurface residual stress fields and correlation with microstructural conditions in a shot-peened inconel component

Shot-peening is an important surface treatment used in a preventative way to guard against fatigue failures. The residual stress state imparted by shot-peening prevents surface cracks from forming and propagating. In this paper, we describe the measurement of residual stresses in an Inconel, IN100, sample using lattice strains measured using High Energy synchrotron X-ray Diffraction (HEXD) and a Bi-Scale Optimization Method (BSOM). HEXD enabled rapid, non-destructive lattice strain measurements over a large region of the sample. Subsurface strains were obtained using a conical slit setup. The BSOM utilizes a macroscale representation of the sample and a spherical harmonic-based crystal scale representation of crystal orientation space at each experimental point (diffraction volume). A roughly biaxial stress state was predicted with a von Mises equivalent stress between 300 MPa and 400 MPa near the surface. The layer of material with large residual stress induced by shot-peening was found to be approximately 1 mm thick. Diffraction peak width, EBSD, and microhardness measurements were also made on the same sample, which rendered more qualitative measures of the plasticity-related effects of the shot-peening induced residual stress field. All of these measurements were consistent with a decrease in shot–peening plasticity with depth.