Microscopic residual stress distribution measurement on the surface of shot peening

Shot peening, which imparts compressive residual stress, is a very effective means for improving the fatigue strength and is widely used for springs. The relationship between fatigue strength and compressive residual stress by shot peening is well known and is a subject of many previous studies. Residual stress caused by shot peening is generally evaluated using X-ray stress measurement method, and it give rise to compressive residual stress macroscopically when the X-ray irradiation area is not small. On the other hand, in order to investigate the microscopic stress distribution of dents around the vicinity, there are research and experiments with big steel ball simulating a shot, and studies using FEM method confirms that tensile residual stress is generated locally, such as the center of a dent. However, identifying the residual stress generation mechanism by shot peening using calculation alone is a very difficult task because there are many factors involved. In this study, with the aim of clarifying experimentally the microscopic residual stress distribution of peening surface with CCW(conditioned cut wire), which is widely used for the production of spring, we have developed an X-ray diffractometer which makes it possible to measure the microscopic stress distribution. We prepared as specimen SUP9 (JIS G 4801: spring steels), shot peened at coverage of 10%, 60% and 300%. The microscopic residual stress distribution is measured in an area of 0.8mm x 0.8mm with 0.1mm step on each specimen surface. Obtained results are summarized as follows: Shot peened surface has average compressive residual stress present in large irradiation areas, but the tensile stress and compressive stress of the yield stress order has been distributed in local areas of 100μm or less. It became clear that, even if the coverage of the shot peening is increased, there is not much difference of macroscopic residual stress, and an overall uniform compressive stress distribution is not obtained; but there is a large residual tensile stress in the local area. There is no correlation between surface shape and residual stress distribution due to shot peening. In the stress distribution around the dent by single shot, unlike previous research results using shot peening of the sphere, it clearly indicates a residual stress distribution with no regularity. ©2014 The Society of Materials Science.