ばね鋼へのショットピーニングにおける有向性加工層の残留応力

摘要

High durability is required for automotive engine valve springs. To ensure the performance of spring, it is necessary to evaluate and measure the residual stress distribution of spring surface and a downward direction from the top surface. Generally, it has been assumed that the residual stress of a material surface treated by the shot peening is uniform. The round and spiral shape of a coil spring brings various peening angles corresponding to the surface location, then the directional shot angles may leads to a non-uniform residual stress on the coil spring surface. It is well known that the material received the directional deformation shows the non- linear 2 theta - sin~2 psi diagram (psi-split) due to the triaxial stress state. In this study, the residual stress distributions of spring materials deformed by the shot peening with different angles were measured and, also the microstructure especially for carbide precipitates was examined by using field emission type scanning electron microscope (FE-SEM). The non-linearity in the 2 theta - sin~2 psi diagram was revealed for the shot peening samples. The extent of psi-split became larger with the increasing shot peening angle and was dependent not only on the mass fraction of carbide particles but also on the carbide particles size distribution.