Effect of Shot Peening on Surface Chracteristics of Carbon Steel with Different Heat Treatments

Abstract

The shot peening process is one of the surface treatments. The peening effects are characterized by the fact that the surface layer undergoes large plastic deformation due to the collision of shots. This action imparts compressive residual stress on the surface, thus improving the fatigue life of the component. Therefore, this process has been utilized in order to improve the performance of engineering components. Researchers have been found a number of new phenomena in the shot peening process. It is well known that the peening effects are greatly influenced by the processing history or the thermal history of material. The hardness near the surface of the deformed material or heat treated material was often decreased by shot peening. Presently, little is known about the relation between hardness of the shot peened surface and the processing history of materials. In the present study, the effects of shot peening conditions on the surface characteristics of medium carbon steels with different heat treatments were investigated. In the experiment, the shot peening process was performed with an air‐type machine using cast steel balls. Air pressure is in the range from 0.4 to 0.8 MPa and coverage is from 200 to 2000%. The workpiece was used the commercial medium carbon steel JIS‐S45C. These are annealed at 900° C for 7.2 ks and quenched and tempered from 850° C in oil. Hardness, surface roughness, and compressive residual stress in the peened workpieces were measured. When the tempered workpiece was shot peened, the hardness of the surface was considerably lowered. The hardness distribution shows work softening near the surface. This amount increases with increasing coverage. This is due to the influence of processing heat generated by the plastic deformation during shot peening. The maximum residual stress appears about 840 MPa at about 0.180 mm in depth from the surface. It was found that the difference of the thermal history of the workpiece influences the hardness of the peened surface.