Microstructured surface layer induced by shot peening and its effect on fatigue strength

The shot peening process under normal conditions produces a compressive residual stress on the surface of a material without inducing a phase transformation. However, the shot peening process produces a phase transformation (e.g. nanoferrite-phase and metal flow layer) on the surface of carbon steel under the intensified peening conditions of high peening velocity and the use of a high-hardness shot media. Previously, we reported that the fatigue strengths of springs having a nanocrystalline phase were higher than those that did not have a nanocrystalline phase. In this study, we investigated the microstructure of the surface layer produced by shot peening and its effect on fatigue strength. The test specimens were compressive coil springs made of oil-tempered wire.The test springs were all manufactured by the same process, except that the shot peening times were varied. The shot peening times used were 100, 300, 500, 1000, 2000, 3000 and 6000 s. The shot media was steel cut wire having a diameter of 0.25 mm. The surface roughness and residual stress distribution of the test springs were measured. Microstructures were observed using an optical microscope and a scanning electron microscope. In addition, fatigue tests were carried out using a spring fatigue testing machine. The residual stress distribution and the surface roughness were almost the same for springs having shot peening times in the range 300-6000 s. The spring shot-peened for 300 s consisted of a matrix phase and a surface metal flow phase, while the spring shot-peened for 1000 s had a white layer on a portion of its surface. For shot-peening times greater than 3000 s, the white layer covered almost the entire surface. The fatigue strength of the springs increased with shot-peening time up to 1000 s, above which it remained constant.