Microstructured Surface Layer Induced by Shot Peening and its Effect on Fatigue Strength

Abstract

The shot peening process in normal conditions produces a compressive residual stress on the surface of a material without phase transformation. But, 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 higher peening velocity and the use of a high-hardness shot media. Previously, we reported that the fatigue strength of springs having a nanocrystalline phase were higher than without nanocrystalline phase. This study investigates the microstructural surface layer induced 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 manufactured by the same process except for the shot peening time. The shot peening time was varied as 100, 300, 500, 1000, 2000, 3000, 6000 s. Shot media was steel cut wire of diameter 0.25 mm. Surface roughness and residual stress distribution were measured. Microstructures were observed with an optical microscope and a scanning electron microscope. And, fatigue tests were carried out by using a spring fatigue test machine. The experimental values of the residual stress distribution and the surface roughness s are almost the same for 300-6000. The spring shot-peened for 300 s consists of the matrix phase and the surface metal flow phase. The spring shot-peened for 1000 s contains the white layer on a part of the surface. For the shot-peened time greater than 3000 s, the white layer almost covered full of surface. The fatigue strength increased with peening time until 1000 s. For the specimens shot-peened for more than 1000 s, the fatigue strength remained at a constant level.