Effect of impact energy on residual stresses induced by hammer peening of 304L plates

Residual stresses induced by welding may reach the yield strength of the material and decrease the life of the components by premature cracking. The hammer peening process has been used to improve the fatigue resistance in welded components, especially at the weld toe. It is expected that the powerful hammer peening process produces a deeper compressive layer than other currently used processes and thus a better improvement in fatigue life for heavy welds. To better understand residual stress relaxation by hammer peening, this study evaluates the effect of each hammer blow on the residual stresses field under the treated surfaces. An impact machine was used for hammering and generating unitary impacts at the same location. 304L steel plates were tested under 10-impact with various energy levels. Both initial stress free plates and plates with residual stresses were tested. Strain gages were used for recording strains at the surface in the near field of the impact, whereas in-depth residual stresses at 8 mm from the impact center were measured using the hole-drilling technique. The micro-hardness profiles were also measured to evaluate the hardening affected zone of the hammer peening. It was found that in initial stress free plates, hammer peening introduces compressive residual stresses. In plates with initial tensile residual stresses, hammer peening either released them or turned them into compression. It was found that increasing energy improves the hammer peening efficiency and that the first three or four hammer blows were the most effective. It was also found that the extent of the hammer-peening induced compression was around 1 mm. Microscopic observations of samples from the tested plates did not reveal cracks on the treated surfaces.