Mechanism of Compressive Residual Stress Introduction on Surfaces of Metal Materials by Water-Jet Peening

Water-jet peening (WJP) has been applied to several Japanese nuclear power plants as a method of preventive maintenance against stress corrosion cracking. WJP introduces compressive residual stress reaching hundreds of micrometers in depth, comparable with shot peening (SP), and much smaller plastic deformation at the processed surfaces than SP does. The causes of these features are investigated from the perspective of the impact processes on the surfaces. Pulse-load propagation simulation through elasto-plastic calculations using a finite-element method program was applied to analyze the effects of various parameters of the impact processes on the depth profiles of the residual stress and the amount of plastic deformation on the surface of austenitic stainless steels processed with either WJP or the SP. The calculated depth profiles of residual stress and plastic deformation were similar in some degree to the experimental results of an XRD residual-stress analysis and a plastic-strain analysis using both cross-sectional hardness measurements and EBSD analysis. The analysis reveals that the depth of the compressive residual stress tends to increase as the size of the loaded spot during impact increases. The average and maximum observed load spots using WJP were 0.25 and 0.95 mm in diameter, respectively. These diameters were respectively 1.3 and 4.8 times as large as the calculated diameter of a load spot using SP. The reason that the depth of the compressive residual stress using WJP is comparable with that using SP is considered to be the fact that the sizes of the load spots during the impact with WJP are in the same range as those with SP. Shots impact the surface during the SP process, while shock waves generated by the extinction of cavitations impact the surface during the WJP process. The analysis reveals that the shots deform the surface locally with much higher surface pressure in the early stages of the impact, while shock waves deform the surface evenly throughout the wave passage across the surface. It is inferred from these analyzed results that the media impacting the surface make a difference in the hardness and microstructure of the processed surface.