Injection water jet peening of carburised 18CrNiMo7-6 steel surfaces

A compressive residual stress field (CRSF) can improve the fatigue life of machined parts. Injection water jet peening is a new way of inducing compressive residual stress in the surface layer. A carburised 18CrNiMo7-6 gear steel was tested using injection peening under different conditions. The main variables included pressure, nozzle velocity and stand-off distance. The surface hardness was improved up to 63.4 HRC from its original hardness of 56.3 HRC. The maximum residual stress reached −1,240 MPa at a depth of 150 μm. The results also showed that pressure has the most dominant effect on the CRSF. Metallographic photos showed that the grain size was refined in the subsurface layer, a phase change having occurred because of the peening. It was also found that the average surface roughness was inversely influenced by peening, suggesting a new role for water jets in the finishing process.