Hard milling of carburized and waterjet peened 18CrNiMo7-6 steel

Post-mixed waterjet peening is an effective treatment method for compressive residual stress field. After peening, the surface quality of the parts is low, and the hardness is usually higher than 60HRC. A fine manufacturing process should be carried out to obtain a satisfactory surface roughness without side effect on existing residual stress. In this article, the 18CrNiMo7-6 steel after carburization was subjected to post-mixed waterjet peening, the surface hardness was 66HRC and the maximum compressive residual stress was about 1242.86 MPa. Hard milling with CBN inserts were used to remove the rough surface layer of 18CrNiMo7-6. The hardness after milling has changed about 1-2HRC lower than that before cutting. When the cutting speed vc is 753.6 m/min, the feed speed vf is 0.04 mm/min and the cutting depth ap is 0.02 mm, compared to the original Ra 1.204 μm, the best surface roughness can reach Ra 0.0823 μm. When ap exceeds the maximum residual stress location point of 60 μm, the cutting effect will significantly change the residual stress. Considering the hardness, surface roughness and residual stress, the recommended milling parameters vf and ap should use the lower values, the range of vc should be 500–700 m/min and sufficient coolant is required during the cutting process.