Experimental Study On Surface Modification Of 18CrNiMo7-6 Carburizing Steel By Post-mixed Waterjet Shot Peening

Fatigue is one of common failure types for the key parts and components in machines.Surface modification,based on the transformation of external energy,is a key technique in anti-fatigue manufacturing area which purpose is to improve the fatigue life of parts effectively by forming a certain residual stress field with proper depth and amplitude on the surface of parts.At present,after the important components be processed by the existing modification methods,the anti-fatigue properties are far less than the fatigue limit of materials,and the modification methods and process parameters can’t meet the need of practical engineering application.In this thesis,the effects of the post-mixed waterjet modification process on hardened surface of commonly used gear steel 18CrNi Mo7-6 were studied.The mainly research work include the following aspects.Research on the theory,using finite element modeling（FEM）analyzed the distribution cloud map of speed,pressure and streamline,and the variable curves of speed and pressure,then got the jet impact force and tested it,the results were basically the same and it proved that the FEM model is correct.Research on the surface quality,studied the effects of jet pressure,stand off distance and nozzle moving speed on the surface roughness of specimen.The results showed that the surface roughness of specimen increased with the increase of jet pressure,first increased then decreased with the increase of stand off distance,and decreased with the increase of the nozzle movement speed.Based on the analysis of test data,the weight of surface roughness variation caused by the three factors was different,from large to small:jet pressure>nozzle moving speed>stand off distance.Research on the residual stress,studied the effects of jet pressure,stand off distance,nozzle moving speed,nozzle diameter,nozzle moving spacing,pellet diameter and jet times on the residual stress distribution of specimen.The results showed that the modified specimen had residual compressive stress within the depth of 500μm,and the residual compressive stress first increased then decreased with the increase of the depth.Different test parameters had different?_srs,?_mcrsandZ_m,It was predicted that the residual compressive stress induced by the post-mixed waterjet shot peening had the maximum value?_mirs,that was,the yield residual stress limit.Research on the material organization,studied the effects of post-mixed waterjet shot peening on microstructure,microhardness,residual austenite and half-width values of the X-ray diffraction profiles.The results showed that after water jet peening,the internal grains got refined,some of the residual austenite changed into martensite,and there were cyclically hardened near lays and the microhardness of the specimens was related to the half-width values of the X-ray diffraction profiles.