Microstrcture And Performance Of Hybrid Modified Layers Of M50 Steel By Shot Peening And Plasma-baesd Ion Implantation

Wear and fatigue are the cause of premature failure during precision bearing service time, and the nitrogen plasma-based ion implantation is an effective surface modification technology for the bearings, which can significantly improve the wear resistance and fatigue resistance performance without changing the dimensional precision and bearing raceway roughness. In order to increase the thickness of the modified layer and achieve better surface strengthening, nitrogen plasma-base ion implantation and shot peening and plasma-base ion implantation were performed on M50 steel, and the organizational structure and residual stress gradient and wear properties of the modified layer were studied.Nitrogen plasma-base ion implantation were performed on M50 steel. The variations of nitrogen concentration were investigated using glow discharge spectrometer(GDS), while the phase composition was investigated with X-ray diffraction(XRD). The residual stress gradient was measured using X-ray diffraction method. The friction coefficient of all specimens was measured using a ball-on-disk wear tester. The results reveal that with the increasing of temperature the nitrogen concentration distribution increase, and nitrogen mainly exists in the lattice clearance as the form of supersaturated solid solution. The nano-hardness of the modified layer usually improves, and the value increases from 11 GPa to 16 GPa. N PBII can introduce certain values of residual compressive stress on surface, but it has little influence on the stress gradient. The friction coefficient and wear track width of modified sample is lower than the untreated,and wear track width reduces 37%. The injection uniformity is better with elevated temperature. The nano-hardness of the sample placed on outer circle of the toroid is higher than the one placed on inner circle of the toroid.Shot peening and plasma-base ion implantation were performed on M50 steel. The results reveal that shot peening produces a large number of dislocation at the interface between martensite lath for nitrogen atom diffusion channel,which can accommodate more nitrogen atoms. The saturation concentration of nitrogen increases and nitrogen concentration is still more than 10at% at 30μm. The hybrid modified layer have Cr N new phase and martensitic diffraction peak intensity significantly decreases, which structure is refined. There is carbide or nitride precipitation of Fe, Mo, Cr and V in grain boundary. The surface residual compressive stress is unusually high after shot peening and grinding, which make the residual stress with depth distribution in the modified layer steep. Shot peening with a diameter of small steel shot and at high temperature can obtain larger plastic deformation, therefore the distribution of residual compressive stress is larger and deeper than others. N PBII after shot peening introduces larger and deeper distribution of residual compressive stress gradient than shot peening indicating the coupling effect of those two modification technology and the value increases 37%,and the depth increases 50%. The friction coefficient of modified sample is lower than shot peening and N PBII,and the wear track width reduces 51%.