Fine Particle Bombardment (FPB) is a surface strengthening technique in which a large number of micron-sized pellets are carried by high-pressure air to impact the surface of the workpiece, introducing residual compressive stress and microstructure deformation (such as grain refinement and dislocation density increase). Compared with traditional shot peening, it has the advantage of a smaller particle size (50-100 μm), deeper residual compressive stress layer, more uniform strengthening effect and lower surface roughness. GCr15 bearing steel is a kind of high carbon chromium steel widely used in the field of machinery manufacturing, because of its excellent hardness, wear resistance and fatigue properties, and also applied in the manufacture of bearings, balls, rollers and other key components. However, under actual working conditions, bearing steel often undergoes complex alternating loads and micro-dynamic friction and wear, which will lead to gradual failure of the material surface, thereby shortening its service life. For GCr15 bearing steel, Fine Particle Bombardment can effectively improve the surface hardness, while maintaining high precision surface quality (Ra<0.2 μm), especially suitable for high-load, high-speed bearing strengthening needs. In this work, the surface modification of GCr15 bearing steel was carried out with this technique, and the strengthening mechanism and micro-dynamic friction and wear characteristics were studied.
GCr15 bearing steel was cut into ϕ24 mm×8 mm round cake by EDM cutting, and the processed sample was polished and ground. The round cake sample was treated with FPB for 120 s under 0.3 MPa air pressure. The diameter of particles ranged from 50 μm to 100 μm. The distance between the nozzle and the sample was kept at 100 mm. Scanning Electron Microscope (SEM) and Electron Back Scatter Diffraction (EBSD) were used to analyze the grain structure and grain boundary distribution of the strengthened sample. The residual stress distribution was determined by X-ray diffraction (XRD) analysis. The results showed that serious plastic deformation occurred on the material surface, forming a nanocrystalline layer of about 20 μm and a deformed layer of tens of microns. After FPB treatment, the density of low-angle grain boundaries increased from 5.8% to 12.6%, the surface hardness increased significantly to 1 005HV0.2, an increase of 56.5%, and the density of high-angle grain boundaries increased to 20.1%. The residual compressive stress from the surface to the bottom increased first and then decreased, and the residual compressive stress reached its maximum value in the subsurface. Under different displacement amplitude (20 μm, 50 μm, 80 μm) and different normal loads (40 N, 60 N, 80 N), the samples treated with FPB were tested for microkinetic friction and wear to simulate the friction and wear behaviour under actual working conditions. The fretting cycle diagram (Ft-D-N) was drawn to analyze the fretting operation state under various parameters. The morphology and element distribution of the wear marks were observed by scanning electron microscope and energy dispersive spectrometer (EDS), and the friction and wear mechanism were analyzed. The average friction coefficient reached the minimum value of 0.12 at 20 N and 80 μm and reached a maximum of 0.52 at 40 N and 80 μm.
The hardness and wear resistance of the material can be effectively improved by fine particle bombardment. The average friction coefficient and wear volume of the sample after peening increase with the increase of displacement amplitude. Under the same displacement amplitude, the average friction coefficient decreases gradually with the increase of normal load.
Key words
fine particle bombardment /
GCr15 bearing steel /
microstructure /
fretting wear /
wear resistance /
wear mechanism
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Funding
Shandong Provincial Natural Science Foundation (SZR2138); Key R&D Program of Shandong Province (SGG2313)
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