Structure And Properties Of Gear Materials By SP /HCPEB Composites

As the basic components of modern machines, gears have been widely used. Manufacturing industry has developed rapidly in our country, but products surplus caused by large production and inferior quality is the relatively prominent problem in gear industry at present. As the surface engineering technology being continuously innovated, using the new surface modification technology with the traditional methods of surface strengthening has become the development tendency of improving comprehensive mechanical properties of the material surface.Based on part of the national natural science fund project which named “studies on surface modification technology and properties of gears based on the electron beam ”, treatments on commonly used gear materials by strong current pulse election beam(HCPEB) combined with shot peening were studied, which provided experimental basis for the improvement of comprehensive mechanical properties of gear surface by strong current pulse election beam(HCPEB) combined with shot peening surface strengthening technology.In this paper, the gear steel 40Cr(hardening and tempering) and 20CrMo(carburizing and quenching tempering at low temperature) have been employed as research objects and have been treated by different HCPEB process parameters. Observation and analysis results of surface topography, roughness and surface hardness have indicated that melting pits were produced on surfaces of 40 Crand 20CrMo after irradiating with electron beam and the quantity of melting pits decreased, the size became larger and the roughness became lower with the increase of accelerating voltage and pulse frequency of the electron beam. The surface hardness of 40 Cr reached to an optimum value 529.1HV0.03 after 25 times electron beam processing under 30 KV which increased by 72.58% over the matrix; the surface hardness of 20 CrMo reached to an optimum value 838HV0.03 after 25 times electron beam processing under 30 KV which increased by 19.44% over the matrix.Based on the studies above, a compounded disposal method composed of HCPEB technology and shot peening strengthening has been carried out. Microstructure observation, phase analysis, cross section hardness gradient analysis and residual stress test were conducted on 40 Cr and 20 CrMo after different processes of compounded processing and compared with the single processing approach. Results show that the surface roughness can be improved and “self-quenching” and remelting hardening happened on the surface layer with electron beam processing. The remelting hardening produced by shot peening after electron beam processing was the most obvious and after 25 times this kind of processing under 30 K, the surface solidified layers of 40 Cr and 20 CrMo were the thickest, reaching to 5.52μm and 9.49μm,relatively. Residual tensile stresses were generated on surfaces of 40 Cr and 20 CrMo after electron beam processing, and the maximum tensile stress reached to 572 MPa on the surface of 40 Cr 25 times electron beam processing under 24 KV. And an ideal compressive stress layer was formed on the surface of the material through the following shot peening. The cross section hardness values of 40 Cr and 20 CrMo after electron beam processing decrease first then increase, and tend to be stable finally with the layer depth, with a “valley” appearing in 40μm and 70μm respectively. Besides, the hardness values are lower than that of the matrix. However, the hardness layers are more than100μm with shot peening and the internal stress is uniform with no “valley” appearing by the compound surface treatment of electron beam first then shot peening.Finally, friction and wear properties and corrosion resistance of 40 Cr and 20 CrMo after electron beam treatment shot peening and compound treatment of electron beam and shot peening processing were studied. The results indicate that the average coefficient of frictions of 40 Cr and 20 CrMo processed by electron beam first then shot peening are 0.34 and 0.19, volumetric wear losses are 12.39×10-3 mm3 and 5.55×10-3mm3 and scratch widths are 609μm and 578μm, respectively, all of which are the smallest. Meanwhile, the self-corrosion potential is the highest and the self-corrosion current is the lowest. This suggests that the wear properties and corrosion resistance of the gear material can be improved though the compound strengthens process of electron beam and shot peening.