Processing Study On Improving The Homogeneity Of Carbides In Bearing Steel

The mechanical properties of a bearing and the life under a cyclic impact load are strong influenced by the homogeneity of carbides in bearing steel. When network carbide occurs in high carbon-chromium bearing steel, the steel will become brittle, so that cause the early failure. In order to improve the percent of pass for GCr15 due to the continuous carbide network in Fushun Special Steel, the present study is to investigate the influence factors on carbide homogeneity and impacting of controlled rolling and cooling, as well as its subsequent spheroidizing annealing on steel structure and property under melting and casting. The results are obtained as follows:(1)The chemical composition of steel ingot and its segregation are the key point to form carbide network. In order to reduce composition segregation, the carbon and chromium content should be controlled to lower limits. For preventing network carbide and achieving homogeneous annealing in high temperature to steel ingot or billet, the ingot case with slow solidifying speed will be suggested under casting.(2)The superheating of liquid steel can be quickly eliminated by electromagnetic stirring process, Controlling central segregation within certain limit is effective to increase the equiaxed crystalgrain and improve density of continuous casting billet, which such effect will be increased with decreasing of superheat and casting speed.(3)The optimum process of controlling rolling and cooling for GCrl5 are:the heating billet is heated at 1070℃, and the final rolling temperature kept at a range of 850-900℃, after twice hard cooling, the holding temperature for steel surface is controlled at a range of 630-640℃. With the controlled rolling and cooling process, the thin carbide network, pseudo-pearlite and fine lamellar pearlite can be obtained.(4)The microstructure of GCr15 before its spheroidizing annealing is strongly influenced with the parameters of controlled rolling and cooling. The time of spheroidizing annealing can be shorted with the finer carbide network particles, smaller spacing among lamella pearlite and smaller size of pearlite or sorbite.(5) With increasing of both spheroidizing heating temperature or annealing time, the amount of carbide network in GCr15 can be decreased. so that the spheroidized microstructure will become finer, globular and homogeneity. The best spheroidizing annealing process is heat to 820℃, then kept at 725℃for 4hours.