Spheroidizing Treatment And Austenitizing Research Of GCr15 Bearing Steel

Rolling bearing is one of the key basic components in major equipment, and its service life has a close relationship with the heat treatment. Since GCr15 bearing steel is the most widely used high-carbon chromium bearing steel, its heat treatment processes including spheroidization and austenization were investigated in this study by utilizing optical microscopy(OM), scanning electron microscopy(SEM) and mechanical property tests.In order to obtain a mixed structure in which small spherical cementite particles are uniformly dispersed throughout the ferrite matrix, two kinds of spheroidizing process, high temperature tempering and intercritical annealing, were researched. The effect of temperature, holding time and cooling mode on carbide morphology and particle size distribution was systematically studied after the solution treatment at high temperature; meanwhile the mechanisms of the two types of spheroidization were discussed. For high temperature tempering, the spheroidizing process includes the preciptitation, coarsening and spheroidization of the carbides. The best spheroidized structure was obtained for holding 2 hours at 740℃. For intercritical annealing, carbides precipitated from the martensite was spheroidized during intercritical heating and got further spheroidized during the divorced eutectoid transformation. Good spheroidized structure can be obtained for holding 30 minutes at 780℃ and then continuous cooling at the speed of 25℃ per hour or isothermal annealing at 720℃ for 4 hours.Carbide dissolution, carbide coarsening and austenite grain growth during the austenitizing process have great influence on the final microstructure and properties of GCr15 bearing steel. A model of carbide dissolution and coarsening was established by iteration of the two processes. Comparatively accurate prediction of the mean diameter and volume fraction of the carbides were given during the austenising process. In addition, the regulation of austenite grain growth was studied based on the experimental results. When the heating temperature was below 900℃, the effects of heating temperature and holding time on the grain size were not significant due to the obstruction of undissolved carbides.Finally, the effect of different spheroidizing technologies on the final microstructure and properties after the same final heat treatment was studied. Three kinds of spheroidization, high temperature tempering, continuous annealing and isothermal annealing after the solid solution pretreatment, were compared with the traditional isothermal annealing, and it was found that all these three kinds of spheroidization can effectively improve the bending strength of GCr15 bearing steel, in which continuous annealing is the optimum technology.