Research On Heat Treatment Process For Improving The Properties Of Bearing Steels

Recently, with the fast development of wind electric power projects and high speed railway, Rolling Bearing is widely used in engineering, and the service life is required to become higher and higher. This paper based on improving the properties of bearing steels through heat treatment, determined the microstructure and mechanical properties of several domestic and external bearing steel samples, and the retention of austenite of different samples were measured by means of XRD, SEM was used to analysis the difference in structure of several bearing steel samples. On this basis, the carbide was refined by various pretreatment process, and the microstructure and mechanical properties of samples were measured after different heat treatment. The residual austenite content was controlled in different ranges by ultimate treatment technology, the microstructures and the mechanical properties are examined, and the influence of residual austenite content on properties of bearing steels have been analyzed. The microstructures and mechanical properties of bearing steel were investigated after austempering for different isothermal holding time. Bainite transformation rate and its effect on bearing steel properties were researched, SEM was used to analysis fracture mechanism and XRD was used to measure the residual stress of samples after martensitic quenching and bainite hardening.Based on the research work, some conclusions are drawn as follows:The martensite content of shaft sleeve and axostyle of external bearing steel are controled respectively in different ranges. Comparing with domestic bearing, the external bearing steels have well distributed finely carbides, and lower residual austenite content. Indeed, the residual austenite was eliminated completely in some external bearing steels. Experimental results indicate that the external bearing steel exhibits a better wear resistance than domestic bearing.The optimum carbide superfining pretreatment process was the complete austenitizing austempering and solid solution hardening and medium temperature tempering. The fine carbides were evenly dispersed in its microstructure and austenite grains were refined. Because of the slightly higher retained austenite content and lower carbide fraction and irregular carbide particle, the comprehensive mechanical properties of bearing steel samples were decreased slightly after carbide refining treatment. The bearing steel had the best comprehensive properties by spheroidizing annealing together with ultimate treatment technology. The spherical degree of carbide particle and the content of retained austenite must be controlled well when carbide superrefining.The residual austenite in GCr15 was stabilized after austenitizing at 870℃for 15min and austempering at 160℃for 20min, and the residual austenite content increased to 14.2%. With the increase of the isothermal holding time to 60min, the residual austenite contents remained essentially constant. Martensitic quenching and immediately cold treatment can effectively eliminate the residual austenite content of GCr15. The excess of residual austenite content could decrease the strength and hardness of bearing steels, and it could not obviously improve the impact toughness of GCr15. After the martempering 60min, some remained austenite transformed into isothermal martensite, the wear resistance of sample was improved 32.1%.After cooling treatment, the strength and toughness of bearing steel sample decreased somewhat, but the hardness and wear resistance were increased, especially the wear resistance increased 35.7% .During austenitizing at 870℃for 15min and austempering at 240℃for different isothermal holding time, then cooling in air or oil, the microstructure was acicular martensite + low bainite + little retained austenite + retained carbides. The bainite phase transformation was faster at the initial time and then slower gradually, and the bainate grower up firstly and then growled no more for certain time. The bearing steel samples showed a better performance of various properties, which bainite transformation amount is about 45.1%.Its anti wear property improved 47.5% than martensitic quenching tempering sample. The fracture mechanism of GCr15 was microvoid accumulation fracture by martensitic quenching and low temperature tempering process. During bainite hardening, the fracture mechanism transformed into cleavage fracture, and the surface residual compressive stress was higher than martensitic quenching tempering sample.