Effect of Isothermal Quenching Process on the Microstructure and Wear Properties of GCr15SiMo Steel

To explore the influence of isothermal quenching process on the microstructure, friction and wear of GCr15SiMo bearing steel. The GCr15SiMo bearing steel was heat treated under different isothermal quenching temperature (190, 210 and 230 ℃) and different holding time (4, 8 and 24 h). Then, the microstructure, phase composition and hardness were characterized by using the optical microscope (OM), scanning electron microscope (SEM), X-ray diffraction (XRD), transmission electron microscope (TEM), Rockwell hardness tester and so on. Also, the friction and wear properties of different samples were investigated by using the friction and wear testing machine. The results showed that the microstructure of GCr15SiMo bearing steel which was treated under the traditional oil quenching process mainly was martensite, the microstructure of samples which were treated under the isothermal quenching process were mainly composed of bainitic ferrite, retained austenite, a small amount of martensite and undissolved of carbide. With the isothermal quenching temperature was risen and the holding time was prolonged, which resulted in the diffusion ability of carbon atoms was enhanced and the bainite transformation cycle became shorter, thus the content of bainite was increased, the content of martensite, undissolved carbides and residual austenite content were gradually reduced. The microstructure of sample which was treated under the traditional oil quenching process mainly was brittle martensite, and the martensite was easily peeled off by the form of fine fragments during the friction and wear process. When the fine fragments as hard particles which moved on the worn surface would resulted in the micro-cutting phenomenon. So, the wear mechanisms of the sample which was treated under the traditional oil quenching process were mainly micro-cutting and oxidation wear. Under the isothermal quenching process, when the isothermal quenching time was fixed as 8 h, with the increasing of the isothermal quenching temperature, the content of bainite was increased and the toughness of the materials was improved, thus the wear mechanism of the material gradually changed from micro-cutting to adhesive wear. When the isothermal quenching temperature was fixed at 210 ℃, with the holding time was prolonged, the content of bainite in the material was increased significantly, the toughness of the material was improved and its hardness was decreased at the same time, so the wear mechanism of the material gradually transformed from adhesive wear to severe abrasive wear. Finally, the best isothermal quenching process in this study was that the isothermal quenching temperature was 210 ℃ and the holding time was 8 h. And, under the best isothermal quenching process, the volume percentage of each phase in the sample were retained austenite 4.3%, undissolved carbide 4.8%, martensite 32.3% and bainite 58.6%, respectively. the material had good strength and toughness combination, the minimum wear rate is 0.38 × 10⁻⁴ mg/(N·m).