Effect of Compound Isothermal Quenching on Bainite Transformation and Wear Properties of GCr15Si1Mo Steel

Nano-bainite bearing steel has excellent comprehensive properties, but the long isothermal transition period is the main problem that limits its wide application. In order to accelerate the nano-bainite transformation of GCr15Si1Mo bearing steel, a compound three-step (157 ℃+190 ℃+250 ℃) isothermal quenching process was designed in this paper as a comparison with the conventional one-step (190 ℃) isothermal quenching process, and the test steels were heat-treated under different processes. The microstructure and mechanical properties of GCr15Si1Mo bearing steel were characterized by scanning electron microscope (SEM), X-ray diffractometer (XRD), transmission electron microscope (TEM) and Rockwell hardness tester. The tribological properties and wear mechanism of the test steel were investigated by using UMT friction, wear testing machine and 3D topography profilometer. The results showed that the compound isothermal quenching process accelerated the transformation of nano-bainite from the two stages of gestation and transformation. Compared with the conventional isothermal quenching process, the phase transition time of the compound isothermal quenching process decreased by 5 h when they both transformed the equivalent content of bainite (about 53%~55%). Therefore it could significantly improve the transformation efficiency of nano-bainite. In the compound isothermal quenching process, the residual austenite content in the microstructure was obviously reduced, and the block residual austenite was largely transformed into thin film residual austenite, thus the impact toughness of the material which was treated by using the compound isothermal quenching was obviously improved. With prolonging the compound isothermal quenching time, the impact toughness of the material firstly increased and then decreased. When the compound isothermal quenching process parameter was 157 ℃×5 min+190 ℃×2 h+250 ℃×1 h, the strength and toughness matching and wear resistance of the material were the best, and the wear resistance of the material was better than that of the conventional isothermal quenching process. The results indicated that the dominant wear mechanism of the test steel was abrasive wear after the compound isothermal quenching and the conventional isothermal quenching, but the degree of abrasive wear of the material under the compound isothermal quenching process was reduced and accompanied by slight adhesive wear and fatigue wear.