Research On Microstructure And Properties Of G55SiMoV Steel Effected By Austempering Process

The researchers are trying to get bearing steel with better strength and toughness and get high quality bearing. The composite phase of high strength and high toughness can greatly improve the toughness of metal materials under the premise of keeping high strength. Martensite/bainite (M/B) duplex microstructure has high impact toughness, high yield strength and high fracture toughness. However, there is no agreement about the effect of microstructure on the mechanical properties in duplex microstructure steel. In addition, the influence of pre-deformation and stress on the bainite transformation is a new research hotspot for the theoretical study. Various conclusions have been obtained based on different experimental conditions since the mechanism of bainite transformation is complicated. In some researches, the pre-deformation was generally presumed to shorten the incubation period of the bainite transformation, but the fraction of bainite decreased at the end of isothermal holding. In the other researches, the incubation period of the bainite transformation decreased with the deformation increasing while the final fraction of bainite increased. These studies mostly focused on the continuous cooling process rather than the isothermal transformation process. The investigation of bainite transformation under pre-deformation and stress not only can be used to analysis the mechanism of bainite transformation, but also provide a theoretical support on the application of bainite for thermomechanical treatment. According to the above problems, G55SiMoV steel which is an impact-resistant bearing material was trial-produced, and the influence of heat treatment parameters on the microstructure and properties of G55SiMoV steel was investigated systematically in order to obtain the austempering heat treatment specification. The toughening mechanism and the tempering stability of the M/B duplex microstructure were clarified via discussing the effect of various microstructure on properties in G55SiMoV steel. The influence of pre-deformation and externally applied stress on isothermal transformation in G55SiMoV steel was discussed via microstructure observation and kinetic analysis. The research contents and conclusions are described as follows:1. G55SiMoV hot rolling bar satisfying the working condition of rolling mill was trial-produced. The dynamic CCT curve of the G55SiMoV steel was plotted based on the results of thermal simulation test. The parameters of the transformation kinetics of G55SiMoV steel were determined according to the CCT curve and the effect of alloy elements. The influence of main parameters (austenitizing temperature, austempering temperature and austempering time) on the mechanical properties was obtained from the orthogonal experiment. And then, the optimum heat treatment specification of the austempering process applying to G55SiMoV steel was determined:austenitizing temperature is910℃, austempering temperature is280℃, austempering time is30～45min and quenching medium is50%KNO3+50%NaNO2. The M/B microstructure with the bainite (mainly lower bainite) fraction of15～25%could be obtained in G55SiMoV steel after the improved austempering process. G55SiMoV steel with M/B microstructure has excellence performance:impact toughness is>20J/cm2and the hardness is HRC≥57. Furthermore, the improved austempering process is suitable for the actual large size production (≤Φ36mm) with high hardness (HRC≥55) and uniform M/B microstructure for the G55SiMoV bearing steel specimen.2. The mechanical properties and fractography of the M/B microstructure and the single martensite in G55SiMoV and GCr15steel were comparatively investigated in order to clarify the toughening mechanisms of M/B microstructure. The results indicate that the toughness of M/B duplex microstructure is enhanced significantly due to the refinement of cleavage unit and reduction of three-way tensile stress on the crack tip caused by plastic deformation of bainite. The size of bainite lath and the content of retained austenite increase with austenitizing temperature increasing. The fracture morphology of cleavage plane with large size has been detected in the samples austenitized at1050℃, and the size of cleavage plane decreases with decreasing the austenitizing temperature. The average sizes of bainite are6μm and11μm when the austempering temperature are270℃and290℃, respectively. The coarser and longer bainite formed at higher austempering temperature can divide the austenite grain effectively and improve the toughness more significantly. When the bainite content is low, the improvement of toughness caused by the refinement of martensite is counteracted by the significant strain strengthening on bainite caused by martensite. The toughness of the material is strongly improved when the bainite content is high, that is caused by the reduction of strain strengthening on bainite.3. The austempered samples were tempered at different temperature and for different time. And the temper resistance of M/B microstructure was investigated by microstructure observation, mechanical property testing and resistivity test. The results show that the G55SiMoV steel with M/B microstructure has better temper resistance. The microstructure and the hardness of G55SiMoV steel after austempering are quite stable tempered below300℃. Temper brittleness occur at300℃in M/B microstructure, and the temper brittleness temperature is hardly affected by the bainite content, but the toughness reduction can be decreased with increasing the bainite content. When the austempering time is prolonged, more carbon diffuse to the un-transformed austenite, which leading to the increase of austenitic carbon content and the improvement of austenitic stability. When bainite content are25%and30%, the austenitic carbon content increase from1.1%to1.43%and1.52%, respectively. The stability of M/B micrstructure is better than the single martensite during the cryogenic process. The steel with M/B micrstructure applying appropriate cryogenic treatment can preferably satisfy the working condition of rolling mill.4. The influence of pre-deformation and externally applied stress on the isothermal bainite transformation was investigated in the G55SiMoV steel via microstructure observation and kinetic analysis. It is found that the refined microstructure can be obtained after pre-deformation. The size of bainite lath increases with increasing the austenitizing temperature when the same deformation is applied. The promoting and inhibiting factors affect bainite transformation comprehensively. There is a critical value of deformation reduction between promotion and inhibition. With increasing the deformation reduction and decreasing of the deformation temperature, the inhibition is strengthened and the promotion is weakened. The bainite transformation becomes faster and the final fraction of bainite increases in each applied stress condition. The promotion is more significant when the bainite is transformed at290℃, and the bainite distribution becomes to be non-random in the G55SiMoV steel when the stress increases to150MPa.