Study On Bainite Quenching Process Of 8Cr4Mo4V Steel

With the development of aviation industry,the performance of the internal connecting parts of the aero-engine is becoming more and more demanding,and the quality of bearing performance affects the service life of the engine directly.8Cr4Mo4 V steel has become a common material for aviation bearings.This material has high service temperature,up to 315?,good machining properties,but it has high strength and poor plasticity,and there are a lot of eutectic carbides in this material,the carbides resulted in increased fracture sensitivity,brittleness,poor toughness and lower fatigue resistance.Bainite quenching is a heat treatment process that can improve the toughness of high-strength steel.In this paper,combined with the structural characteristics and material properties of 8Cr4Mo4 V steel,the changes of microstructure and mechanical properties under different heat treatment process conditions were studied,and the optimal isothermal treatment process parameters of materials were determined.Based on not reducing the hardness of this material,the toughness of this material was improved,and the reliability of 8Cr4Mo4 V steel was improved.The impact samples were insulated at different heating temperatures,and then the impact strength of the samples in normal temperature was measured.By observing the impact fracture morphology characteristics of 8Cr4Mo4 V steel under different holding temperatures and combining with the phenomenon of molten micropores,the initial melting temperature of 8Cr4Mo4 V steel was determined to be1170?.By carrying out solution treatment as well as observing and measuring the grain size of the 8Cr4Mo4 V steel at different temperatures,it is determined that the upper limit of solution treatment parameters should be lower than 1095?.According to the calculation results of austenitizing and bainite transformation kinetics,the activation energy Q of bainite transformation of 8Cr4Mo4 V steel is 96.5k J/mol,and the incubation period of bainite transformation is short between 320? and 240?.According to the 8Cr4Mo4 V steel was had treatment at different austempering temperatures and times,and the precipitation of carbides after austempering was analyzed by scanning electron microscope.The changes of hardness and impact properties of the steel under different treatment conditions were analyzed.The optimum austempering temperature and holding time for obtaining martensite+bainite mixed structure of 8Cr4Mo4 V steel were determined,and the optimum austempering temperature is 200? and holding time is 2h.Based on the above research results,the quenching process parameters of bainite+martensite mixed structure of 8Cr4Mo4 V steel with the best mechanical properties are determined as solution heating at 1090? and austempering at 200?for 2h.