Study On Microstructure And Properties Of 45CrMoVE Ultrahigh Strength Low Alloy Heat Resistant Steel

Recently in United States, the45CrMoVE steel was widely applied on some components of aero-engines and space-engines for its excellent and reliable properties. However, the lack of investigation on its smelting process, microstructure transformation and mechanical properties limits its domestic application. In this paper, the characteristics of microstructure and property changes under different heat treatment were determined. Therefore, the references on industrial application could be provided, and the domestic use of this steel could be extended.In this study, the effects of heat treatment on microstructure and mechanical properties of45CrMoVE steel were investigated. In addition, with fracture morphology and microstructure, the principles of toughness were analyzed. The results show that:(1) After thermodynamic calculation, the main precipitation of this steel was proved to be M23C6phase;(2) Under water quenching and oil quenching, both microstructures of this steel consisted of lath martensite. The cooling after normalizing was controlled in different rates. Some microstructural changes could be observed. With decreased cooling rate, the matrix formed from lath bainite to a mixture of granular bainite and pro-eutectoid ferrite. Furthermore, the size and amount lath bainite went down, when those of granular bainite increased simultaneously, with shaped into strips. In addition, the pro-eutectoid ferrite with small amount still remained stable.(3) After rate controlled cooling, the samples were conducted to tempering treatment, and the microstructure was proved to be a mixture of granular bainite and pro-eutectoid ferrite. With decreased cooling rate, the bainite showed less lath feature through recovery process, while M/A islands brock into a mixture of ferrite and cementite.(4) The effect on mechanical property mainly shows on yield ratio, which increased from0.63to0.71with dereased cooling rate. Furthermore, after tempering, the ratio rised to0.87.(5) Another group of heat treatment was conducted through normalizing (followed with air cooling) and tempering. The microstructure was proved to be a mixture of lath bainite, granular bainite and pro-eutectoid ferrite. With the increased tempering temperature, several microstructural changes could be seen:granular bainite integrated and grew up; the size of cementite increased gradually, with the shape transformed from acicular to ellipsoidal; M/A islands, which changed from block to granular, brock into a mixture of ferrite and cementite, with the decreased size and amount; the fine pro-eutectoid ferrite still remained stable.(6) As tempering temperature increased, the characteristics of mechanical properties were revealed as follows:strength decreased gradually, with increased ductility and toughness; in particular, the toughness increased sharply when tempered above650℃.(7) In the aspect of fracture morphology, the evolution of from cleavage fracture to dimple fracture could be observed. When tempered from200-600℃, the morphology was proved be cleavage fracture. Through the microstructure analysis, three types of matrix were deemed to take effect on the toughness. The fine lath bainite dedicated the improvement; on the contrary, the blocky M/A islands contributed to the degradation; the fine pro-eutectoid ferrite made little effort on it. When tempering was conducted under650℃to700℃, the recovery of ferrite occurred, while M/A islands decomposed completely for ferrite and carbide. The microstructural could account for the sharp development of toughness.