Microstructural Evolution And Mechanical Properties Research Of SA508 Gr.3 Steel For Nuclear Reactor Pressure Vessel

At present,China has been devoting major efforts to developing the nuclear power industry,thus the research on the microstructural evolution and mechanical property in the process of manufacturing and servicing for nuclear reactor pressure vessel has significant importance.In this dissertation,the phase transformation kinetics and microstructural evolution of the SA508 Gr.3 steel for nuclear reactor pressure vessel were studied.The relationship between microstructure and mechanical property was established by analyzing the impact fracture and high temperature creep behavior.The purpose of the dissertation is to provide guidance for understanding the relationship between heat treatment process,microstructure and mechanical properties for nuclear large forgings.Firstly,the phase transformation kinetics of the SA508 Gr.3 steel was studied by means of dilatometer method,the TTT and CCT curves including the start/finish temperature,transformation amount and time kinetics information were established and the microstructural evolution during the undercooled austenite isothermal transformation and continuous cooling transformation was researched.The result showed that as the isothermal temperature decreases,the isothermal transformation microstructure are proeutectoid ferrite,flake ferrite,carbonless bainite,upper bainite and lower bainite respectively.As the cooling rate increases,the continuous cooling transformation microstructure are proeutectoid ferrite and bainite and matrensite,bainite,martensite and bainite,lath martensite respectively.Secondly,the instrumented impact experiment at different temperature of furnace cooling sample,air cooling sample and oil cooling sample was conducted.The result showed that as the cooling rate increases,the impact energy increases and the ductile-brittle transition temperature decreases.According to the load-displacement curve,the macro and micro fracture surface observation and the feature of secondary cracks,it can be concluded that the block size of microstructure and the density of big angle boundaries are the key factors affecting the crack initiation and propagation for nuclear reactor pressure vessel.Finally,the creep tests at different temperature and stress showed that creep behavior is difficult to occur at low temperature.As the temperature or stress increases,the creep strain increases and the creep behavior becomes more and more obvious.The scanning electron microscope and electron backscattered diffraction analysis showed that the microstructure is refined and plenty of sub-structures appear in the grain after creep test,which reveals the dislocation movement mechanism is the main creep deformation mechanism.In addition,the point defect diffusion and grain boundary sliding make contribution to creep deformation at high temperature.