Structure And Mechanical Properties Of Steel For Reactor Pressure Vessel

Currently, the most widely used material for commercial nuclear pressure vessels is SA508-3 steel. However, with the development of reactor pressure vessel to large-scale and integration, performance requirements for materials further enhance, but SA508-3 steel is difficult to guarantee uniformity and stability on thick section. In this case, SA508-4 steel, with high strength and high toughness, that can help introduce advanced PWR systems with higher safety and economy in the near future.Firstly, by comparing SA508-3 steel has been widely used and SA508-4 steel has not been used but is the fourth-generation nuclear power plant alternative material because of microstructure and mechanical properties, learned the differences and connections between SA508-3 steel and SA508-4 steel. By changing heat treatments and chemical compositions of SA508-4 steel to study the effects on microstructure and mechanical properties of SA508-4 steel and obtain the chemical composition and heat treatment making SA508-4 steel has the best comprehensive properties. The main research is about the effect of chemical compositions and heat treatments on the structure and mechanical properties of SA508-4 steel, and the heat treatment process includes austenitizing temperature, quenching cooling rate, tempering temperature and tempering holding time. In this paper, using thermodynamic software of Thermo-Calc for thermodynamic calculations, as the theoretical basis for research precipitates; using optical microscopy, scanning electron microscopy and transmission electron microscopy to observe the structure of materials after heat treatment; using MH-6 type micro hardness tester, universal testing machine and impact testing machine to test micro Vickers hardness, tensile properties at room temperature, impact toughness at low temperature, and analyze effects of the chemical composition and the heat treatment on mechanical properties; using scanning electron microscope to observe tensile fracture morphology and impact fracture morphology to judge the fracture mode; using Image- Pro plus software for statistical distribution of precipitates, and to study the relationship between precipitations and mechanical properties; using transmission electron diffraction spots microscope and X-ray diffraction to identify the type of precipitated carbides.By contrasting SA508-3 steel and SA508-4 steel, found that the hardness, strength, ductility and impact toughness of steel SA508-4 after heat treatment are better than SA508-3 steel. Heat treatments and chemical compositions affect the mechanical properties by affecting precipitates and hardenability of the steels. With increasing of tempering holding time, the strength decreases, toughness and plastic gradually increase, hardness basically unchanges; with increasing of tempering temperature, the strength first reduces and then increases, toughness and plastic first increase and then reduce, hardness gradually decreases; with increasing of tempering parameters P, the change of strength, toughness, toughness, plasticity and hardness basically is the same with tempering temperature increasing and when P is 19.6, could obtain the extremum of mechanical properties; with austenitizing temperature increasing, the strength and hardness basically unchange, toughness decreases, plasticity slightly decreases; quenching cooling rate has no effect on mechanical properties of SA508-4 steel; with contents of Cr and Ni increaseing, the strength and toughness gradually increase, hardness and plasticity basicaly unchange.