Research On The Spray Forming Process Of SA508-3 Steel And High Temperature Deformation Characteristics

SA508-3 steel is one of the main materials of nuclear power pressure vessel.However,with the development of nuclear power pressure vessel towards thick wall and large size,it is more difficult for traditional casting-forging technology to guarantee the uniformity and stability of structure on extra-thick section,which results in that the quality and service life of forgings can not keep up with the requirements of nuclear power plant.More attention has paid on the spray forming process because of its uniform microstructure.SA508-3 steel was prepared by spray forming technology.Firstly,the spray forming atomization process was simulated,then the spray forming experiments were guided by the parameters obtained from the simulation,and the optimum actual processing parameters were determined.The influence of spray forming on SA508-3 steel is explored by comparing the microstructures and mechanical properties.Finally,the high temperature rheological stress model of spray forming SA508-3 steel was established.The main conclusions are as follows:（1）Computational Fluid Dynamics（CFD）numerical simulation technology was used to systematically study the influence of process parameters on atomization effect in spray forming by comparing the flow field characteristics under different atomization parameters（liquid temperature,medium and pressure,diameter of liquid guide tube）.The reasonable atomization parameters were obtained:the initial temperature of liquid metal,1550℃;the diameter of the guide pipe,4.5 mm;the atomization gas is N₂;and the atomization pressure,3.0 MPa.（2）Spray forming experiments were carried out according to some process parameters obtained by numerical simulation.By comparing the cross-section quality and microstructure of SA508-3 steel under different process parameters,the influence of each process parameter on the quality of spray forming deposited billet was analyzed,and the spray forming process parameters were further optimized.The results showed that the spray gas was N₂,the spray pressure was 3.0 MPa,the diameter of the guide pipe was 5.0 mm,and the deposition height was 450 mm.SA508-3steel was prepared by optimizing parameters.The results show that the spray forming microstructure is granular bainite,the grain is equiaxed and the size is small,which is obviously better than the as-cast structure.The yield strength of Spray-Formed SA508-3 steel can reach 601 MPa after forging and heat treatment.However,the impact toughness of spray formed SA508-3 steel is similar to that of as-cast steel because the voids in spray formed material are not completely eliminated in hot forging.（3）Hot compression experiment was carried out on spray formed SA508-3 steel.Two-stage high temperature rheological stress model and dynamic recrystallization size model of spray forming material were established,and the influence of thermal deformation parameters on microstructure was systematically studied.It is found that spray forming SA508-3 steel has obvious dynamic recrystallization softening characteristics at high temperature.When the deformation temperature is higher or the strain rate is lower,the nucleation and growth of dynamic recrystallization are more complete;and larger strain rate can inhibit the recovery and growth of recrystallized grains,so when the deformation temperature is constant,a larger strain rate is needed to obtain fine dynamic recrystallized grains.