Theoretical And Experimental Study On Loose Tool Forging Process Of Step-by-step Loading For Large Head With Wall Thickness

The upper head of reactor pressure vessel（RPV）of marine nuclear power platform with high flange and thick wall is an key part on nuclear power plant pressured water reactor,which possessed high requirements for pressure bearing capacity.Researching the controllable forging technology under complex geometric conditions and realizing the integrated control in shape/properties of forgings with similar structures are benefit to improve the progressiveness and scientific level of the manufacturing of large forgings in China.Taking this forging as the research object,a less-loading new loose tooling forging of step-by-step loading is putted forward in this paper,which can provide a reference for the profiling forging of large forgings with complex shape.The specific research contents and results are as follows:The single pass thermal compression experiments of SA508 Gr.3Cl.1 steel at deformation temperature of 950～1200℃strain rate of 0.001s^-1～1s^-1 are performed using 1500 hot thermophysical simulator.The hot deformation behavior is studied based on the tested stress-strain curve.The test results show that the softening mechanism of the steel are closely related to the thermal deformation parameter Z.The dynamic recrystallization kinetic model is build based on improved Avrami equation;two-stage constitutive model is established based on the dislocation density theory during different deform stage;the hot working maps of the steel with strain of 0.1～0.6 are drawn and the best forging process range of SA508 Gr.3Cl.1 steel is obtained by analyzing the working maps.The tensile tests SA508 Gr.3Cl.1 steel at deformation temperature of800～1200℃and strain rate of 0.001s^-1～1s^-1 are done.The high temperature plasticity of the steel is analyzed and the damage model at the high temperature based is established by introducing thermal deformation parameter Z.The accuracy of model established is verified by comparing the calculated and experimental values of the critical damage value and the length of the sample after tensile fracture.The results show that the model can accurately predict the damage behavior of SA508 Gr.3Cl.1 steel during hot deformation.The high temperature damage model established is embedded into the DEFORM post-processing program through the secondary development interface of DEFORM software and the dangerous damage areas of the head forging can be predicted during the process of thermal deformation.The microstructure evolution rules between deformation and heat treatment interface are studied using plane deformation test and after-forging normalizing test.The test results show that all seriously uneven grains（9.3～385μm）after deformation become fine and uniform grains（17～22μm）after normalizing under the deformation temperature of 1050℃～1250℃.Among them,the large grains with size of 228～385μm become fine and uniform,which grain size is of21.48～18.9μm.That is,the SA508 Gr.3Cl.1 steel has no inheritance of grain size and the coarse non-uniform grains contained in the forgings after hot deformation can be refined by heat treatment for SA508 Gr.3Cl.1 steel.Therefore,the key problem in the forging process is how to improve the dimensional accuracy for the heavy forgings made of SA508 Gr.3Cl.1 steel and the grain size and uniformity can be resolved heat treatment technology.Thus,the forming difficulty of complex heavy forgings made of this steel can be reduced and it is of important engineering application value.Through combining the method of numerical simulation and optimization design,the metal flow behavior during thermal deformation of the large head with high flange variable and thick wall is explored;the effects of forging process parameters and die parameters on the hot forming processing are analyzed and the quantitative relationship between forming load and main forging process parameters and die parameters is established;and the forging process scheme that can meet the quality requirements is obtained under the condition of limited load.According to the structural characteristics of RPV upper head and the existing equipment,a new loose tooling forging process of step-by-step loading is proposed.Because of the forming load far exceeding the equipment capacity,taking the forming load as the control target,the main forging process parameters and die parameters affecting the forming load are obtained through analyzing the range and variance of the orthogonal test results based on DEFORM numerical simulation tests.Further more,the main parameters are optimized using response surface method and comprehensively considering the damage,the forging size and forming load,the forging process parameters and die structure parameters which satisfied the requirements are obtained.The results of the scale test and the product of the experimental forging indicate that the loose tooling forging processing of step-by-step loading proposed in this paper is feasible in technology and application and the profiling forging of forgings can be realize by this new process.