Simulation On Microstructure Evolution And Deformation During Heat Treatment Of Water Chamber Head Of Nuclear Stream Generator

The large forging of 16MND5 steel steam generator water chamber head is the important part of the third generation nuclear power units,and heat treatment determines the final performance of the water chamber head.Through simulation of heat treatment process of large forging,we can study the microstructure evolution and deformation,and provide a basis for the design and optimization of heat treatment process of large forging.In this paper,by studying the transformation kinetics and material parameters of 16MND5 steel for nuclear power pressure vessel,a multi-field coupled temperature-phase transformation-stress/strain model of water chamber head spray quenching process is constructed.And the prediction of temperature,microstructure evolution,stress and deformation of water spray quenching cooling process of a typical nuclear power steam generator water chamber head is achieved.In this paper,the phase transformation kinetics and microstructure evolution law of the 16MND5 steel during cooling were studied,and the TTT curves for isothermal transformation and the CCT curves for continuous cooling were established by thermal expansion tests.The JMAK model and the K-M model were used to describe the bainitic transformation and martensitic transformation during the cooling process,respectively.And the temperature-dependent phase transformation model parameters n,lnb and α were determined.The temperature interval from Ms point to 600°C of 16MND5 steel was clarified as the bainitic transformation zone,and the martensitic phase transformation started below 400°C.The microstructure under different cooling rates in the continuous cooling process was analyzed,and the critical cooling rate of 16MND5 steel is 20℃/s for martensite phase transformation only and 0.2℃/s for no pearlite transformation.Based on the results of spray quenching of the barrel-shaped test piece(CFHI),numerical simulations and inverse algorithms were used to optimize the integrated heat transfer coefficient of the spray cooling process for large forging.The material database of nonlinear material property parameters and heat transfer coefficients was finally established.A multi-field coupled model of temperature,microstructure and stress/strain was constructed to simulate the spray quenching of 16MND5 steel water chamber head forgings numerically,and the prediction of temperature,phase transformation and deformation during the spray quenching of water chamber head forgings was realized.The results showed that the surface microstructure of the water chamber head at the maximum wall thickness position was mainly martensite and bainite,the inner zone was bainite,the core zone was bainite and a small amount of pearlite.The influence of phase transformation on the stress field of the head during cooling process is obvious,the influence of temperature on the stress field is also more obvious,and the opening deformation of the water chamber head is large,the maximum deformation rate is about0.4%～0.5%.Based on the numerical simulation to obtain the temperature and microstructure fields,the heat treatment verification test of the head forging was carried out.The test results showed that the numerical simulation results were more accurate,and the tempered verification specimens had ferrite and carbide,and the yield strength was 651 MPa at the surface,475 MPa at 1/4 thickness,and 483 MPa at the heart,and all the mechanical properties tested met the process requirements.