Microstructure - Mechanical Properties Relationship And Numerical Simulation Of Quenching And Tempering Heat Treatment Process For Reactor Pressure Vessel Heavy Forgings

With the improvement of life and safety requirements of the third generation design of nuclear power plant,reactor pressure vessel forgings showing the large-scale trend and integration.At present,demand for nuclear power is strong in China.Research the microstructure evolution and the correlation with properties during hot manufacturing process,to ensure the stable production of reactor pressure vessel integrated forgings is imperative.In the dissertation,the phase transformation kinetics and microstructure evolution of SA508Gr.3 steel are studied,and the quantitative relationship between microstructure and properties is established.The finite element method is used to simulate the quenching and tempering treatment of typical reactor pressure vessel integrated head cover heavy forging.The optimization of the heat treatment process which based on the temperature-microstructure-properties prediction is realized.First,the phase transformation kinetics of the SA508Gr.3 steel has been studied.the continuous heating transformation?CHT?curve,austenitic isothermal heating transformation?IHT?curve,TTT and CCT curve about SA508Gr.3 steel during the phase transformation have been tested and perfected,the kinetic equations which can describe the SA508Gr.3 steel's temper process have been established by set up the relationship between hardness and temper temperature and time.In this paper,the JMAK equation is taken to describe the diffusive transformation,such as austenite transformation,bainite transformation,and tempering.The index n,the pre-exponential factor lnk₀ and phase transformation activation energy Q of JMAK equation have been determineed,respectively.The martensite phase transformation's description is taked by K-M equation,the temperature related parameter?also has been fitted.Second,the SA508Gr.3 steel's microstructure evolution during the phase transformation has been researched.As to the austenitizing process,with the increase of temperature,the nucleation and growth of austenite is in the grain boundary and lath boundary,then the carbon-riched bainite area will take place of austenite phase transformation,finally the carbon-poored ferrite will occur the austenite phase transformation.With regard to the continuous cooling process,the morphology and formation mechanism about martensite,the mixed microstructure of martensite and bainite,the mixed structure of bainite and the mixed structure of proeutectoid ferrite and bainite have been analyzed under the different cooling rate.The results indicate that the critical cooling rate of martensitic transformation is higher than 50?/s,and the critical cooling rate of the ferritic transformation is lower than 1?/s.For the tempering process of quenching microstructures,the microstructural evolution during the tempering process mainly includes the decomposition of M/A constituents,the precipitation and coarsening of Fe₃C and Mo₂Ccarbides,and the recrystallization of bainitic ferrite under high temperature by means of the thermodynamic calculation and the microstructure characterization.Then,the effect of austenite grain size,the size of bainite packet,the volume fraction of M/A constituent,the size and distribution of carbides on the properties are studied.The results show that the coarse austenite grain is not conducive to the impact toughness.The austenite temperature should be lowered as far as possible under the premise of ensuring the complete austenitizing.In order to ensure the impact property,the material should avoid the polygonal ferrite.There is a direct relationship between the impact toughness of the material and the size and distribution of M/A phase and the carbides in the microstructure,especially the M/A phase has great influence on the impact properties.The process window of heating,quenching and tempering for the SA508Gr.3 steel is determined,namely the heating window of austenitizing should be determined between890?and 910?at which it doesn't cause the significant growth of austenite.The critical engineering quenching cooling rate of the impact property of the material is ensured 1?/s,the optimum tempering temperature is between 640?and 655?and the tempering time is5-10 hours.The strength prediction model based on experiments is established by applying the weighted average method and the prediction of quenching hardness field is achieved.Based on that,the hardness field after tempering is calculated by adopting the kinetics equation of tempering,and then realizing the prediction of strength after tempering.On the basis of clarifying the impact fracture mechanism of the material,fitting the instrumented impact curve and establishing the relationship between the key parameters and hardness,to obtain the method of predicting the impact properties.Finally,by using the established dynamic model and performance prediction method,the computer simulation of the temperature-microstructure-performance multi-field coupling in the process of quenching and tempering heat treatment for SA508 Gr.3 steel with integrated head cover forging is carried out,which reflects the inhomogeneity of temperature-microstructure-performance field in the process of heat treatment of integrated head cover,and the process of quenching and tempering is optimized.Compared with the upwards quenching process and the downwards quenching process,the sampling position of downwards process can't reach the engineering critical cooling rate.For the upwards process,the cooling rate of sampling location is about 1.1?/s,reaching the engineering critical cooling rate.The performance of tempering for the forgings after the upwards and downwards quenching is predicted.The results show that the downward quenching forging is in poor performance.The tempering process for the upward bowl forgings is optimized.After tempering 8 hours at 650?,the relatively uniform performance field is obtained in the flange section of maximum wall thickness and sphere segment.In this dissertation,the study which is based on the physical laws of the temperature-microstructure-properties relationship during the phase transformation of SA508 Gr.3 steel,realizes the numerical simulation of the whole process of quenching and tempering heat treatment by mathematical modeling,obtains the laws of microstructure evolution of integrated large forgings in the process of austenite heating,quench cooling and tempering by calculation and determines the temporal and spatial distribution of temperature-microstructure-performance in the quenching and tempering heat treatment process of large forgings.The quenching and tempering heat treatment process which has been verified by production practice for large forgings with integrated head cover is also optimized.