Mathematical Model Of Grain Evolution For Nuclear Power SA508-3 Steel During Hot Forging Process And Its Application In The Development Of Heavy Pressure Vessel Head Forming

The nuclear power pressure vessels is one of the key components of nuclear power equipment.The heavy upper head of nuclear pressure vessure is considered as the "heart" of pressurized water reactor nuclear power station equipment.Heavy forging is usually adopted in industry.Forging usually have a complex shape and outline.Heavy forging usually consists of multi-pass and each pass consists of multiple reductions.It belongs to typical large forgings non-uniform thermal deformation of extreme manufacturing process.Hot forging process must ensure the required shape and size,and the most important issue is that the forging scheme must meet the parts made in respect of their performance.Therefore,in order to understand the microstructure evolution law of metal during muti-pass thermal deformation,we mainly study the microstructure evolution between micro and macro interaction rules of SA508-3 steel,and establish the corresponding mathematical model and develop the reliable key technology and numerical simulation tool.The main research content is as follows: 1.Study of rules microstructure and establishment of the mathematical model during the whole process of thermoplastic deformation for large forgings(1)Austenitic grain growth rules of SA508-3 steel have been investigated by the experimental heating treatment in terms of the change of heating temperature,holding time and heating rate.Austenite grain growth kinetics has been investigated under different heating conditions,involving heating temperature,holding time as well as heating rate.Based on the experimental results,the mathematical model was established by regression analysis.The model predictions present a good agreement with the experimental data.Meanwhile,grain boundary precipitates and pinning effects on grain growth were studied by transmission electron microscopy(TEM).The results from this study provide the basis for the establishment of large-sized ingot heating specification for SA508-3 steel.(2)The hot deformation behaviors of SA508-3 steel were investigated by isothermal hot compression tests with different forming temperature and strain rate,and the corresponding flow stress curves were obtained with Gleeble thermo-mechaincal simulator to study the DRX behavior.According to the experimental results,quantitative analysis of work hardening and dynamic softening behaviors was presented.Based on classical stress-dislocation relation and kinematics of DRX,the flow stress constitutive equations of the work hardening-dynamical recovery period and dynamical recrystallization period were established for SA508-3 steels,respectively.Comparisons between the predicted and measured flow stress indicate that the established physically-based constitutive model can accurately characterize the hot deformations behaviours for the studied steel.The DRX kinetics model and grain size model of this steel were proposed by using quantitative metallography.The predicted values of model are in good agreement with the experimental results.(3)The heavy forgings productions includes the process of multi-pass,high temperature deformation,in a time interval softening behavior happens.The softening behavior of SA508-3 steel was investigated using double pass compression tests with Gleeble thermo-mechaincal simulator.The static recrystallization(SRX)and metadynamic recrystallizaiton(MDRX)behaviors are the main mechanism of static softening process.The thermal deformation of multi-pass after the softening of interval time and deformation has important effect on grain size evolution.Therefore,the dynamics equation of SRX and MDRX and evolution models for grain size are established respectively.2.Development of reliable key technologies and multi-scale numerical simulation technology based on DEFORM software These models,such as the dynamic and static softening dynamics and grain size growth mathematical models,were established under steady state experiment conditions.based on DEFORM software platform to study the SA508-3 steel models correction principle,the key technologies of numerical simulation were put forward in program development process and unsteady multi-pass temperature field,stress field and microstructure evolution.Based on DEFORM software,numerical simulation platform are set up to carry on simulation prediction and experimental validation.(1)To adjust model under the condition of unsteady flow stress and strain after the continuity,flow stress was carried on partial differential treatment of strainand strain rate,respectively.The instantaneous value of temperature and strain rate were adoped by the infacter of Zener Hollomon parameter.(2)For the thermal deformation between the softening and the effect of static softening technology processing multi-pass time interval by residual strain and the correction method to calculate the equivalent strain,so as to realize the rheological stress and strain and the microstructure evolution model in multichannel hot deformation process more physical field simulation.(3)Temperature changes with the hot metal deformation process accompanied by deformation time.These models are based on the constant temperature condition for related parameters,and further put forward the method to take the temperature compensating equivalent time into account.The transient temperature process is divided into several time increment steps,and then embedded in the finite element mathematical model to calculate the average temperature each time increment steps of units,so the accumulation of equivalent time is implemented in the process of unsteady temperature.(4)Based on grain coarsening experiment and numerical simulation analysis,the mechanism of coarse grains and process control of large forgings forming method was invested.3.Numerical simulation calculation for engineering application and technological innovation(1)Based on simulation software to study the nuclear CAP1400 steam generator of large head forging homogenization method.In order to achieve the forgings with grain homogenization,the numerical simulation technology is adopted to study the different heating temperature and holding time,heating process on the temperature field and grain size distribution of laws.Finish forging forming process of nuclear power CAP1400 steam generator of big head was simulated,and then,to analyze the forming process of various physical fields.Through the analysis of simulation and the actual forming process,a new technological scheme for hot forging and numerical simulation study of grain homogenization was optimized control of forging forming technology.(2)A nuclear reactor AP1000 with large complex water nozzle chamber head forging products overall forming process was studied by numerical simulation technology and to "forming and control" as the goal for process optimization.Based on the product form features,the shortages of traditional forming process and put forward combined tire die forging blocking and hot stamping forming process,and through the numerical simulation to optimize the product forming blank and die forgings,simulation forecast hot stamping forming load and related physical fields.