Study On Grain Evolution Of 316H Stainless Steel During Forging Process Based On Homogeneity

316H stainless steel not only has excellent creep and fracture resistance,but also has excellent corrosion and radiation resistance at high temperature.These properties match the requirements of nuclear power materials.Therefore,this steel is widely used in the fourth generation nuclear power system.Nuclear power steels have more stringent requirements on mechanical properties and grain size of materials.However,with the traditional forging process,316 H stainless steel forgings often have obvious coarse grains and mixed grains,which seriously affect the quality of forgings.In order to solve the problems of grain inhomogeneity in 316 H stainless steel forgings,it is necessary to study the grain evolution law and establish a series of prediction models about the grain evolution.Therefore,it can lay a foundation for the numerical simulation and process optimization of 316 H stainless steel forging process.In this paper,the 316 H stainless steel is systematically studied from the aspects of grain growth,hot deformation behavior,dynamic recrystallization and static recrystallization,and the deform numerical simulation has been carried out for the purpose of grain homogeneity.The conclusions can be drawn as follows:(1)Through the study of the true stress-strain curve of 316 H austenitic stainless steel under hot compression,it is found that the true stress-strain curve of the steel begins to show dynamic recrystallization at above 1000℃,and the activation energy of hot deformation of 316 H austenitic stainless steel is470.216 k J.The constitutive equation of 316 H stainless steel is established by using the modified Arrhenius relation.(2)By analyzing the evolution law of dynamic recrystallization(DRX)of316H stainless steel during hot compression,a calculation method was proposed to characterize the dynamic recrystallization fraction of different stress softening degrees by using the DRX fraction at the maximum strain of compression experiment as a reference value.Based on this method,a Serajzadeh DRX dynamic model of 316 H stainless steel is established.In order to meet the requirements of DRX dynamic model in DEFORM,Yada DRX dynamic model is established.Comparing the curves of strain and DRX fraction obtained by the above two models with the verified experimental values obtained from metallography,it is found that the average absolute error between the predicted values and the verified experimental values is small,which proves the accuracy of the two models.The relationship between DRX grain size and Z parameter is established by analyzing the DRX grain size obtained from metallographic structures at different temperatures and strains.(3)By comparing the effects of deformation parameters on the double pass stress-strain softening curve and metallographic microstructure,the effects of holding time,pre-strain,strain rate and deformation temperature on static recrystallization(SRX)are obtained,and the SRX dynamic model and SRX grain size model of 316 H stainless steel are established.(4)By analyzing the average grain size obtained at different temperatures and holding time,it is found that the grain growth rate is significantly accelerated when the temperature rises to 1150℃.Therefore,it is recommended that the initial forging temperature of 316 H stainless steel be 1150℃.The Sellars model of grain growth is given by multivariate nonlinear fitting.(5)DEFORM numerical simulation is carried out for the thermal compression experiment,and the simulation results are compared with the experimental results to verify the accuracy of DEFORM simulation results.Using DEFORM to simulate and optimize the compression process in the actual production process,it is found that the optimal ratio of overall upsetting and step upsetting is: overall upsetting 30%,step upsetting 20%;In addition,both bidirectional rotary upsetting and curved anvil can promote grain refinement and inhibit coarse and mixed grains.