Research On Hot Deformation Behavior Of Martensitic Stainless Steel And Multi-Physics Numerical Simulation Of Turbine Blade Hot Forging

The mechanical properties of forgings are mainly depends on the microstructure evolution during hot forging process, during which the microstructure undergoes dynamic recovery, dynamic recrystallization, static recovery, static recrystallization and metadynamic recrystallization. The macroscopic factors such as deformation temperature, strain rate and strain play a key role in the microstructure evolution when the material composition is given. In recent years, many researchers have paid much attention to the control of microstructure evolution in order to get good mechanical property. In this paper, the hot deformation behaviors of X20Cr13 martensitic stainless steel used for turbine blade are studied. The mathematical models for the prediction of microstructure evolution are developed based on the hot compression test. A coupled analysis system of flow stress field, temperature field and microstructure evolution for hot forging is developed based on the commercial software. The main research content of this paper is as follows:Hot deformation behavior of X20Cr13 martensitic stainless steel is investigated by conducting hot compression tests on Gleeble-1500 D thermo-mechanical simulator. The flow stress curves of X20Cr13 martensitic stainless steel is obtained by the single hot compression tests. The material flow behavior under different deformation temperature and strain rate is studied. Constitutive model is developed based on the Arrhenius equation by introducing the effect of the strain compensation. The average absolute relative error is 3.30%, which indicates the efficiency of the developed model. The calculated activation energy of dynamic recrystallization is 359.402 k J/mol. The processing maps of X20Cr13 martensitic stainless steel are developed using Prasad instability criterion based on the dynamic materials model theory. The variation of strain rate sensitivity with the deformation temperature, strain rate and strain has been analyzed. The optimal range of processing parameters is proposed according to the analysis of both the power dissipation efficiency and instability parameter.The equations of peak strain and stress, steady-state strain and stress are obtained by regression analysis. The mathematical expression of critical strain for onset of dynamic recrystallization is developed. The critical strain value calculated by this model is close to the value obtained by the work hardening rate-stress curves.The dynamic recrystallization kinetics and grain size model are built based on the microstructure analysis. The constitutive model for X20Cr13 martensitic stainless steel is developed by using the dislocation density and grain size as the internal state variables, at the same time the influence of dynamic recrystallization on the dislocation density is considered. The recovery parameter is decoupled with the temperature and strain rate dependencies and fitted by the stress data before the critical stress under different deformation conditions. The change of dislocation density induced by the dynamic recrystallization is assumed to be a function of average grain size. The proportionality parameter is decoupled with the temperature and strain rate dependencies and fitted by the stress data after critical stress using cubic polynomial. The influence of microstructure change to the macroscopic stress is considered in the developed constitutive model which has a good prediction capability.The metadynamic recrystallization and static recrystallization behaviors of X20Cr13 martensitic stainless steel are investigated using double pass compression test. The results show that the static recrystallization grain size is mainly affected by initial austenite grain size and strain. The metadynamic grain size is mainly affected by the deformation temperature and the strain rate. The activation energies of static recrystallization and metadynamic recrystallization of X20Cr13 martensitic stainless steel are calculated. The kinetics model and grain size model are developed.The multi-physics numerical simulation system of X20Cr13 martensitic stainless steel for forging process has been developed. The numerical results of dynamic recrystallization volume fraction and average grain size are compared with the test results of unstable deformation conditions. The comparison results show that the developed constitutive model and microstructure evolution model are relatively reasonable. The forging process of X20Cr13 martensitic stainless steel turbine blade is simulated by the developed system. The influence of initial temperature of billet and deformation velocity on the load and finial temperature of forging part is analyzed. The variation of strain, temperature, dynamic recrystallization volume fraction and grain size has been analyzed through the forging process simulation. The numerical simulation results show that the average grain size of martensitic stainless steel turbine blade is between 18.7and 25.8 ?m. Based on the simulation results, the optimum hot forging process for complex forging parts is obtained.