Hot Deformation Behavior Of New Heat-resistant Steel For High Steam Condition Thermal Power Units

Electricity is the main component of energy.China has always relied on thermal power.Increasing the operating parameters of thermal power units can significantly increase the efficiency of power generation.Therefore,higher requirements were imposed on the heat transfer tubes for ultra-supercritical thermal power units.Super304H austenitic heat-resistant steel is widely used as pipes,however,under high temperature conditions,coarse precipitates will accumulate on the grain boundary of Super304H,which will affect its structural stability and limit its service life.In order to solve this problem,the research group has optimized the composition of Super304H and developed a new type of austenitic heat-resistant steel?code name:CHDG-A steel?with better service performance at high temperature.In this paper,the deformation characteristics of CHDG-A steel at a deformation temperature of 900¹100?and a strain rate of 0.01¹0 s^-1 were studied through hot compressive experiments.Using optical microscope?OM?,transmission?TEM?and electron backscatter diffraction?EBSD?analysis techniques to study the structural evolution of CHDG-A steel during thermal deformation,revealing its dynamic recrystallization?DRX?nucleation mechanism,thus lay the foundation for the hot forming process of new austenitic heat-resistant steel.The main conclusions and innovations are as follows:The effect of deformation conditions on the true stress of CHDG-A steel was systematically studied.The results showed that the true stress of new material increases as the strain rate increases and the deformation temperature decreases.Based on the true stress-true strain curve,the Arrhenius hyperbolic equation of CHDG-A steel at peak strain was constructed,and its thermal deformation activation energy was 515.618kJ/mol.The results showed that the model can accurately describe the flow stress of CHDG-A steel during thermal deformation.Based on the DRX theory,the values of peak stress,critical stress,peak strain and critical strain for DRX of CHDG-A steel was determined.The ratios of?_c/?_p and?_c/?_p are also identified,which are 0.52 and 0.98,respectively.By the analysis of experimental data and microstructures under different deformation conditions,it was showed that?_c decreases with decreasing strain rate and increasing temperature.The DRX kinetics established based on the Avrami equation showed that under large strain,high temperature and low strain rate,the value of X_DRX increases.DRX is the principal softening mechanism of CHDG-A steel during hot working.The main DRX nucleation mechanism is dDRX with bulging of the original grain boundaries,cDRX plays a supporting role.Embedding the high temperature constitutive equation and DRX kinetics to Deform software to conduct the hot forging numerical simulations for CHDG-A steel,it was found that stress and strain in the specimen unevenly distribute and change with the amount of deformation,strain rate and deformation temperature.The experiments and calculations of DRX volume fractions is very similar which proves the accuracy of models.