Research On The Hot Deformation Behavior Of Austenitic Heat Resistant Steel For Ultra-supcrcritical Boilers

HR3C and super304H austenitic heat resistant steels have become thepreferred materials of supercritical (SC) and ultra-supercritical (USC) generatingset due to its high creep rupture strength, good steam oxidation resistance andflue gas resistance. So far, there is still a large gap in the production technologyof HR3C and super304H in China compared with foreign countries. The productyield and quality is influenced by the technological problems, such as the crackgenerated in the process of rolling and non uniform structure. The key to solvethe problems is to obtain uniform fine recrystallization structure by controlingthe hot working processes, and then improve its performance. The research ofhigh temperature plastic deformation behavior is beneficial to optimize the hotworking parameters, grasp microstructural evolution in hot deformation processand improve the combination property of materials.To obtain the stress-strain curves of HR3C and super304H steels in thedifferent deformation conditions, the single pass compression tests wereconducted by using a Gleeble1500D thermal-mechanical simulator. Theinfluence of deformation temperature and strain rate on flow stress was studied.Finally, the processing map of HR3C and super304H were obtained by using thedynamic materials model (DMM). The main conclusions are as follows: (1) The characteristic of stress-strain curves of HR3C and super304H is as follow:the flow stress increases with the increase of dislocation density at the beginning of the deformation; when the deformation reaches a critical strain, dynamic softening effect start and gradually enhanced.When dynamic softening effect is equal to the work hardening effect, the stress reaches a maximum. With further deformation, dynamic softening effect is better than work hardening, and the stress decreases gradually until stabilized.At the same time, the dynamic softening effect and work hardening are in dynamic equilibrium.(2) The flow stress of HR3C and super304H austenitic heat resistant steels decreases with the increase of strain rate and the decrease of deformation temperature. Both the influence of strain rate and deformation temperature on the peak stress can be described by the hyperbolic sine Arrhenius relationship.(3) The dynamic recrystallization structure of HR3C and super304H is influenced by its strain rate and deformation temperature. The decrease of strain rate and increase of deformation temperature are beneficial to the nucleation and growth of dynamic recrystallization grain. In other words, the occurrence of dynamic recrystallization needs high temperature and low strain rate.(4) When the deformation temperature ranging from950℃to1250℃and strain rate from0.001s-1to1.0s-1, the hot deformation equation of HR3C austenitic heat resistant steel is as follow: ε=8.79x1018x[sinh(0.0091σp)]5.11exp(-558x103/R7),its peak stress can be described by the following Z parameter:(5) When the deformation temperature ranging from850℃to1250℃and strain rate from0.01s-1to1.0s-1, the hot deformation equation of super304H austenitic heat-resisting steel is as follow: ε=3.62x1017x[sinh(0.0070σp)]4.78exp(-482x103/R7), its peak stress can be described by the following Z parameter:(6) Combined the analysis of processing map of HR3C and super304H with microstructural evolution,we found that:The best technological parameters of HR3C austenitic heat resistant steel are the deformation temperature ranging from1180℃to1250℃, strain rate from0.1s-1to1.0s-1, and the deformation temperature ranging from1100℃to1180℃, strain rate from0.007s-1to0.03s-1, while the best technological parameter of super304H is the deformation temperature ranging from1150℃to1250℃, strain rate from0.1s-1to1.0s-1(7) The hot deformation activation energy of HR3C and super304H austenitic heat resistant steel is558kJ/mol and482kJ/mol, respectively. This is not only because of the high concentration of alloying element of HR3C heat resistant steel, but also because of the higher contents of Cr, Ni elements in HR3C than in super304H, which have strong effect on the increase of activation energy.