Study On The Hot Deformation Behavior And The Microstructure Of 253MA

Research on the hot deformation behavior of Ni-saving austenitic stainless steel 253MA aimed at revealing the evolution of flow stress and microstructure during hot deformation combining processing-microstructure-property of materials. The hot deformation behavior of 253MA stainless steel was investigated by isothermal compression tests on Gleeble-3500 thermal simulator in the temperature range of 900 to 1150 ℃, the strain rate range of 0.01 to 20s-1 and the amount of compression of 20%,40% and 60%. The effects of deformation heat friction on flow stress were analyzed; the constitutive equations of 253MA and dynamic materials model processing maps were established utilizing the flow stress data corrected for deformation heating and friction. The effects of strain were introduced by considering its influence on the material constants.The effects of deformation parameters on the microstructural evolution were studied by optical microscopy (OM) and electron back scattering diffraction method. Present the major results of this investigation as follows:(1) During deformation process, the flow stress increased rapidly at first. And as the dynamic recrystallization mechanism initiated, flow stress reached its peak and became steady. Flow stress increases with the increase of the strain rate and the reducing of deformation temperature, while the critical strain for dynamic recrystallization decreases.(2) Deformation heat can lower the experimental flow stress. And the influence increases with the increase of temperature, reducing of strain rate and strain. The friction between dies and specimen increase the experimental flow stress. Because of the preferable lubrication condition, the influence of friction on the flow stress is not that much.(3) On the basis of the Arrhenius-type hyperbolic sine model, the constitutive equations of 253MA steel were developed by utilizing the flow stress data corrected for deformation heating and friction. The strain was introduced by considering its influence on the material constants. The reliability of the developed constitutive equations was assessed by means of both numeric comparison and statistical analysis. The results suggest that the developed constitutive equations can adequately describe the relationships between the flow stress and deformation parameters.(4) The degree of dynamic recrystallization and size of recrystallized grain elevated along with the increase of deformation temperature and the reducing of the strain rate. Moreover, the microstructure of different deformation conditions was analyzed combining with Zener-Hollomon parameter and found out that dynamic recrystallization can also be effected by grain boundary migration hindrance such as the existence of precipitations.(5) The dissipation map of DMM processing map was establish with the corrected flow stress data. And three different instability maps based on three different instability criterions which were Kumar-Prasad, Murty-Rao and Babuinstability criterion were compared. Study on the processing map with microstructures of corresponding microstructural evolution mechanisms was carried out and acquired the optimum processing conditions of temperature from 1050 to 1150℃, strain rate from 0.1 to 1s-1.