Research On Deformation Behavior Of Super Austenitic Stainless Steel With High Mo And N

Super austenitic stainless steels (SASS) are able to provide excellent resistance to pitting and crevice corrosion, which are considered as alternative materials of nickel-based alloys and titanium alloys. The development of high alloy super austenitic stainless steels is one of the most important directions currently. Recently, a super austenitic stainless steel with high Mo and N (HHSASS) has been developed by Outokumpu. This material exhibits higher corrosion resistance and has been applied in several aggressive fields. However, HHSASS is susceptible to form secondary phase and edge crack because of the high Cr and Mo content. Therefore, it is able to provide guidance for hot working processes of super austenitic stainless steels, by studying the hot deformation behavior.In this thesis, the hot deformation behavior of HHSASS was studied through hot compression test on a Thermecmastor-Z thermal simulating machine. The constitutive behavior, microstructural evolution and processing map of HHSASS were studied by Thermo-Calc software, optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM).The flow curves show that the flow stress are influenced by the deformation temperature and the strain rate. It increases with decreasing temperature and increasing strain rate. Austenite undergoes dynamic recrystallization (DRX) at low strain rate and exhibit work hardening at high strain rate. The consistutive equation was established, the apparent activation energy (Q) at whole range of temperatures was determined to be530kJ/mol. The method to predict critical strain of DRX for HHSASS was studied through the three polynomial fitting methods.The microstucture evolution was examined optical microscopy. Increasing deformation temperature and decreasing strain rate promotes dynamic recrystallization of austenite. The formation of precipitates during the hot deformation were examined by SEM. The volume fraction of the precipitates decreases with the increasing deformation temperature. The drag effect of precipitates restricts dynamic recrystallization. It was found that σ phase is the the main precipitate during hot deformation of HHSASS. Based on the prediction of Thermo-Calc, Cr and Mo promote the formation of a phase and N suppresses the precipitation of σ phase.The basic theory of dynamic material model (DMM) was studied. The processing maps of HHSASS with different strains was established. In order to optimize hot working processing, the distribution of safe regions and instable regions of processing maps was analyzed combined with the microstructure. The best hot working temperatures range from1150℃to1250℃and the strain rates range from0.001s-1to0.01s-1.