Research On Hot Deformation Behavior Of Hyper Duplex Stainless Steel

The high alloy hyper duplex stainless steel(HDSS),is one of the important development direction of high-quality stainless steels.It is characterized by excellent resistance to pitting,crevice corrosion and stress corrosion cracking in chloride containing environments.Due to its excellent combination of desired strength and high corrosion resistance,hyper duplex stainless steel has been used in petroleum refining,ocean industries and several aggressive fields,instead of austenite stainless steel.However,different hot deformation behaviors between austenite phase and ferrite phase,as well as the second phase,can easily cause the defects of microstructure and the degradation of propertyies during hot deformation,which will seriously affect the quality of products and the subsequent processing.Therefore,the hot deformation behavior and soften mechanism of hyper duplex stainless steel were studied in this paper,attempting to establish the theoretical basis for optimum hot working condition,which is significant to improve the properties and yield of materials.In this thesis,hot deformation behavior of hyper duplex stainless steel was investigated by conducting hot deformation tests at deformation temperatures of 900-1250 ? and strain rates of 0.01-10 s-1.The hot deformation soften mechanism were studied.The consistutive equation was established.The microstructure evolution was well studied by optical microscopy(OM),scanning electron microscopy(SEM),transmission electron microscopy(TEM)and X-ray diffiracromer(XRD),compined with results made by Thermo-Calc software.The processing maps that based on the dynamic materials model were established.The results show that the flow stress increases with the decrease in temperature and the increase in strain rate.The constitutive equation of peak stress was established on the basis of Arrhenius-type hyperbolic sin equation,and the apparent activation energy(Q)at whole range of temperatures was determined to be 574.452kJ·mol-1.The material constants in the model such as a,n,Q and InA were functioned with the strain fitted by sixth order polynomial.The constitutive model with the compensation of strain was developed to predict the flow stress curves under different deformation conditions.The results showed that the developed constitutive model had excellent predictability of flow stress.The influence of deformation conditions on the microstructure evolution of hyper duplex stainless steel is significant.The soften mechanism can be summarized that the ferrite takes occurrence of continuous dynamic recrystallization by the rapid transformation from low-angle grain boundaries to high-angle grain boundaris,and the austenite takes occurrence of dynamic recovery.The strain induced precipitate prompts the formation of a phase at low deformation temperature.The pinning effect of a phase restricts dynamic recrystallization.Based on the calculational results of Thermo-Calc,Cr and Mo promote the formation of ? phase and N has the opposite effect.Based on the dynamic materials model,the processing maps of hyper duplex stainless steel with different strains were established.In order to optimize hot working processing,the distributions of safe regions and instable regions of processing maps were analyzed,combined with the microstructure.The optium hot working domain was obtained to be in the deformation temperature range 1160-1250 ? and the strain rates range 4s-1-10s-1.