Research On High Temperature Oxidation Mechanism And Homogenization Treatment Of Super Austenitic Stainless Steel S32654

The development of high alloy super austenitic stainless steels is one of the most important directions of high quality stainless steels.Due to the high contents of chromium,molybdenum and nitrogen,the steels have very excellent resistances to pitting and crevice corrosion and remarkable mechanical properties in extreme service environments,which make them becoming alternative materials of titanium alloys and nickel-based alloys.However,due to their high Cr and Mo contents,super austenitic stainless steels are prone to produce the serious dendritic segregation during the solidification,which affects the uniformity of dendritic structure and reduce the hot workability.Homogenization treatment is often used to eliminate the dendritic segregation and improve the hot ductility of steels.However,super austenitic stainless steels have another severe problem of high temperature oxidation during the homogenization treatment,which restricts the production and application of the steels.Therefore,studying the high temperature oxidation behavior and homogenization treatment to elucidate the oxidation mechanism and determining optimum condition of homogenization treatment is able to provide guidances for hot working processes and application of super austenitic stainless steels.The S32654 super austenitic stainless steels were prepared by 25kg pressure induction furnace.The high temperature oxidation mechanism and the homogenization treatment were well studied by Thermo-Calc software,laser scanning confocal microscope(CISM),scanning electron microscope(SEM),electron probe micro-analyser(EPMA),X-Ray Diffraction(XRD)and transmission electron microscope(TEM).According to investigate the oxidation kinetics curves,the macro-morphology of the samples after oxidation at high temperature,the micro-morphology and component of oxide layer and the type of precipitates,the oxidation law and mechanism of super austenitic stainless steels S32654 were analyzed.In addition,the optimum homogenization treatment was determined through analyzing the dendrite morphology,segregation coefficient,residual segregation index and the precipitates before and after being homogenized.The results of high temperature oxidation tests show that the surfaces of samples were oxidized lightly and the oxidation follows a parabolic law at 900?.The oxide layer can be divided into two levels,the outer layer is MnMoO4 and the inner layer is Fe-Cr-Ni spinel,and no significant spalling occurs.The two layers can prevent the outward diffusion of metal cations and the inward diffusion of oxygen,which results in the reduction of the oxidation rate.At 1000 and 1200?,oxidation is very serious and obeys a liner law.The porous oxide layer is composed of Fe-Cr-Ni spinel,MoO2,Mn2O3 and NiMoO4.The formation and volatilization of MoO3 compromise the integrity of the oxide layer.The strip-shaped Cr2N caused by the air nitriding reduces the ability to form a protective layer of Cr2O3.The synergies of MoO3 and Cr2O3 led to the occurrence of catastrophic oxidation.The calculation results of Thermo-Calc software show that,with the decreasing of the liquid volume fraction,each element deviates from its equilibrium composition in the pure liquid phase,and the segregation of Mo element is the most serious.The as-cast microstructure analysis show that,developed dendrites have formed in the edge,R/2 and the center parts of the ingot,and that in the edge of ingot is the most fine.From the edge to the center of the ingot,the secondary dendrite spacing,the amount and the size of precipitates increases gradually.The precipitates whose types are Laves phase and sigma phase mainly distributed in the interdendritic and grain boundaries.According to the dendrite component analysis,the segregation coefficient of Mo element is the highest.The results of homogenization experiment show that,when homogenized at 1200? for 16h,obvious dendrite Microstructure is found in the center of ingot.While the segregation ratio is less than 1.2,the residual segregation index is less than 0.2,and all the precipitates dissolved into the matrix,which suggests that the composition of ingot has reached homogenization.When homogenized at 1250? for 8h and 1280? for 4h,both the dendritic microstructure and composition of ingot have reached homogenization,and all the precipitates have dissolved.Therefore,the optimum homogenization treatment is 1250? for 8h or 1280°? for 4h.