Effect Of Rare Earth And Silicon On High Temperature Oxidation Behavior Of Super Austenitic Stainless Steel 654SMO

Super austenitic stainless steel possesses significantly higher alloy content(Cr,Ni,Mo,N,etc.)than traditional stainless steel.It exhibits excellent corrosion resistance and good comprehensive mechanical properties in extremely harsh service environments,such as petrochemical industry,energy conservation and environmental protection,marine engineering and other fields.It is an ideal substitute material for nickel-based alloy and titanium-based alloy.However,the molybdenum content of super austenitic stainless steel is very high.During high temperature homogenization,hot working and heat treatment,the synergistic effect of MoO3 volatilization and Cr2N precipitation induced by air nitriding easily lead to catastrophic oxidation,greatly increasing slab grinding loss rate and seriously affecting the surface quality and corrosion resistance of hot-rolled and cold-rolled plates.Adding a small amount of rare earth or appropriately increasing silicon content is an efficient,reasonable and low-cost method to improve high temperature oxidation resistance,but it has not been applied in super austenitic stainless steel at present,and the related mechanism is still unclear.Therefore,it is of great significance to systematically study the effects of rare earth and silicon on the high temperature oxidation behavior of super austenitic stainless steel.In this paper,super austenitic stainless steel 654SMO with different rare earth elements and different silicon contents smelted in 25 kg induction furmace is taken as the research object,and the influence law and mechahism of rare earth and silicon on high temperature oxidation behavior at 1200℃ are explored,respectively.The main conclusions are as follows:The addition of rare earth reduces the oxide layer thickness,oxidation weight loss of 654SMO,inhibits the formation and volatilization of MoO3 and the precipitation of lamellar Cr2N,enhances the compactness and adhesion of the oxide layer,thus improving high temperature oxidation resistance.The ability to improve high temperature oxidation resistance of the three rare earth elements is Y>La>Ce.This is due to the segregation ability at grain boundaries of the three:Y>La>Ce,Y is most likely to segregate at grain boundaries,inhibiting Mo segregation at grain boundaries,hindering the rapid diffusion of Mo outward along grain boundaries,reducing the formation and volatilization of MoO3,enhancing the compactness and protection of the oxide layer,and thus inhibiting the precipitation of Cr2N induced by air nitriding.Meanwhile,Y has the strongest affinity with oxygen and it is most easily oxidized preferentially,and Y2O3 has the smallest volume,which promotes the nucleation of Cr oxide,refines oxide particles and enhances the compactness and adhesion of the oxide layer.In addition,Y is easy to segregate at the oxide grain boundary,which hinders the diffusion of Cr3+ and slows down the oxidation rate,further enhancing the compactness and adhesion of the oxide layer.Under the above effects,Y promotes the formation of dense Cr2O3 layer and blocks the synergistic effect of MoO3 volatilization and air nitriding,significantly improving high temperature oxidation resistance.However,in actual production,too high rare earth content is easy to block the nozzle,which seriously restricts the smooth smelting.To sum up,the reasonable Y content in this steel is 0.005%-0.01%.As the silicon content increased,the oxide layer thickness and oxidation weight loss of 654SMO gradually decrease,and the MoO3content in the oxide layer gradually decreases.Meanwhile,the lamellar Cr2N in the substrate almost disappears,the compactness and adhesion of the oxide layer gradually increase,and the high temperature oxidation resistance is significantly improved.This is because silicon has the strongest affinity with oxygen and it is easy to preferentially oxidize to form a dense SiO2 layer,hindering element diffusion,inhibiting MoO3 generation,reducing the damage of MoO3 volatilization to the oxide layer,enhancing the compactness and protection of the oxide layer,and further inhibiting Cr2N precipitation induced by air nitriding.Additionally,some preferentially formed SiO2 particles are scattered in oxide layer and provide nucleation sites for Cr oxide,refining oxide particles and further enhancing the compactness and adhesion of the oxide layer.The PBR value of Cr2O3 is higher,which is easy to crack during its growth process and reduces the protection of the oxide layer.Raising silicon content increases the SiO2 content in the oxide layer and reduces the growth stress and cracking tendency of the oxide layer,futher enhancing the protection of the oxide layer.Therefore,increasing silicon content promotes the formation of dense SiO2 and Cr2O3 protective layers and blocks the synergistic effect of MoO3 volatilization and lamellar Cr2N precipitation induced by air nitriding,thereby significantly improving high temperature oxidation resistance.However,too high silicon content will promote the precipitation of harmful intermetallic compounds,which will lead to easy cracking during hot working,increase the burden of heat treatment and reduce the comprehensive properties of steel.To sum up,the reasonable Si content in this steel ranges from 0.7%to 1.0%.