Study On The Oxidation Behavior Of 20Cr-25Ni-Nb Austenitic Stainless Steel And Methods For Its Improvement

Austenitic stainless steel(ASS)is considered an ideal candidate as a cladding material for the next generation of nuclear power systems due to its excellent combination of mechanical properties,oxidation resistance and creep resistance at high temperatures.Compared to Zr alloys,which are often used as cladding materials in Generation III reactors,ASSs have higher strength and better oxidation resistance at high temperatures.Additionally,ASSs have a relatively smaller neutron absorption cross-section.As a fuel cladding material for Advanced Gas Cooled Reactors(AGRs),20Cr-25Ni-Nb ASS has excellent mechanical properties,oxidation resistance,and structural stability in high-temperature CO2.AGR’s long operating experience shows that this steel has significant application in future nuclear power plants.Through a systematic study of the manufacturing,microstructure,mechanical properties,and oxidation resistance of 20Cr-25Ni-Nb,this study provides important data support and theoretical basis for the development of excellent heat-resistant ASS and the selection of materials for Generation Ⅳreactors in China.Based on the AGR clad material,20Cr-25Ni-Nb ASS was produced in this study by adjusting the alloy composition,i.e.,reducing the C and O contents and increasing the Nb and N contents,meanwhile satisfying the ’stability ratio’ between C,Nb and N.The microstructure,mechanical properties,and high-temperature oxidation resistance of the as-produced 20Cr-25Ni-Nb have been systematically investigated.In addition,the effects of Mo addition,surface mechanical abrasive treatment(SMAT),and laser shock peening(LSP)on the oxidation behavior of the steel in high-temperature CO2 were also investigated.The main conclusions are as following:(1)The 20Cr-25Ni-Nb as produced in this study has excellent tensile properties and structural stability.This steel has better tensile properties than 310S,the tensile strength,yield strength and elongation of which reached 608 MPa,334 MPa and 43%,respectively.After thermal aging,no harmful second phase precipitated in the matrix and the grain size was maintained at～30 μm.NbC particles in 20Cr-25Ni-Nb steel can inhibit M23C6 carbide precipitation and grain boundary migration during thermal aging.The dispersed distribution of NbC in the matrix can act as a strengthening phase,inhibiting dislocation movement and thus improving the mechanical properties of stainless steel.(2)The oxidation resistance of 20Cr-25Ni-Nb in high-temperature CO2 can be improved by the addition of Mo.Nb plays an important role in the oxidation process of 20Cr-25Ni-Nb steel.The dispersed distribution of NbC in the matrix can retard the outward diffusion of metallic elements such as Fe,and the small amount of Nb decomposed from NbC can promote the diffusion of Cr,all of which improve the oxidation resistance of steel at high temperatures.The addition of Mo can further improve the oxidation resistance of 20Cr-25Ni-Nb because the volatilization of Mo during oxidation can promote the outward diffusion of Nb,leading to the formation of a continuous Nb-rich layer at the oxide/matrix interface,which can effectively hinder the outward diffusion of Fe and prevent the formation of the Fe-rich outer oxide layer.(3)SMAT improves the oxidation resistance of 20Cr-25Ni-Nb in hightemperature air,but causes carburization and deteriorates the oxidation resistance of it in high-temperature CO2.The weight gain of the SMAT samples in hightemperature air was～3 5%lower than that of the untreated samples,showing better oxidation resistance,which was due to the SMAT-induced nanograin accelerating Cr diffusion and thus promoting the formation of dense Cr-rich oxide layers.However,the surface micro-cracks induced by SMAT accelerate CO2 penetration,which,together with the high C diffusion rate caused by nano-grains,leads to the formation of large amounts of carbides(Cr23C6).Carbide formation not only reduces the Cr content in the matrix,but also promotes oxide spallation,which deteriotates the oxidation resistance of 20Cr-25Ni-Nb in high-temperature CO2.(4)LSP improves the oxidation resistance of 20Cr-25Ni-Nb in both hightemperature air and high-temperature CO2.In both the high temperature air and the high temperature CO2,the weight gain of the LSP samples was less than that of the untreated samples by more than 60%.Due to the ultra high strain rate,LSP tends to produce high density dislocations rather than significantly refine the grains in the stainless steel.The high density of dislocations promotes Cr diffusion in the matrix to some extent,facilitating the formation of dense Cr-rich oxide scales and thus improving the oxidation resistance of stainless steel in air.Due to the relatively small plastic strain,LSP does not produce defects such as cracks on the surface,and does not significantly accelerate the carburization process.Therefore,20Cr-25NiNb stainless steel after LSP process shows excellent oxidation resistance in hightemperature CO2.