Study On Precipitation Behavior Of Strengthening Phase And Microstructure Thermal Stability Of Austenitic ODS Steel

Oxide dispersion strengthened(ODS)steel has excellent high-temperature performance and corrosion resistance.It has broad application prospect and development space in the key fields such as boiler steel for thermal power ultra supercritical(USC)units,high temperature structural materials for nuclear power,hot end parts of aeroengine.In this paper,austenitic ODS steel based on Super304H stainless steel composition was designed and prepared by powder metallurgy preparation process.The as-HIPed microstructure of austenite ODS steel after HIP process was studied and analyzed,including matrix grain distribution characteristics,micron-scale large size precipitated phase,nanoscale oxide particles,and the strength mechanical properties of as-HIPed alloy were measured.The strengthening mechanism of the austenitic ODS steel was obtained from the point of view of microstructure.In addition,the high temperature microstructure thermal stability of austenitic ODS steel after HIP was studied,and the microstructure change of matrix and hardness properties were analyzed.Based on the contrast analysis of the matrix microstructure and hardness properties,the hardness change of this austenitic ODS steel at high temperature was obtained.The research results showed that the original as-HIPed microstructure of austenitic ODS steel has bimodal grain characteristics.There were a large number of nano-particles Y₂O₃which had semi-coherent relationship with the matrix in the grain,and the addition of Cu element promoted the formation of Cu-rich phase.As the nucleation center of Y₂O₃,in which the existence of Cu-rich phases were beneficial to the precipitation of Y₂O₃,and presented a gathering structure that the Y₂O₃phase was wrapped around the Cu-rich phase.The mechanical properties of RT and 650?showed that the strength of this austenitic ODS steel was greatly improved when compared with commercial Super304H stainless steel,and the ultrafine grain structure and dispersed oxides were the most important reasons for the strengthening of austenite ODS steel.The results of thermal stability study showed that the matrix grains were coarsened during aging,and the M₂₃C₆precipitated along the grain boundary,and the number of precipitation increases with the increase of aging time.At the same time,the white Nb(C,N)phase is precipitated in the crystal.Aging treatment weakened the coherent relationship between oxide and matrix,so that the interface energy of oxide can be increased,and then coarsening and growing.Meanwhile,the coating structure of Y₂O₃wrapped around the Cu-rich phase gradually disappeared,and a large number of small Cu-rich phases were precipitated in matrix.The change of microhardness during aging was related to the grain size and the number and density of the second phase precipitation.The hardness of micrometer-sized grains(MGs)region did not change much,but the overall hardness of ultrafine grains(UFGs)zone increased with aging time.