Study On Creep Property And Oxidation Behaviour Of 11Cr-5.5Co-2W-1Mo Martensitic Heat Resistant Steel

Based on our country's abundant coal resources,thermal power generation occupies a major position in power production.The efficiency of power generation can be effectively improved by improving the steam parameters of the generator set.9-12%Cr martensitic heat-resistant steel has the advantages of good thermal conductivity,low thermal expansion coefficient,and high cost performance,which is used as an alternative material for manufacturing steam pipes for ultra-supercritical generator sets.Increasing the content of Cr can effectively improve the oxidation resistance of heat-resistant steel.The generation of Z phase in traditional11% Cr heat-resistant steel during long-term creep or aging will significantly reduce its creep strength,in order to reduce the nucleation driving force of Z-phase,it is necessary to reduce the N content in steels,and to further increase its creep strength by adding other alloying elements in a reasonable proportion.In this subject,a new type of 11%Cr martensitic heat-resistant steel with low N contents was designed,and its creep properties and oxidation behavior were studied.The conclusions are as follows:(1)A kind of 11Cr-5.5Co-2W-1Mo martensitic heat-resistant steel was designed.The matrix structure of the test steels is tempered martensite after heat treatment,and the structure has a higher dislocation density.The martensite lath structure is obvious,and there are mainly two precipitated phases,M₂₃C₆ and Nb-rich MX phase.Besides,the Laves phase precipitates during the creep process and is mainly distributed on the original austenite grain boundary and the martensite lath boundary.(2)The creep curve of the test steel presents a typical three-stage characteristic.By extrapolating the traditional creep life evaluation method,the long-term creep life is obviously over-predicted.The new creep equation is used to predict the creep endurance strength at 650 ?/105 h to be 66.2 MPa,and the creep performance is better than P92 steel,which is currently widely used.The actual allowable temperature of the test steel is slightly higher than 625 ?.(3)All the samples under the creep test conditions are ductile fractures.The Laves phase and M₂₃C₆ phase have different degrees of coarsening during the creep process.No obvious coarsening of the MX phase or transformation into the Z phase is found.In the higher temperature range,the martensite lath structure is significantly degraded,and the microhardness value of the sample is significantly reduced.The change of the stress index n in the new creep equation in different stress ratio zones is mainly related to the coarsening of the Laves phase and the degradation of the martensite lath structure during the long-term creep process.(4)The test steel exhibits excellent oxidation resistance for a long time in the air and 20%water vapor environment at 650 ?;in the 600-hour oxidation test at 600-700 ? and 20% water vapor environment,the oxidation resistance varies with temperature The rise first strengthens and then weakens,and a protective Cr₂O₃ film can be formed at 650-700 ?.