| The development of ultra-supercritical power generation technology is the basis of clean and efficient operation of units,and the development of ultra-supercritical technology at 630℃ and above is of great strategic significance for realizing the“double carbon” goal of energy saving and emission reduction.High nitrogen martensitic heat resistant steel is a new type of heat resistant alloy with nitride as the main strengthening phase.There is a lack of systematic research on the durability and oxidation resistance of alloys under different heat treatment processes.In this paper,a variety of heat treatment processes were developed for 9Cr alloys(N-0.25,N-0.27,N-0.30)with different nitrogen content,combined with J Mat Pro simulation phase diagram.The results show that with the increase of homogenization and normalizing temperature,the microstructure of the experimental steel is martensite and a small amount of δ-ferrite biphase structure.The presence of δ-ferrite will reduce the toughness of the alloy.In order to reduce the content of δ-ferrite,a heat treatment process of 1010℃ homogenization,1020℃ normalizing and 760℃tempering has been developed.The oxidation kinetics behavior and microstructure evolution of oxide films of three different nitrogen content test steels and 9CrMoCoB steel(CB2)at 650℃,750℃ and 850℃ were compared.The results show that,compared with CB2 steel,the oxide film formed by high-nitrogen martensitic heat-resistant steel has higher permeability and stronger adhesion,with fewer voids and gaps on the surface,mixed spinel phase on the outer layer and Cr-rich oxide layer on the inner.According to the distribution of Cr in the oxidation process,the formation mechanism of oxide layer gap in the test steel is discussed,and the reason of high nitrogen martensitic heat-resistant steel has good oxidation resistance is explained.The durability of the N-0.30 test steels under different heat treatment conditions was studied at 650℃ and 155 MPa.According to the results,the optimal heat treatment process was selected as 1050℃ homogenizing,1080℃ normalizing and760℃ tempering.Under this heat treatment process,the high nitrogen martensitic heat-resistant steel was tested at different temperatures(620℃,650℃ and 695℃)and170 MPa.It was found that the MN type precipitated phase still has strong thermal stability under high temperature and high stress,and the Laves phase is easy to precipitate and coarse-grained at high temperature. |
