| During the service of Super304H steel in Ultra Supercritical Units in China,there are few cases and data on the study of the microstructure and high temperature resistance of Super304H steel after high temperature aging.However,the parts of Super304H steel tube used in Ultra Supercritical Unit boilers,such as high temperature superheater and high temperature reheater,should be replaced periodically according to the evolution of the microstructure and properties of the materials.Because Super304H steel contains a lot of alloy elements,the composition of precipitates and their interaction are also complex.Therefore,the research of Super304H steel should focus on the interaction of precipitates after high temperature aging and the evaluation criteria of structure and properties.In the present paper,comprehensive characterization of the Super304H was performed by using optical microscopy,scanning electron microscopy,transmission microscopy,X-ray diffractometer,room-temperature test,high temperature creep test and electrochemical test.The main focus of this study is the Super304H steel combine micro mechanical properties and microstructure analysis,to analyze the performance impact of microstructure and change rule under the condition of limitation of the regional metallographic structure,further investigation of macro mechanical properties and the relationship between the precipitation.The effect of precipitation on the corrosion resistance of the alloy under high temperature ageing was clarified by observing the evolution of microstructures and intergranular corrosion.The metallographic structure of Super304H steel is gamma-phase and precipitated phase,which mainly consists of copper-rich phase and Nb(C,N).When the temperature is less than 1100℃,,the growth rate of gamma phase grains is slow.When the temperature is higher than 1100℃,the grains grow rapidly.The precipitated phases are mainly copper-rich phase,NbCrN,Nb(C,N)and M23C6 during high temperature aging.Super304H steel has strong resistance to intergranular corrosion.The susceptibility to intergranular corrosion increases with aging at high temperature.This change is closely related to the characteristics of precipitated phase M23C6(including shape,arrangement and distribution,precipitation number,etc.).At the same time,the susceptibility to intergranular corrosion varies at different temperatures.The precipitation of M23C6 at grain boundaries and in grains during high temperature aging will result in chromium depletion in the surrounding region,which further aggravates intergranular corrosion.The hardness of Super304H steel reaches its maximum at 750℃,and then decreases with the increase of temperature.In the high temperature aging experiment,the impact energy decreases with the increase of temperature in the initial stage and reaches the lowest value at 650℃.Then,with the increase of aging temperature,the impact energy increases gradually and reaches the peak at 850℃.The welding seam of dissimilar steel butted with Super304H and T91/92 needs stress relief annealing.The treatment can reduce the depth and hardness of shot peening layer,and then affect the steam oxidation resistance of Super304H steel.Local heat treatment is recommended. |
PDF Full Text Request