Research On Microstructure Stability Of740H Alloy Used For A-USC Boilers

740H is a precipitation hardening alloy with Ni-Cr-Co matrix. It hashigh creep strength at elevated temperature. The γ’ phase is formed duringthe aging process by adding elements Al and Ti. In addition, traceelements Nb and B are added in the alloy.As an important candidate superheater material of A-USC boilers, tospeed up its use in boiler products, we must master various performancedata and microstructure evolution of this material under conditions ofservice. One of the key factors to ensure the material’s security of serviceis microstructure stability on long-term aging.In this paper, long-term aging test and creep rupture test at elevatedtemperature were carried out to investigate the mechanical property underlong-term service. Thermodynamic calculation, SEM, TEM and XRDwere used to study on the samples which were obtained from long-termaging test and creep test. On this basis, we analyzed740H’smicrostructure, precipitate rules of strengthening phases, and structurestability at elevated temperature. In addition, this paper researched theinfluence of stress on the precipitation’s growth and structure stability by comparing the samples obtained from long-term aging test and creep test.The research showed that under long-term aging at770℃,740H hasgood high temperature strength and mechanical properties in10000hours.The γ’ phase in the grain precipitates stably, and its growth corresponds tothe LSW rules. Besides, there are carbides precipitate on the grainboundary, and the precipitations in the grain are stable, so the alloy hasgood structure stability on long-term aging at service temperature. Allsamples are ruptured along the grain boundary in the creep test. Theprecipitation on the grain boundary is the main factor which causes thegrain boundary weakening.By comparing the samples of aging and creep test, it shows thatstress don’t affect the growth of γ’ phase obviously, but it affects theprecipitation of carbides on grain boundary obviously. The stress whichleads to sharp growth of precipitation on grain boundary is the mainfactor of final creep rupture.