Research Of The Thermal Stability And Behavior Of Oxidation Of K282 Alloy

K282 alloy is a new cast Ni-based superalloy for large castings of 700? advanced ultra-supercritical coal-fired power plants unit developed based Haynes282 alloy.Large-scale castings for power stations should be used for a long time at high temperatures,therefore the excellent thermal stability is the guarantee of long-term stable operation for castings.In this study,The thermal stability of K282 alloy at 700?,750?,800? and 850? during long-term aging with the corresponding tensile properties at 750? and the behavior of oxidation at 700?,800? and 900? were studied.There are spherical ?' precipitates,strip-like MC carbides and flower-like M23C6 carbides in the alloy after standard heat treatment.The shape of phase is spherical during the process of aging at 700 and 750? while evolves into square at 800 and 850?.The average size of y phase increase with the increasing aging time,which obeys the LSW theory.The growth and coarsening rate of ?' precipitates increase obviously above 800? compared with 700 and 750?.Meanwhile,the size discreteness also increases.A large number of ? phase were found near MC carbides aged for 3000 hours at 700? and 1000 hours at 800? and 850?.After aged 3000h at at 800? and 850?,the number of ? phase decreases,but coarsening occurs Degeneration of primary MC carbide occurred aged at 800? and 850? for 3000h,and the degeneration reaction was MC+??M6C+M23C6+?.The stability of the microstructure decreased with the increase of temperature,especially above 800?.Compared with after heat treatment,the yield strength of the alloy increases after aged at 700and 750?,while the yield strength decreases significantly after aged at 800?.The elongation at 700? and 750? decreased firstly and then stabilized,while decreased firstly and then increased at 800?.During long-term aging,the alloy at 700? and 750? can keep a higher yield strength and the elongation tends to be stable,which show the good mechanical properties.The change trend of mechanical properties corresponds to the structural stability,and the high stability corresponds to high mechanical properties.Higher structural stability,higher mechanical properties.The tensile fracture mode for all alloy is mainly transgranular cluctile fracture.And the oxidation kinetic curves of K282 obey the parabolic law at 800 and 900?.At 700?,there is only the external oxide scale composed of Cr2O.Different from 700?,the internal oxidation of the alloy occurred forming internal oxide Al2O3 at 800? and 900?,and the external oxide scale are composed of Cr2O3 and TiO2.Gain weight rate is depended on the diffusion speed of metal ion and oxygen ion.The diffusion speed increases with the increasing of temperature,causing a faster gain weight rate.