Study On Carbide Transformation Mechanism Of Hastelloy N Alloy After Post-weld Heat Treatment

Hastelloy N,a solid-solution strengthened nickel-based superalloy,which possesses superior strength,excellent oxidation resistance and corrosion resistance,is the candidate structural material for molten salt reactors.When the temperature exceeds 1300 ?,the primary carbides in the Hastelloy N alloy will be transformed into the eutectic carbides,and the welding temperature ranged from room temperature to the melting point(above 1450 ?)contains the temperature point of carbide eutectic transformation,so the eutectic carbides exist in both the heat affected zone(HAZ)and the weld zone(WZ).In addition,the welded joints become the weak areas because of the element segregation,microstructure inhomogeneity and the welding residual stress The evolution behavior of carbides during welding and post-weld heat treatment(PWHT)of Hastelloy N alloy,the effects of PWHT on the micro-hardness,tensile properties,stress rupture properties and residual stress of the welded joints were studied,and the changes of microstructure and mechanical properties of PWHT were obtained Thus,the carbide transformation process and mechanism of PWHT and relationship between microstructure evolution and mechanical properties change were revealed.The main conclusions are as followsFirst,the effects of PWHT temperature on the microstructure and mechanical properties of Hastelloy N alloy welds were systematically investigated.The microstructure of the as-welded sample shows that some eutectic-like lamellar M6C carbides and nano-sized granular M2C carbides appeared in the interdendritic regions,whereas some finer M6C carbides were precipitated at the dendritic boundaries.The nano-sized granular M2C carbides in the interdendritic regions were dissolved during the PWHT.When the PWHT temperature reached 950?,these nano-sized granular M2C carbides completely disappeared.With the increase of PWHT temperature,the micro-hardness of the welds decreased remarkably,the yield strength decreased obviously at first and then decreased slowly,the tensile strength increased slightly,while the elongation gradually increased.The elongation of the weld after PWHT at 870? and 950? became significantly higher than that of the as-welded sample,which were 28.0%and 34.0%,with an increase of 34.0%and 62.7%,respectively.The micro-hardness and yield strength decrease because of the dissolution of the nano-sized granular M2C carbides and the annihilation of the dissolution during the PWHT,whereas the tensile strength and elongation increase because the size and quantity of the M6C carbides precipitated at the dendritic boundaries increase during the PWHT With the increase of PWHT temperature,the stress rupture life and elongation of the weld both increase slowly at first and then increase rapidly.The stress rupture life of the weld after PWHT at 870? and 950? was 1674.2h(5.7 times of the as-welded)and 3347.7h(11.5 times of the as-welded),respectively.The dendritic boundary carbides grow up obviously under the stress during the stress rupture test,which further hinders the grain boundary slip,enhances the stress rupture property and increase the elongationSecond,in order to study the effects of PWHT time on the microstructure evolution and mechanical properties of the welded joint of Hastelloy N alloy,the PWHT temperature was fixed on 870°C.The boundary carbides in the different zones of the welded joint increased with the PWHT time.After the PWHT at 870? for 0.5h,the yield strength of the welded joint decreased by 43MPa(the as-welded 272 MPa)while the elongation increased by 69.6%(the as-welded 24.0%).The welding residual stress is concentrated in the dendritic boundaries and subgrain boundaries within dendrites due to welding thermal cycle,which makes it easy to become the crack source.In the process of tensile test,cracks expand along these boundaries,which causes the welded joint to be fractured in the WZ.The welding residual stress of the WZ decreases after PWHT at 870?,and the microstructure of the WZ shows some dendrites.Thus,the base metal(BM)with lower yield strength first enters the yielding and plastic stage,and the fracture occurs in the BM.The stress rupture life and elongation of the welded joint both increased obviously after the PWHT at 870? for 0.5h,increased first and then decreased with the increase of the PWHT time.The improvement of the stress rupture properties of the welded joint after PWHT at 870? is due to the decrease of local stress and strain,the precipitation of the boundary carbide which can pin dislocations and hinder grain boundary slipThe eutectic reaction occurs in the WZ of Hastelloy N alloy because of the element segregation between dendrites.Eutectic reaction process of the carbide in the WZ is written as:?+M6C(P)?L??y+M6C(E).Due to the welding heat cycle,constitutional liquation occurs at the primary M6C carbide-matrix interface in the eutectic zone of HAZ(HAZ(E)),eutectic carbides are formed by local liquid phase eutectic reaction Eutectic reaction process of the carbide in HAZ(E)can be written as?+M6C(P)??+L??+M6C(E).The spheroidization process of the eutectic carbides after high temperature heat treatment indicates that,the high interfacial energy promotes the spheroidization and obtains a stable system.Besides,the decrease of volume energy promotes the inward diffusion of Si into globular M6C carbides during the high temperature heat treatment.The total concentration of Si and Cr in the M6C carbides during welding and the high temperature heat treatment remains stable.The outward diffusion of Mo from the M6C carbides leads to the decrease of the lattice parameter of the carbides in the different zones of the welded joints during welding and the high temperature heat treatment.Based on the first principle calculation,this is due to the small atomic radius of Si,Ni,Cr atoms,compared to Mo atom,which leads to the reduction of M6C carbide lattice parameters.