Mechanical Properties Of Ni 690 Strip Weld Metal

Alloy 690 has been widely used in nuclear power equipment for its excellent high temperature performances,and excellent resistance to stress corrosion cracking?SCC?and pitting corrosion.Therefore,it is important to study mechanical properties of Ni690 weld metal.Commercial Ni690welding strip was deposited onto a low alloy steel plate using the strip cladding process.Elevated temperature tensile tests?500¹000??were carried out on a Zwick/Roell Z050 mechanical testing machine.Evolution of precipitates at grain boundary?GB?and their effects on mechanical properties of Ni 690 strip weld metal at elevated temperature were studied.The post-weld heat treatment?700?×20h,700?×50h?was carried out on the weld-metal.The effect of post-weld heat treatment on the microstructure and mechanical property at room temperature were also studied.The precipitates at GBs were identified as M₂₃C₆ by energy dispersive spectroscopy?EDS?and X-ray diffraction?XRD?,and the microstructure evolution and fracture surfaces were also investigated using scanning electron microscope?SEM?.The morphologies of M₂₃C₆ carbides distributed at the GBs changed with different temperatures,including continuous carbides below 700?,lamellar carbides at 700?,and discrete carbides above 700?.It was found that the stress-strain curves at 700?and below had an apparent long strain harden stage after yielding due to dislocation strengthening mechanism and the fracture mode was dimple fracture.The dimples initiated from grain interior.Higher temperature above 700?,grain boundary slip at grain boundaries controlled the plastic deformation and fracture mode was inter-granular fracture.Cracks initiated from dispersed carbides at GBs.The discrete M₂₃C₆ at GBs accelerated the detachment of the interface between the matrix and precipitates,leading to ductility dip at high temperature.The solid solution?SS?treatment was carried out on the weld-metal.The effect of GB M₂₃C₆ carbides on mechanical property was studied.At500-700?,the strength of the SS specimen is better than that of the weld-metal.It was due to the amount of GB M₂₃C₆ carbides in weld-metal is more than that in SS specimen,the precipitation of carbides consumed a lot of Cr.The loss of Cr softened the matrix,which caused the deterioration in yield strength.Above 800?,the difference between the yield strength and the elongation was not obvious,because most GB M₂₃C₆ carbides had been dissolved into the matrix.Heat treatment at 700? with different time was carried out.A lot of second phases were observed by SEM.Besides M₂₃C₆ carbides were observed at GBs,needle-shaped M₂₃C₆ carbides were also precipitated in the matrix.It can be concluded that the heat treatment increased the yield strength of the weld-metal.Because needle-like M₂₃C₆ carbides were precipitated in the matrix,which made the dislocation difficult to move,easy to gather at GBs,and caused stress concentration.The fracture mode changed from dimple fracture to the mixed mode of ductile fracture and inter-granular fracture.