Investigation On Welding Technology And Post Welding Heattreatment Of Laser Welding Of P92 Steel And Inconel 625 Alloy

Ultra-super-critical(USC)technology has made great progress in recent years due to its high thermal efficiency of power generation plants and low emission of exhaust gas.The technology is characterized by an extra high operating pressure and service temperature viz.the pressure greater than 25 MPa and temperature higher than 600 oC.Therefore,the high-temperature properties of materials used in this technology are critical to the service life of the USC power plant.Ferritic/martensitic steels have been widely used in USC technology to manufacture the structural components.In comparison,nickel-based Inconel 625 superalloy with a better high-temperature performance can be applied in a more aggressive condition.However,the failure,called Type IV cracking,often occurs at the heat affected zone of the welded 9Cr steel joint during a long term exposure in an elevated temperature condition,due to the high heat input when arc welding technology employed.Ultra narrow gap laser welding with filler wire technology has been developed in recent years,which is an efficient method applied to weld thick plate.Laser welding has obvious advantages in the technology because of low power and intensive energy density.The experiments of laser welding and ultra narrow gap laser welding with filler wire was carried out with thickness of 5mm and 25 mm P92 steel and Inconel 625 alloy in this paper,respectively.Microstructure and properties of welded joints were optimized through post weld heat treatment.In the laser welding process,the geometry of the joint in the transverse-cross section presents an outstanding narrow and deep nature of the laser welding technique.The dendritic morphology in the weld is in the form of equiaxed dendrites in the upper region of the bead and a transition from a mixture of equiaxed and short columnar dendrites to predominantly long columnar dendrites at the center of the fusion zone.The width of HAZ in 9Cr steel side is about 0.8 mm without softened zone,composed of coarse grain heat effected zone and fine grain heat effected zone.The final fracture took place in the segment of Inconel 625 that is away from the fusion zone.The impact toughness of weld is smaller than that of Inconel and larger than that of 9Cr steel.In the ultra narrow gap laser welding with filler wire,the weld thermal cycle generates a dendrite morphology in the fusion zone and several Laves precipitates are distributed along the interdendritic region.The width of HAZ in 9Cr steel side is about 1 mm without softened zone,composed of coarse grain heat effected zone and fine grain heat effected zone.The tensile test indicated that the dissimilar butt joints ruptured away from the fusion zone in the base metal of Inconel 625 under room temperature and ruptured in P92 steel under high temperature.The stress-rupture specimens are broken in P92 steels.The impact toughness of fusion zone is smaller than that of Inconel 625 and larger than that of P92 steel.In this work,dissimilar joints of P92 steel and Inconel 625 superalloy is fabricated by laser welding and ultra narrow gap laser welding with filler wire.The present work provides understanding of microstructural features in dissimilar ferritic/martensitic steel and nickel-based superalloy weld fabricated by advanced laser welding technology for the USC applications.