Research On The Microstructure And Property Of Electron Beam Welded IN738LC Nickel Based Superalloy By Presetting Interlayer

As a precipitation hardened nickel-based superalloy,IN738LC superalloy is particularly used in the hot section components of aerospace engines,which is ascribed to its high-temperature strength coupled with excellent corrosion resistance.Since the IN738LC welded joints cannot be directly applied to hot section components,post-weld heat treatment?PWHT?is required.However,IN738LC joint is highly susceptible to strain-age cracking?SAC?during PWHT.In this paper,the formation mechanism of SAC in electron beam welded IN738LC alloy during PWHT was studied.The method of adding interlayer was adopted to control SAC.Meanwhile,the microstructure and mechanical properties of the joints were investigated.Through the direct welding of IN738LC alloy,the weld is composed of rod-like and Chinese-script like MC carbides,nano-scaled ?'-Ni₃?Al,Ti?and ?-?' eutectics.The micro-hardness of the weld is high while the ductility is poor.The extent of dissolution of ?' particles in different locations of the HAZ is distinctly different because of the welding thermal cycle.The weld and the HAZ are in high stress after welding.During the subsequent solution heat treatment of the joint,fine ?' particles precipitate near the grain boundary?GB?of the weld,causing the aging shrinkage stress and intragranular secondary hardening.Cracks occur when the welding residual stress and aging shrinkage stress superimpose on the GB of the weld.In addition,the brittle oxides formed near the GB accelerate the growth of the cracks.Based on the analysis of the welding residual stress and microstructure of PWHT joints,this article proposed a method of presetting nickel-based alloy interlayer with low?Al + Ti?content to control the SAC.When four kinds of interlayers with low?Al + Ti?content are preset for welding,the micro-hardness and welding residual stress of the FZ decrease and the ductility increases.There is almost no precipitation of ?' participates near the GB at the stage of heating of PWHT.Therefore,the level of aging shrinkage stress is low and there is intragranular secondary hardening,which greatly reduces the cracking susceptibility of the weld and the crack-free PWHT joint is obtained.After solution treatment and aging treatment,MC carbides in the FZ are partially dissolved and transformed into particles,M₂₃C₆ carbides are formed at GB and ?' particles are precipitated from the matrix and grow up.The tensile strength at room temperature,the tensile strength at high temperature and the creep life of the PWHTed joints basically increase with the increase of the?Al+Ti?content of interlayer.When using GH738 interlayer,the volume fraction of ?' particles of the FZ is the highest.The M₂₃C₆ carbides at the GB are fine-grained and continuously distributed.The tensile strength of the joint at room temperature is 985 MPa,which is equal to the base metal.The creep life of the joint at the condition of 900oC/350 MPa reaches 2.04 h.