Microstructural evolution and liquation cracking in the partially melted zone of deposited ERNiCrFe-13 filler metal subjected to TIG refusion

The microstructure of ERNiCrFe-13 multipass weld metal has been shown to contain Laves/γ or σ/γ eutectic constituents that can increase susceptibility to solidification and weld metal liquation cracking resulting from the low eutectic reaction temperature. Under poor heat dissipation conditions such as on the edge of large thickness welded components, a partially melted zone (PMZ) may form in the weld metal during multipass welding. The microstructural evolution and liquation cracking susceptibility of this PMZ in ERNiCrFe-13 multipass welds have received little attention. In the present study, a tungsten inert gas (TIG) refusion process is used to simulate a thermal cycle with a long elevated temperature dwell time in order to investigate the microstructural evolution and liquation cracking in the weld metal PMZ. The results show that the eutectic microstructures in the PMZ evolve into three eutectic morphologies after TIG refusion, including long linear chains extending perpendicular to the boundary between the refusion zone and PMZ, skeletal structures, and fine lamellar networks. This evolution contributes to constitutional liquation occurring at the γ/Laves and γ/σ interface. Nb and Mo play a leading role in the constitutional liquation of γ/Laves and γ/σ eutectic microstructures, respectively. Liquation cracking in the PMZ is shown to occur along the linear chain grain boundaries resulting from constitutional liquation.

Computer Prediction of Phase Fraction in Multipass Weld of Duplex Stainless Steel - Proposal of Microstructural Improvement Welding Process -

Computer simulation of the α/γ phase transformation in multipass weld of duplex stainless steel was made for predicting the distribution of the γ phase fraction in the weld metal (WM) and HAZ. The kinetic equations including rate constants of the dissolution behaviour as well as precipitation behaviour of γ phase were determined by isothermal heat treatment test. Based on the kinetic equations determined, the distribution of the γ phase fraction in multipass weld of duplex stainless steel was calculated applying the incremental method combined with the heat conduction analysis in welding process. The γ phase fraction was reduced in the higher temperature HAZ and WM, however, that in the reheated HAZ and WM was increased and recovered to the base metal level. Microstructural analysis revealed that the calculated results of the γ phase fraction in multipass weld were consistent with experimental ones. Based on the computer prediction, the microstructural improvement welding (“reheat bead welding”) process, with analogous concept to the temper bead welding technique, was newly proposed for recovering the γ phase fraction in weld even in the as-welded situation.