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Byline: N.L. Richards, M.C. Chaturvedi, Y.G. Liu, K. Mount A further study to our previous work has been carried out in an attempt to understand the welding parameters affecting heat-affected zone micro-fissuring in Incoloy 903. Electron beam welding (EBW) parameters such as travel speed, beam current, focus current and beam oscillation were varied, while the accelerating voltage and working distance were kept constant. The six independent variables were formulated into a response surface statistical design in order to investigate the effect of these factors on the dependent variable. The results of the experimentation showed that the main factors affecting micro-fissuring were beam current, focus current, welding speed and oscillation across the welding direction. These factors were combined into a regression equation showing the effect of each independent variable on the micro-fissuring behaviour.

Byline: M.C. Chaturvedi, N.L. Richards Optical, SEM/EDX and TEM/EDX techniques have been used to characterize the parent, weld and heat affected zone (HAZ) microstructures in electron beam welded (EBW) Incoloy 903. The as-forged microstructure showed a duplex grain size of large warm-worked grains decorated by finer grains. MX carbides, fine MC carbides and MNP phosphides were observed on grain boundaries. The weld metal solidified in a cellular-dendritic mode: with segregation of Nb and Ti to interdendritic regions; no micro-fissures were observed originating in the weld metal. The HAZ, however, showed extensive micro-fissuring, especially on the long warm-worked grain boundaries. The micro-fissuring was attributed to constitutional liquation of the MX carbides (partial), fine MC and MNP phosphides on the grain boundaries. Micro-fissuring was, however, absent on the fine grain boundaries, though the boundaries exhibited a thickened appearance attributed to liquid film migration (LFM). The LFM was found to influence the solidification mode of the grain boundary liquid phase. Liquid solidifying via a non-LFM mode, resulted in a terminal solidification products of γ + MC carbide. These carbides were in extensive sheets along the grain boundary. Areas associated with LFM solidified to a Nb rich austenite and exhibited no evidence of micro-fissuring. The LFM resulted in a smaller solidification temperature range for the grain boundary liquid and resulted in a less sensitive microstructure for the weld cycle thermal stress to act on. LFM was also concluded to be the reason for the beneficial effect of fine grains on minimizing HAZ micro-fissuring.