Intergranular liquation cracking in heat affected zone of a welded nickel based superalloy in as cast condition

The microstructures of the as cast and the heat affected zone (HAZ) of tungsten inert gas welded IN738 superalloy were studied. A different secondary solidification microconstituent was observed in front of some of gamma-gamma' eutectic in interdendritic regions of the as cast alloy, which is likely formed by an invariant eutectic type reaction during the terminal stage of the ingot solidification. On welding, the alloy in as cast condition, suffered extensive grain boundary liquation cracking in the HAZ regions while the fusion zone was free of exclusive centerline cracking. Subsolidus liquation reactions of terminal solidification microconstituent, MC type carbides and gamma' precipitate particles present in the preweld alloy in considerable volume fraction, were observed to be major causes of HAZ intergranular liquation and the attendant cracking. Good wetting behaviour of the grain boundary liquid film possibly aided by surface active boron atoms released by the liquation of the terminal solidification reaction product is suggested as an important factor contributing to high susceptibility of the alloy to HAZ cracking in the as cast condition.