The effect of initial microstructure on heat-affected zone microfissuring in Incoloy 903

Studies were undertaken on electron beam welded thermomechanically processed Incology 903 with varying proportions of duplex grain size distributions, to determine quantitatively the effect of variation in the volume fraction of fine grains to warm worked grains on heat-affected zone (HAZ) microfissuring. Extensive microfissuring was observed in samples consisting primarily of large warm worked grains, which, however, could be minimized by increasing the proportion of the fine recrystallized grains outlining the large warm worked grains. Microfissuring was also observed to be reduced by decreasing the welding speed for any given value of electron beam current. A solidification structure composed of γ/carbide eutectic reaction was observed on the boundaries that developed microfissures. The fine grain boundaries that were devoid of microfissures exhibited liquid film migration and formation of niobium-enriched γ phase and were devoid of γ/carbide eutectic constituents. It is suggested that one of the major causes of minimal microfissuring on fine grains in Incoloy 903 is due to the occurrence of liquid film migration towards curvature on some of the grain boundaries. The existence of substantial mean grain boundary curvature on the fine grains is thus the major factor responsible for the reduced microfissuring in Incoloy 903.