THE EFFECT OF INITIAL MICROSTRUCTURE ON HEAT-AFFECTED ZONE MICROFISSURING IN INCOLOY-903

Studies were undertaken on electron beam welded thermomechanically pro cessed Incoloy 903 with varying proportions of duplex grain size distr ibutions, to determine quantitatively the effect of variation in the v olume fraction of fine grains to warm worked grains on heat-affected z one (HAZ) microfissuring. Extensive microfissuring was observed in sam ples consisting primarily of large warm worked grains, which, however, could be minimized by increasing the proportion of the fine recrystal lized grains outlining the large warm worked grains. Microfissuring wa s also observed to be reduced by decreasing the welding speed for any given value of electron beam current. A solidification structure compo sed of gamma/carbide eutectic reaction was observed on the boundaries that developed microfissures. The fine grain boundaries that were devo id of microfissures exhibited liquid film migration and formation of n iobium-enriched gamma phase and were devoid of gamma/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 bounda ries. The existence of substantial mean grain boundary curvature on th e fine grains is thus the major factor responsible for the reduced mic rofissuring in Incoloy 903.