THE INFLUENCE OF SOLIDIFICATION MODE ON HEAT-AFFECTED ZONE MICROFISSURING IN A NICKEL IRON BASE SUPERALLOY

The heat affected zone (HAZ) microfissuring of thermomechanically processed Incoloy 903 with a duplex grain structure, has been examined with a view to understand the mechanism(s) of, and to reduce the incidence of microfissuring. Extensive formation of liquid films on the HAZ grain boundaries primarily due to the complete constitutional liquation of preexisting MNP phosphides, MC carbides and partial constitutional liquation of primary MX carbide insolubles was observed. The liquid films resulted in considerable microfissuring on the long warm worked grain boundaries and extensive grain boundary liquid film migration (LFM) on the fine recrystallized grain boundaries. The liquid films formed on the warm worked boundaries were observed to have undergone normal solidification accompanied with the formation of dendritic gamma and terminal interdendritic constituents. The exclusive occurrence of LFM only on the fine grains and not on warm worked grains is attributed to the increased driving force and velocity for LFM arising due to substantial interface curvature of the fine grains. It is shown that HAZ microfissuring is minimized if the liquid films on the HAZ grain boundaries can partially equilibrate by LFM instead of by normal solidification.