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

The heat affected zone (HAZ) microfissuring of thermomechanically proc essed Incoloy 903 with a duplex grain structure, has been examined wit h a view to understand the mechanism(s) of, and to reduce the incidenc e of microfissuring. Extensive formation of liquid films on the HAZ gr ain 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 fi lms resulted in considerable microfissuring on the long warm worked gr ain 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 t erminal interdendritic constituents. The exclusive occurrence of LFM o nly on the fine grains and not on warm worked grains is attributed to the increased driving force and velocity for LFM arising due to substa ntial interface curvature of the fine grains. It is shown that HAZ mic rofissuring is minimized if the liquid films on the HAZ grain boundari es can partially equilibrate by LFM instead of by normal solidificatio n.