SOLIDIFICATION AND WELDABILITY OF NB-BEARING SUPERALLOYS

The solidification and weldability of experimental superalloys contain ing systematic variations in Fe, Nb, Si and C were studied using diffe rential thermal analysis (DTA), microstructural characterization techn iques and Varestraint testing. Microstructural evolution during solidi fication of DTA samples and gas tungsten are welds generally occurred by a three-step solidification process. Solidification initialed by a primary L --> gamma reaction, which enriches the interdendritic liquid in Nb and C until a eutectic-type L --> (gamma + NbC) reaction occurs over a rather broad temperature range. Solidification terminated by a second eutectic-type reaction, L --> (gamma + Lave), which occurred o ver a narrow temperature range. Carbon additions increased the start t emperature of the L --> (gamma + NbC) reaction and induced a concomita nt decrease in the temperature interval of the primary L --> gamma sta ge of solidification. As a result, carbon additions often promoted a s ignificant improvement in solidification cracking resistance. The resu lts from solidification studies are combined with detailed examination s of microstructural morphologies in fusion zone solidification cracks to propose a microstructure morphology classification scheme. The cla ssification scheme provides a phenomenological explanation of the rela tion between alloy composition, solidification temperature range, micr ostructural morphology and cracking propensity. This interpretation al so leads to an improved correlation between the solidification tempera ture range and fusion zone cracking susceptibility.