The present dissertation repofts the findings of a study on liquation cracking of cast IN 738LC superalloy welds. Microstructures of the base alloy, heat affected zone (HAZ) and fusion zone of welded specimens and Gleeble simulated HAZs were examined and characterized by the use of standard metallographic techniques involving optical microscopy, electron microprobe analysis, analytical scanning electron microscopy and analytical transmission electron microscopy. The hot ductility of the alloy in solution heat treated and overaged heat treated conditions was evaluated by the use of Gleeble thermo-mechanical simulation system.

Contrary to the generally accepted view that solidification in IN 738LC is completed by γ-γ' eutectic formation, terminal solidifìcation reaction products consisting of M₃B₂, Ni-Zr and Ni-Ti based intermetallic conlpounds were observed in fiont of y-γ' eutectic within interdendritic regions of the as-cast alloy, which was observed to be responsible for the incipient melting during high temperature heat treatment of the alloy. Likewise, analytical transmission electron microscopy revealed the occurrence of M₃B₂ and Ni₇Zr₂ ahead of interdendritic y-γ' eutectic in the fusion zone of welded specimens, consideration of which, are suggested to be pertinent to the development of optirnum post weld heat treatment for the alloy. A pseudo-ternary solidification diagram based on Ni-Ti-C system is adapted for IN 738 alloy, which adequately explains the as-solidifred microstructures observed in the as-cast alloy as well as in the weld-metal zone.

Careful analytical electron microscopy examination of welded materials revealed formation of resolidified constituents along microfissured grain boundaries in the HAZs thereby indicating that HAZ cracking in this alloy involves liquation cracking. Constitutional liquation of secondary solidifîcation constituents (MC carbides, M₂SC sulphocarbide and M₃B₂ borides, γ-γ' eutectic) and γ' precipitate particles, which were present in tlie pre-weld specimens in considerable volume fraction, were found to be the vital cause of grain boundary liquation and the resultant intergranular microfissuring in the HAZs. Constitutional liquation of γ' precipitate particles, which is the major strengthening phase of this alloy and in most other precipitation hardened nickel-based superalloys, was observed and reported for the first time in the present work. Liquation of these phases, as well as characteristics of the intergranular liquid film contributing to the alloγ's low resistance to HAZ cracking were discussed. Contrary to the generally accepted view that primary γ' precipitates undergo complete solid state dissolution in the HAZ regions that experienced peak temperatures above γ' solvus temperature, it was found that the particles constitutionally liquated above this temperature in IN 738. increased the volume of intergranular liquid film, and restricted the occurrence of grain boundary liquid film migration. These factors consequently, resulted in either microfissuring and/or formation of re-solidified eutectic constituents along such liquated grain boundaries, which could be the potential sites for crack initiation and/or propagation during subsequent post weld heat treatment. Gleeble hot ductility test results confirmed the effect of grain boundary liquation in damaging the on-cooling ductility of the alloy, and thus increasing the alloγ's propensity to microfissuring under induced welding stresses.

Appropriate consideration of the different types of Iiquation phenomena discussed in this disseftation in conjunction with the effects of rapid γ' precipitation kinetics is indispensable towards development of appropriate pre-weld and post weld heat treatments to minimize HAZ microfissuring in IN 738 weldments.