| The present dissertation reports 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 gamma-gamma' eutectic formation, terminal solidification reaction products consisting of M3B 2, Ni-Zr and Ni-Ti based intermetallic compounds were observed in front of gamma-gamma' 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 M3B 2 and Ni7Zr2 ahead of interdendritic gamma-gamma ' eutectic in the fusion zone of welded specimens, consideration of which, are suggested to be pertinent to the development of optimum 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-solidified 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 solidification constituents (MC carbides, M2SC sulphocarbide and M3B 2 borides, gamma-gamma' eutectic) and gamma ' precipitate particles, which were present in the 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 gamma' 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 alloy's low resistance to HAZ cracking were discussed. Contrary to the generally accepted view that primary gamma' precipitates undergo complete solid state dissolution in the HAZ regions that experienced peak temperatures above gamma' 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. (Abstract shortened by UMI.)... |
