Microstructural responses of a nickel-base cast IN-738 superalloy to a variety of pre-weld heat-treatments
| dc.contributor.author | Thakur, Anurag | en_US |
| dc.date.accessioned | 2007-05-15T15:21:44Z | |
| dc.date.available | 2007-05-15T15:21:44Z | |
| dc.date.issued | 1997-09-01T00:00:00Z | en_US |
| dc.degree.discipline | Mechanical and Manufacturing Engineering | en_US |
| dc.degree.level | Master of Science (M.Sc.) | en_US |
| dc.description.abstract | This dissertation reports an investigation involving a thorough characterization of the microstructural response of cast IN-738 to a variety of pre-weld heat-treatment schemes. The as-received material showed cored dendritic microstructure, containing coarse grains, about 0.5% casting micropores, 0.7% primary carbide particles, and 42% primary $\gamma\prime$ particles respectively. The volume-fraction of primary $\gamma\prime$ particles decreased continuously from 20.1% to about 5% for solution-treatment temperatures (STT) in the range of 1120$\sp\circ$C-1225$\sp\circ$C. The solvus of primary $\gamma\prime$ particles was higher than the 1160$\sp\circ$C-1175$\sp\circ$C range suggested by Steven and Flewitt (27). The brine-quenched samples from the STT manifested extensive intergranular cracking and aging contraction stresses. Aging resulted in coarsening of primary and secondary $\gamma\prime$ particles, degeneration and dissociation of primary carbide particles to form secondary $\gamma\prime$ particles andfine $\rm M\sb{23}C\sb6$ carbide particles on the grain-boundaries. Only long-term aging showed the presence of these secondary carbide particles in the grain interiors. Aging at 845$\sp\circ$C, after solution-treating at 1120$\sp\circ$C showed a distinct bimodal distribution of $\gamma\prime$ particles. The overaged microstructure for the 1175$\sp\circ$C and 1225$\sp\circ$C consisted of a unimodal distribution of degenerate $\gamma\prime$ particles. The activation energy for the coarsening of secondary $\gamma\prime$ particles using the LSEM theory was 241 KJ/molK, in reasonable agreement with the 259KJ/molK calculated by Henderson et. al. (45). Grain-boundary $\rm M\sb{23}C\sb6$ carbide particles have a solvus of about 1025$\sp\circ$C and liquate/dissolve at higher temperatures. The liquation of low-melting continuously linked fine carbide particles on the boundaries was another sole cause for intergranular quench cracks. The coherency strains are usually responsible for heat-affected zone microfissuring in this material and were found to be always lower for the overaged material as compared to the peak-aged and the underaged material. (Abstract shortened by UMI.) | en_US |
| dc.format.extent | 6835371 bytes | |
| dc.format.extent | 184 bytes | |
| dc.format.mimetype | application/pdf | |
| dc.format.mimetype | text/plain | |
| dc.identifier.uri | http://hdl.handle.net/1993/932 | |
| dc.language.iso | eng | en_US |
| dc.rights | open access | en_US |
| dc.title | Microstructural responses of a nickel-base cast IN-738 superalloy to a variety of pre-weld heat-treatments | en_US |
| dc.type | master thesis | en_US |