| Summary: | 碩士 === 長庚大學 === 機械工程研究所 === 87 === It was observed that an acicular phase formed in the matrix of IN738LC turbine blades, which were used at elevated temperatures for a prolonged period of time. As a result, cracking was a common problem encountered during repair welding of IN738LC blades. In this study it was verified that the cracking was heat-affected zone (HAZ) liquation cracking which was due to constitutional liquation of the acicular phase during welding. Specimens were solution heat treated at 1300oC in order to eliminate this detrimental acicular phase and improve the weldability of the material. However, a new (Ta, Ti, Nb, Ni) containing blocky phase formed in the grain boundary areas, and resulted in the formation of a precipitate free zone (PFZ) and subsequent heat treatment cracking during cooling from the solution temperature.
The major objective of the welding experiments was to completely avoid the formation of HAZ liquation cracking during repair welding of IN738LC without using any solution heat treatment or welding preheat. Gas tungsten arc welding (GTAW) and gas metal arc welding (GMAW) were selected as the repair welding processes. It was found that the formation of the HAZ liquation cracking was directly related the welding heat input. By lowering welding heat input, the liquation cracking was successfully reduced or eliminated in autogenous GTAW experiments. However, this heat input was too low to avoid the formation of lack of fusion if filler metal was added. In GMAW experiments, welding heat input was controlled by suitable selection of filler wire diameter and welding parameters. It was found that the HAZ liquation cracking and lack of fusion were successfully avoided in the GMAW weldments.
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