| Summary: | 碩士 === 正修科技大學 === 機電工程研究所 === 95 === On welding of austenitic steel piping, the GTAW technology is a normal application. However, it is not an easy task to deal with the heat input control on this common practice, and the welds and HAZ are always in a state of high tensile residual stresses that can assist IGSCC and pitting failure in welded stainless steel.
In this study, a liquefied CO2 heat sink apparatus is involved in GTAW. That can prevent HAZ sensitization and reduce high residual stress remaining in welds. In utilization of MATLAB, numerical simulation is applied in the study and the applicable heat sink welding parameters can be estimated. In the experiments, the liquefied CO2 forms a solid snow-like phase as it emerges from the backing gas nozzles and then sublimation with welding generated heat. The sublimated dioxide gas is environment friendly and can drop temperature gradient in the welds. Furthermore, it can prevent the IGSCC susceptibility of austenitic stainless steel and reduce the tensile residual stress in welds. Mechanical tests, microstructure examination, blind-hole measurement of residual stresses, SEM composition analysis and IGSCC susceptibility tests are all elaborated in the study.
No matter how Dry Ice located behind the weld torch in 5mm or 10mm in the experiments, it suppresses the chromium carbides formation effectively. HAZ sensitization can be abolished in welding the stainless steel, and the mechanical property of the weldment is not degraded. Lower von Mises residual stresses can be achieved for heat sink welding while compared to commercial GTAW process. This implies less welding deformation in such a welding process. In addition, residual stresses orientated in perpendicular or parallel to the weld can retain in the state of compression and lower than the convention one.
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