| Summary: | 碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 100 === In this study, the gas tungsten arc welding process was used to perform overlay welding of Alloy 52M (a nickel-based filler metal) onto CF8 stainless steels consisted of either 0.006 wt.% (CF8-L) or 0.14 wt.% (CF8-H) sulfur. While direct depositing Alloy 52M on CF8-L substrate, hot cracking in the overlay was observed, in particular, near the weld interface between the Alloy 52M overlay and the CF8-L substrate of the weld overlay. Overall, the hot cracks were most likely to occur at the sites with high dilution rates, e.g., at the weld start/end locations of a single pass or in the first and second passes in multi-pass overlays. It was found that the dilution rate and the formation of eutectic-type constituents (NbC(N)-γ and Laves-γ) both played significant roles in affecting the hot cracking susceptibility of these weld overlays. Nevertheless, hot cracks could be eliminated by proper deposition of a 309LMo buffer layer prior to overlaying with Alloy 52M.
Furthermore, overlay welding of Alloy 52M on a high-S CF8 stainless steel, e.g., CF8-H, was also performed using the same welding process. Hot cracking of such weld overlays was strongly influenced by the S content of the CF8-H substrate. Severe hot cracking was noticed when Alloy 52M was directly overlaid on the CF8-H substrate. To lower the cracking susceptibility, ER 308L was deposited on the CF8-H substrate as a buffer layer before the subsequent deposition of Alloy 52M. Under such circumstances, hot cracks were still present with reduced size. These cracks were most likely to initiate at the weld interface between the 308L buffer layer and the Alloy 52M overlay. The formation of Fe-Ni-S-γ, NbC(N)-γ and Laves-γ eutectic-type constituents at the solidification boundaries could account for the hot cracking near the weld interface. Besides, the fracture surface of the Alloy 52M overlay was covered with coral-like Fe-Ni-Mn sulfide.
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