A Feasibility Study of Applying SS 307Si Buffer Layer for Mitigating the Hot Cracking of Ni-Based Weld Overlay
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JMEPEG DOI: 10.1007/s11665-017-2817-1
A Feasibility Study of Applying SS 307Si Buffer Layer for Mitigating the Hot Cracking of Ni-Based Weld Overlay Kun-Chao Tsai
and Sheng-Long Jeng
(Submitted February 2, 2017; in revised form May 6, 2017) The hot cracking behavior of Ni-based Alloy 52M weld overlay with respective SS 307Si and SS 308L buffer layers was investigated. The dilution level of SS 307Si buffer layer is a little higher than that of SS 308L. However, the hot crack length of overlay with SS 307Si buffer layer is shorter and the SS 307Si layer has higher mechanical properties than that of SS 308L layer. As observed by SEM and EBSD, ferrites precipitated in SS 307Si buffer layer are in vermicular skeletons dotted with lathy precipitates, which have a little higher local stain than that of SS 308L weld. However, Alloy 52M weld around SS 307Si fusion boundary has a lower degree of local distortion. The results generalize that the SS 307Si buffer layer is marginally better for reducing hot cracking susceptibility, owing to its lower local stain and slightly higher mechanical strength. Keywords
307Si, buffer layer, dilution level, weld overlay
1. Introduction High Cr filler alloys of 52 and 52M have been used in weld overlay repairs of the reactor coolant piping systems in nuclear power plants to mitigate primary water stress corrosion cracking (PWSCC) and intergranular stress corrosion cracking (IGSCC) (Ref 1-3). However, ductility dip cracking (DDC) incidents have often been observed during multi-pass welding of Alloy 52 and Alloy 52M (Ref 4). Recently, Alloy 52MSS has been developed with the addition of Nb and Mo to prevent DDC, because Nb-rich precipitates formed in the matrix during the solidification of Alloy 52MSS inhibit grain boundary sliding (Ref 5). The maximum tensile stress of 52MSS could be enhanced by the addition of Nb and Mo with no elongation loss (Ref 6). Nevertheless, increasing Nb has been shown to widen the solidification temperature range in nickel-base alloys, which potentially increases susceptibility to hot cracking (Ref 7, 8). Hot cracking has occurred with Alloy 52M welds overlaid on the stainless steel piping (Ref 7-9). The high P and S of wrought stainless steel piping would diffuse into Alloy 52M welds to induce hot cracking (Ref 7). However, the P and S contents of Alloy 52M welds can be significantly reduced by applying a buffer layer of SS 308L to suppress hot cracking (Ref 8, 10). SS 308L could also decrease the dilution of Alloy 52M overlays and diminish the diffusion of Si into Alloy 52M (Ref 8). Shih reported that the crater cracking of Alloy 52M could be mitigated, when Si content of the sub-grain boundary is lower than the content limit of 0.63 wt.% (Ref 11). However, McCracken et al. (Ref 10) suggested that Si up to 1.0 wt.% does not directly promote the solidification cracking in the Kun-ChaoTsai and Sheng-LongJeng, Division of Nuclear Fuels and Materials, Institute of Nuclear Energy Research, 1000 Wenhua Rd., Longtan District, Taoyuan City 32546, Taiwan, ROC. Contact e-
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