CF8與Alloy 52M異材覆銲之熱裂研究

Abstract

本實驗係針對兩種不同硫含量之CF8不銹鋼 (CF8-L與CF8-H)，以氬銲銲接製程進行Alloy 52M鎳基合金之覆銲。使用Alloy 52M直接覆銲至CF8-L (0.006 wt.% S) 基材上，於兩者銲接界面附近可觀察到凝固熱裂。這些裂紋多發生在高稀釋率位置，例如單道次銲道之起始與銲接終止處，以及多道次銲道的第一、二銲道最容易產生裂紋。實驗結果亦顯示：高稀釋率與共晶組織 (NbC(N)-γ、Laves-γ) 的生成，對於Alloy 52M覆銲層熱裂敏感性有很大的影響。實際覆銲過程中，在覆銲Alloy 52M前先行覆銲309LMo不銹鋼緩衝層，可有效消除熱裂縫。 以氬銲銲接法在CF8-H (0.14 wt.% S) 基材上直接覆銲Alloy 52M，結果發現有源自CF8-H的硫遷移至Alloy 52M覆銲層而導致熱裂現象。若直接覆銲Alloy 52M於CF8-H不銹鋼，則產生嚴重熱裂。為了降低熱裂敏感性，在覆銲Alloy 52M之前，先覆銲308L不銹鋼作為緩衝層，發現熱裂紋主要在緩衝層與Alloy 52M之銲接界面附近生成，凝固晶界上形成之Fe-Ni-S-γ、NbC(N)-γ以及Laves-γ共晶組織，為造成銲接界面附近較高程度覆銲熱裂生成之主因。

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.