| Abstract Scope |
Alloy 709, an advanced nitrogen-stabilized and niobium-strengthened austenitic stainless steel, is under code qualification for fabricating future sodium fast reactor structures, as part of the development efforts under the US Department of Energy, Advanced Reactor Technologies program. An important consideration is the weldability and hot cracking sensitivity of this alloy, in particular the role of the impurity elements, like P, S, and B. In this work, weldability test was undertaken by welding high-phosphorus (140 wppm) commercial plates of Alloy 709 with a high-phosphorus (140 wppm) wire. The welds were analyzed using multiscale characterization techniques. A careful examination of the welds revealed hot cracking in the multi-pass weld metal. The cracks were predominantly located at the border of two over-lapping passes and identified as liquation cracks. The typical characteristics of this liquation cracking were studied in detail. Segregation behaviors (macro and micro) of trace elements, especially phosphorus, in the weld metal were quantitatively measured. The roles of P segregation on liquation cracking susceptibility of these high-phosphorus Alloy 709 weldments were also discussed. Other metallurgical factors (cooling rate, grain size, hardness distribution, stress etc.) were also considered to uncover this liquation cracking mechanism. |