Evaluation of phase relations in weld overlays of 316, 309MoL and SKWAM

Abstract: AREVA NP Uddcomb AB wants to replace the material used for a specific valve seat used in boiling water reactors, BWR. Their solution is a weld overlay of different stainless steels composed of two buffer layers of the steel 309 MoL followed by two layers of the filler material SKWAM welded on type 316 stainless steel or carbon steel. The report focuses on the long term structural effects in the weld overlay due to the operating temperature in BWRs, in this case 270 °C. To investigate the thermodynamic stability in the weld overlay the computer software Thermo-Calc was used and a metallographic examination was carried out. The results from these procedures were compared and possible long term effects were discussed. Most likely spinodal decomposition is the most severe structural change that may appear in the material. At equilibrium conditions at the operating temperature ferrite is decomposed into Fe-rich and Cr-rich ferrite but since the kinetics is not included in the calculations it is not possible to determine the rate of decomposition.