| Molybdenum microsegregation occurs during solidification of welds in 6 wt% Mo superaustenitic stainless steels, which decreases the weld metal corrosion performance. Current techniques for restoring the fusion zone corrosion performance utilize filler metals with a higher Mo content. However, the influence of filler metal composition and welding parameters on the microstructural evolution and the resultant corrosion performance is not well understood. A study was conducted to better understand the relationship between the welding parameters, the microstructural evolution, and the corrosion performance. Welds were fabricated on cast alloy CN3MN with IN686 and IN72 filler metals over the entire range of dilution levels. Welds were fabricated using the GTAW process and 1150°C/1hour or 1205°C/4hour prior and post weld heat treatments were conducted. The weld microstructures were characterized using light optical microscopy and electron backscatter diffraction techniques. The weld solidification behavior was modeled with ThermoCalc Software and the welds were corrosion tested using ASTM G48a specifications. The secondary phase that formed during solidification was identified as sigma phase and the amount of sigma phase was found to be independent of the dilution level due to the offsetting effects of the nominal and eutectic compositions. The corrosion performance of IN686 filler metal welds was restored at lower dilution levels, and it was also restored with a post weld heat treatment of 1150°C/1hour. The IN72 filler metal did not provide adequate weld metal corrosion performance under any conditions. Finally, it was determined that heat treatments should always be applied after welding, if possible, to obtain the best corrosion performance. |
