Effect of Magnetic Stirring on Nickel Alloy Weld Microstructure and Micro-Fissure Response

Nickel filler metals containing high chromium content are widely used in the commercial nuclear power plants to provide resistant to stress corrosion cracking.  However, welding of high-chromium nickel alloys, under heavy restraint conditions, leads to several challenges. These challenges include ductility dip cracking and solidification cracking.  Adding to the complexity, large columnar solidification grains in the welds also reduce the efficiency of detecting these cracks by ultrasonic non-destructive examination (NDE).  Previous research focused on refinement of solidification grain size by magnetic arc stirring to  improve ultrasonic NDE capability, as well as, to reduce the cracking tendency.  For a given welding process (voltage, current, travel speed, and wire feed speed) grain refinement  was achieved at an optimum frequency of 7 Hz. With this grain refinement, the welds show improved (> 50%) signal-to-noise ratio in certain ultrasonic scanning directions.  In this work, the effect of stirring on cracking tendency of Alloy 52M weld deposits on Type 303 plate clad with ER308L-Si stainless steel was evaluated.  Welds made with two power ratios (198 and 249 Kw/in²) are compared with and without magnetic stirring at different dilution conditions. Results showed, in addition to the fine solidification grains, magnetic stirring reduces the tendency for solidification cracking in Alloy 52M. Total number of cracks for the stirred condition is more than 3 times lower than the unstirred condition.  Calculations also show that magnetic stirring at 7 Hz reduces dilution by up to 10%. Mechanisms for grain size refinement and increased cracking resistance will be discussed.