Is R41 connected across the incoming mains power supply to the board? If so, it is likely to be a varistor. These are fitted to protect the internal circuitry from voltage surges/spikes, and would normally result in a blown fuse, rather than the varistor going bang. Does the welder still work?
Looking around at a few bits of information, R41 does indeed appear to be a varistor, probably an MOV (metal oxide varistor). It needs to be fitted to stop mains voltage spikes and surges causing damage to, or glitching the digital circuitry.