I'm not sure, but I think that σιδηροκολλητής might be the more general term, which can be further specified as ηλεκτροσυγκολλητής / ηλεκτροκολλητής when using electricity, οξυγονοσυγκολλητής / οξυγονοκολλητής for acetylene, etc. I'd use the latter for welding with a torch, but maybe someone else will know for sure.