i have welded 4 type ss with mild steel wire. but if 3 type s steel you may want to look at ss wire for this and mild steel will rust in time .all ways put more heat on the thicker steel for s steel you may want a 3 mix on the gas or talk to where you get your welding supplise on the 3 gas and wire i tig all the ss now. but have used mig wire welder many times