Weld Mold 305 description: A 17 chrome, 4 nickel, 4 copper precipitation hardening stainless steel alloy. Weld Mold 305 is used for joining 17/4, 15/5 grades to themselves and dissimilar combinations of these steels. PH steels are used primarily for marine and aerospace applications such as fuel tanks, valves, gears, landing gear covers, pump parts, shafts, bolts, saws, knives and flexible bellows type joints.