Haynes Hastelloy S description: HASTELLOY S RTW fi ller metal is used for the gas metal arc and gas tungsten arc welding of S alloy. The deposited weld metal has excellent thermal stability, low thermal expansion and excellent oxidation resistance to 2000F. S weld metal retains its strength and ductility after aging at temperatures of 800F to 1600F. The fi ller metal also is a great candidate for dissimilar joining in cyclic heating applications and long term temperature exposure.