A New Iron-Base Superalloy for Exhaust Valves

For heavy duty gasoline or diesel engines, exhaust valves of 21-2N or 21-4N are generally coated by cobalt-base hard facing alloys, at faces. But the sluggish supply and the spiraling price of cobalt recently forced automakers to adopt valves of high-grade superalloys without hard facing. Candidate superalloys for high-performance exhaust valves are gamma-prime strengthened nickel-base alloys such as Inconel 751 and Nimonic 80A. Unfortunately above-mentioned alloys are too expensive for automobile components. So authors tried to develop a gamma-prime strengthened iron-base alloy, which bears basically 40% Ni-19% Cr-Al-Ti. To optimize Al and Ti contents the effect of the total amount and the ratio of them was examined thoroughly on hardness, strength and corrosion resistance of experimental alloys at elevated temperatures. Finally authors succeeded in developing a new gamma-prime strengthened iron-base superalloy, RS914, which is inexpensive and satisfies requirements for exhaust valves of both gasoline and diesel engines.