The exhaust valves of 21-4N or 21-2N generally have hardfaced heads for heavy-duty engines. Hardfacing alloys contain a large amount of cobalt (Co), which sometimes becomes very scarce and outrageously expensive. Therefore, some automakers have adopted nickel (Ni)-base superalloys as unfaced exhaust valves, but continued to search for less expensive heat resisting alloy.
THE authors had succeeded in developing an iron (Fe)-base superalloy, RS 914, containing 40% Ni, 19% Cr, 2.8% Ti and 0.7% Al. The mechanical and chemical properties of this alloy had proved to be comparable with those of Ni-base Inconel 751. And unfaced exhaust valves of RS 914 passed endurance tests in turbo-charged diesel engines with minimal damage. This alloy is relatively low cost and easy to fabricate, so that it can be applied for unfaced exhaust valves substituting for Ni-base superalloys.