Iron-base alloys containing a minimum iron content of 45%, a minimum carbon content of 0.50%, a chromium content of not less than 20%, and between 27 and 32% nickel, part of which may be replaced by cobalt, can develop a creep-rupture. strength at 1600°to l800°F approaching that of the cobalt-base alloy H.S. 21 when they are strengthened by not more than 5% additions of Cb, Ta, Mo, and W in certain combinations and relative concentrations. These high-creep-rupture-strength iron-base alloys are equal to H.S. 21 in oxidation resistance at 2000°F, but inferior to the former in short-time cold and hot tensile strength, in cold ductility, and in resistance to thermal shock and thermal fatigue. The possibility of improving the latter by lowering the carbon content and compensating the resulting decrease in rupture strength with suitable modifications in composition has not been explored.
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