On the hot strength of austenitic valve steels with a high nitrogen content

Exhaust valves are made of CrMnNi steels with about 1 wt% of (C + N) and additions of W, Mo and Nb, Remelting under pressure allows to raise N and do without C. After solution annealing the formation of discontinuous N-perlite during aging is suppressed by similar to 1 wt% of Nb whereby the creep strength is increased. The size and distribution of continuously precipitated nitrides in N-steels are finer than those of carbides and nitrides in (C + N)-steels. Thus the creep strength of the former is superior at 700 degreesC. However, at 800 degreesC sigma-phase leads to a break-down of creep strength. The reason is seen in a lower stability of N-austenite as compared to (C + N)-austenite, which shows a higher concentration of free electrons and more short range atomic ordering.