| Abstract: | 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 ∼ 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 °C. However, at 800 °C σ‐phase leads to a breakdown 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. |
