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The really promising way how to improve the creep resistance of

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During creep exposure of modified chromium steels lowering of solid solution strengthening due to precipitation of Laves phase as well as coarsening of all precipitates causes degradation of creep resistance. Two distinct domains of the stress dependence of creep rate and time to rupture have been observed in precipitation strengthened modified chromium steels. The stress characterizing the transition between these domains was found to be closely related to the Orowan stress. This stress consists in these steels of the contribution from large particles on subgrain boundaries (mainly M23C6 and during the limited time also Laves phase) and from small precipitates (Nb(C,N) and VN) inside subgrains. This has to be considered when measuring the interparticle spacing and calculating Orowan stress. Larson‐Miller parametric equation is used to elucidate the necessity of long‐term creep testing. By means of two heats of CrMoVNbN steel it is shown that reliable extrapolation of creep properties is possible only in a stress and temperature domain in which only one creep creep rupture mechanism operate. In the high stress domain Larson‐Miller constant CLM is well above 30 while in the low stress domain this constant does not exceed 25. When the extrapolation is based mainly on short‐term creep tests, the CLM constant is close to that valid in the high stress domain and therefore it overestimates long‐term creep strength.

The creep resistance of advanced chromium steels can be significantly increased due to precipitation of very small particles of vanadium nitride VN. The solubility and precipitation of VN, Nb(C,N) and AIN in austenite and ferrite was analysed using relevant solubility products. The calculated values of nitrogen in solid solution were used for assessment of creep rupture strength of chromium steel (mean considered chemical composition, mass contents in %: 0.18 C; 10.5 Cr; 1.0 Mo; 0.2 V; 0.07 Nb; 0.05 N; 0.01 AI). Increasing N mass contents from 0.03 to 0.07 % leads to increasing creep rupture strength in 100 000 h at 600°C of about 60 %. Lowering AI mass contents from 0.045 to 0.005 % produces higher creep rupture strength of about 30 %. EinfluB von Nb-, V-, N-und AI-Gehalten auf die Zeitstandfestigkeit von 9-12 % Cr Stahlen. Eine betrachulche ErhOhung der Zeitstandfestigkeit warmfester Chromstahle kann man durch VergroBerung der Ausscheidungsverfestigung mit kleinen VN-Teilchen erreichen. Die Loslichkeit im Austenit und Ferrit wurde anhand der Loslichkeitsprodukte analysiert. Mit den berechneten Gehalten an Stickstoff in fester Losunq konnte die Zeitstandfestigkeit des Cr-Stahles (Richtanalyse: 0.18 % C; 10.5 % Cr; 1.0 % Mo; 0.2 % V; 0.07 % Nb; 0.05 % N; 0.01 % AI) abqeschatzt werden. Mit zunehmendem N-Massengehalt von 0.03 bis 0.07 % erhOht sich die Zeitstandfestigkeit bei 600°C nach 100 000 h unqetahr um 60 %. Bei sinkendem AI-Massengehalt von 0.045 bis 0.005 % steigt sie um ungefahr 30 % an.