[en] 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 AlN in austenite and ferrite was analysed using relevant solubility products. Based on results obtained on many heats of chromium modified steels, the calculated values of nitrogen in solid solution - N_ss - were used for assessment creep rupture strength of chromium steel (mean considered chemical composition in wt.%: 0.18C; 10.5Cr; 1.0Mo; 0.2V; 0.07Nb; 0.05N; 0.01Al). Increasing N content from 0.03 to 0.07 wt.% leads to decreasing creep rupture strength in 100000h at 600 C of about 60%. Lowering Al content from 0.045% to 0.005 wt.% produces higher creep rupture strength of about 30%. (orig.)

[en] The superior creep resistance of the microalloyed steels compared to the carbon steels originates in the presence of very fine particles of minor phases based on vanadium or niobium. These steels are delivered either after controlled rolling or in the normalized state. The creep strength of these microalloyed steels reaches nearly that of low alloy 0.15C-0.3Mo steel type. The best creep properties are detected in steels after controlled rolling, while after normalizing creep rupture strength slightly decreases, especially in heats alloyed with niobium. This is the result of the microstructure of the steel when in steel with niobium originally fine particles of niobium carbonitride coarsen during normalizing with corresponding changes in their interparticle spacing. When compared the creep rupture strength attained experimentally with the data stated in the standard CSN 41 2025, it is clear that the data stated in the standard corresponds the creep properties of this steel after normalizing but do not take into account creep properties attained after controlled rolling (normalizing forming). (orig.)

[en] Several models of mechanisms controlling creep of modified chromium steels have been briefly discussed. Solubility and precipitation of VN and Nb(C, N) in austenite and ferrite in 9% Cr Steel has been studied. It was shown that the effect of Cr on the solubility of VN in austenite and ferrite is not negligible. The strengthening of solid solution is caused primarily by the presence of Mo and/or W in solid solution. When assessing the creep properties of modified chromium steel we must take into account the Mo and/or W content in solid solution during time of creep exposure. (orig.)

[en] The model calculating creep rupture strength at 600 C based on proof stress and available nitrogen content was verified on a set of 20 advanced chromium modified steels with Mo and Mo+W addition for 10.000 h. The beneficial effect of W addition was not confirmed at 600 C and only small differences in creep rupture strength up to 10.000 hours were detected. Creep rupture strength at 600 C/10.000 h increases with increasing both proof stress and available nitrogen content. The strong dependence of creep rupture strength of 9-10%Cr steels on proof stress in the new model is on the first view a little surprising. The stronger effect of proof strength is however expected because after 10.000 h at 600 C the microstructure of the steels is far away from the stable state and many structural changes will be still occur in longer term and influence the relationship between the proof strength and the creep rupture strength. The principal role of nitrogen concentration and heat treatment on creep rupture strength of advanced chromium modified steels is confirmed. (orig.)

[en] The aim of the present paper is to analyse different strengthening mechanisms occurring in chromium creep resistant steels. This analysis will be used to explain the observed differences in the creep properties of two heats of the steel X12CrMoVNbN 10 1 1. (orig.)

[en] An improvement in power plant efficiency can be achieved by increasing the operating temperature and pressure of steam turbines. This is dependent on the development of [vanced materials with improved high temperature creep properties, low oxidation rate and good weldability. In order to obtain the material with [vanced properties it is necessary to learn about all processes taking place in materials during long term exposure. Special attention has to be paid to the weld joints which usually represent the weakest point in any power equipment working in the creep regime. This paper summarizes the principal strengthening and degr[ation processes occuring in chromium modified steels, shows the necessity of the correct and rightful extrapolation of creep data. Furthermore, the paper describes the results of creep rupture testing on weldments of P91 steel. (orig.)

[en] This paper presents an outline survey of the principal strengthening mechanisms that can act in 9% to 12% chromium steels. Precipitation strengthening is affected mainly by M₂₃C₆ carbides, while in vanadium and/or niobium modified chromium steels, the effect of M₂₃C₆ carbides is augmented by that of MX carbonitrides. It is shown that the proof stress at room temperature and the content of nitrogen in solid solution are very significant factors controlling the creep resistance of modified chromium steels. It is proposed how to predict the creep rupture strength of these steels by means of the factors mentioned above. (orig.)

[en] The tests effected confirmed the high refractoriness under load and suitable creep properties of steel 17Cr12Ni2Mo with the carbon content up to 0.03% and the nitrogen addition of 0.12 to 0.17%. Thus, the steel is a suitable material for both nuclear and conventional power installations. For rational utilization of alloying elements, a chemical composition can be recommended containing up to 13.5% of nickel, 2.5% of molybdenum, and 0.005% of a boron addition. (Ha)

[en] 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 AlN 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 Al). 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 Al mass contents from 0.045 to 0.005 % produces higher creep rupture strength of about 30 %. (orig.)

[en] The precipitation strengthening of modified chronium steels is effected predominantly by M₂₃C₆ carbides. In molybdenum-modified 9% chromium steels, creep resistance depends on the dispersion of the M₂₃C₆ carbides and on the molybdenum content in the solid solution. There is no point in increasing molybdenum contents of molybdenum-modified steels above approximately 1 wt.%. In vanadium-modified steels precipitation strengthening is effected both by M₂₃C₆ carbides and by VC_xN_y carbonitrides. If the amount of nitrogen in solid solution is insufficient, this reduces the volume fraction of VC_xN_y in the structure and thus impairs the creep resistance of the steel. It is advisable to restrict the aluminium and titanium contents in the vanadium-modified. (orig.)