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.
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.
This chapter deals with the change of the structure and mechanical properties of the austenitic steels grades TP347HFG, Super 304H and HR3C as well as their welded joints after the exposure at temperatures corresponding to the ultra-super critical (USC) and advanced ultra-super critical (A-USC) parameters. Several tube bend radii were investigated in order to understand relation between cold work plastic deformation, mechanical properties and the structural changes with special attention to the generation of sigma phase during long-term high temperature exposure. The effect of post bend solution annealing work on the material properties and structural changes was studied, too, using small punch test (SPT) and miniaturized tensile tests taken both from straight part and extrados of bends under investigation. Creep properties of either base metal either welded joints were tested and evaluated using standard creep specimens at temperatures 650, 700 and 750°C. The obtained results confirmed that the mechanical properties and the structure were significantly influenced even after relatively short-time exposure at elevated temperature. The results of experiments were compared with other results obtained from analyses of tubes after 100,000 h of exposure in a USC block where extensive precipitation of sigma phase was also identified.
This paper deals with microstructural stability of P23/P91 dissimilar welds during long-term creep exposure at 500°C. Detailed TEM investigations on minor phase evolution were performed on specimens after long-term creep exposure. Microstructural changes in the decarburized layer of the P23 steel were studied using EBSD and TEM. Experimental data on minor phase evolution were compared to the results of thermodynamic (Thermocalc) and kinetic (Dictra) simulations. Evolution of minor phases in the P23/P91 welds during creep at 500°C was in a good agreement with results of microstructural modelling. Partial decarburization of the P23 occurred at the expense of dissolving M7C3 phase. Microstructure of the partly decarburized zone in the P23 steel remained bainitic in all specimens investigated. In spite of the partial decarburization in the P23 steel during longterm creep exposure at 500°C, creep failures occurred in the ICHAZ of the P23 steel.
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