Role of dispersed phase in the creep of ferritic steels

Foldyna, Václav; Purmenský, Jaroslav

doi:10.1007/bf01605397

Cited by 6 publications

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“…Two distinct domains of the stress dependence of creep rate and time to rupture have been frequently observed in the case of precipitation strengthened low alloy and modified Cr steels [3; 19]. The values of stress exponent and apparent activation energy are shown in table 1.…”

Section: Mechanisms Of Creep Deformation and Fracturementioning

confidence: 99%

Evaluation of structural stability and creep resistance of 9-12% Cr steels

et al. 1996

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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.

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Section: Mechanisms Of Creep Deformation and Fracturementioning

confidence: 99%

Evaluation of structural stability and creep resistance of 9-12% Cr steels

et al. 1996

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Effect of structural factors on the creep properties of modified chromium steels

Foldyna¹,

Jakobova²,

Riman³

et al. 1991

Steel Research

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The precipitation strengthening of modified chromium steels is effected predominantly by M 23Ce carbides. In molybdenum-modified 9% chromium steels, creep resistance depends on the dispersion of the M 23C 6 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 M 23C e carbides and by VCxN y carbonitrides.If the amount of nitrogen in solid solution is insufficient, this reduces the volume fraction of VCxN 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 chromium steels.ElnfluB der Mikrostruktur auf die Kriechelgenschaften von modifizlerten Chromstiihlen. An der Ausscberdunoshartunq von rnodifizierten Chromstahlen ist vor allem das Carbid M 23Ce beteiligt. In Mo-modifizierten 9%-Chromstahlen hangt die Zeitstandfestigkeit von der Dispersion des Carbids M 23C 6 und vom Mo-Gehalt in fester Losung abo Es gibt keinen Grund, den Mo-Gehalt in diesen Stahlen Ober die 1%-Grenze hinaus zu erhOhen. In V-modifizierten Stahlen beteiligen sich an der Ausscheidungshartung neben M23Ce-Carbiden auch die Carbonitride VCxN y. Wenn in der festen Losung keine ausreichende Stickstoffmenge vorhanden ist, verringert sich im GefDge der VCxNy-Volumenanteil, und dadurch verschlechtert sich die Warmfestigkeit des Stahls. In V-modifizierten chrometahten sollte deshalb der AI-und Ti-Gehalt, der den effektiven N-Gehalt in fester Losung verringert, beschrankt werden.

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The effect of Nb, V, N and Al on the creep rupture strength of 9–12 % Cr steel

Kuboň

Foldyna

1995

Steel Research

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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.

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Role of dispersed phase in the creep of ferritic steels

Cited by 6 publications

References 0 publications

Evaluation of structural stability and creep resistance of 9-12% Cr steels

Evaluation of structural stability and creep resistance of 9-12% Cr steels

Effect of structural factors on the creep properties of modified chromium steels

The effect of Nb, V, N and Al on the creep rupture strength of 9–12 % Cr steel

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