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Creep strain modelling of 9-12 Pct Cr steels based on microstructure evolution
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. KTH, School of Industrial Engineering and Management (ITM), Centres, Brinell Centre - Inorganic Interfacial Engineering, BRIIE.
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology. KTH, School of Industrial Engineering and Management (ITM), Centres, Brinell Centre - Inorganic Interfacial Engineering, BRIIE.ORCID iD: 0000-0002-8494-3983
2007 (English)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 38, no 9, 2033- p.Article in journal (Refereed) Published
Abstract [en]

Creep deformation is simulated for 9 pct Cr steels by using the Norton equation with the addition of back stresses from dislocations and precipitates. The composite model is used to represent the heterogeneous dislocation structure found in 9 to 12 pct Cr steels. Dislocation evolution is modeled by taking capturing and annihilation of free dislocations into account. Recovery of immobile dislocations is derived from the ability of dislocation climb. In spite of the fact that the initial dislocation density is high and is reduced during creep, primary creep is successfully modeled for a P92 steel. Subgrain growth is evaluated using a model by Sandström (1977). The long time subgrain size corresponds well to a frequently used empirical relation, with subgrain size inversely proportional to the applied stress.

Place, publisher, year, edition, pages
2007. Vol. 38, no 9, 2033- p.
Keyword [en]
ferritic steels, stability, behavior
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-7324DOI: 10.1007/s11661-007-9256-9ISI: 000249538900020Scopus ID: 2-s2.0-34548606418OAI: oai:DiVA.org:kth-7324DiVA: diva2:12302
Note
QC20100616Available from: 2007-06-13 Created: 2007-06-13 Last updated: 2017-12-14Bibliographically approved
In thesis
1. Creep modelling of particle strengthened steels
Open this publication in new window or tab >>Creep modelling of particle strengthened steels
2007 (English)Licentiate thesis, comprehensive summary (Other scientific)
Abstract [en]

Materials to be used in thermal power plants have to resist creep deformation for time periods up to 30 years. The role of alloying elements for creep strength of 9-12% Cr steels is analysed. The creep strength in these steels relies on minor additions of alloying elements. Precipitates give rise to the main strengthening and remaining elements produce solid solution hardening. Nucleation, growth and coarsening of particles are predicted by thermodynamic modelling. Phase fractions and size distributions of M23C6 carbides, MX carbonitrides and Laves phase are presented. The size distributions are needed in order to determine the particle hardening during creep. At elevated temperatures the climb mobility is so high that the dislocations can climb across particles instead of passing by making Orowan loops.

By solving Fick's second law the concentration profile around a moving dislocation can be determined. The results show an accumulation of solutes around the dislocation that slows down dislocation movement. When Laves phase grows a decrease in creep strength is observed due to a larger loss in solid solution hardening than strength increase by particle hardening. Solid solution hardening also gives an explanation of the low dislocation climb mobility in 9-12% Cr steels.

Three different dislocation types are distinguished, free dislocations, immobile dislocation and immobile boundary dislocations. This distinction between types of dislocations is essential in understanding the decreasing creep with strain during primary creep. The empirical relation with subgrain size inversely proportional to stress has been possible to predict. The total creep strength can be predicted by adding the contribution from individual mechanisms.

Place, publisher, year, edition, pages
Stockholm: KTH, 2007. 31 p.
Keyword
Particle strengthening, dislocation climb, ferritic steels, dislocation evolution, creep rate modelling
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-4436 (URN)978-91-7178-720-0 (ISBN)
Presentation
2007-06-13, Konferensrum K 408, Materialvetenskap, Brinellvägen 23, 100 44 KTH, 10:00
Opponent
Supervisors
Note
QC 20101112Available from: 2007-06-13 Created: 2007-06-13 Last updated: 2010-11-12Bibliographically approved
2. Creep modelling of particle strengthened steels
Open this publication in new window or tab >>Creep modelling of particle strengthened steels
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Materials used in thermal power plants have to resist creep deformation for time periods up to 30 years. Material evaluation is typically based on creep testing with a maximum duration of a few years. This information is used as input when empirically deriving models for creep. These kinds of models are of limited use when considering service conditions or compositions different from those in the experiments. In order to provide a more general model for creep, the mechanisms that give creep strength have to be identified and fundamentally described. By combining tools for thermodynamic modelling and modern dislocation theory the microstructure evolution during creep can be predicted and used as input in creep rate modelling. The model for creep has been utilised to clarify the influence of aluminium on creep strength as a part of the European COST538 action. The results show how AlN is formed at the expense of MX carbonitrides. The role of heat treatment during welding has been analysed. It has been shown that particles start to dissolve already at 800ºC, which is believed to be the main cause of Type IV cracking in commercial alloys.

The creep strength of these steels relies on minor additions of alloying elements. Precipitates such as M23C6 carbides and MX carbonitrides give rise to the main strengthening, and remaining elements produce solid solution hardening. Particle growth, coarsening and dissolution have been evaluated. By considering dislocation climb it is possible to determine particle strengthening at high temperatures and long-term service. Transient creep is predicted by considering different types of dislocations. Through the generation and recovery of dislocation densities an increase in work hardening during primary creep is achieved. The role of substructure is included through the composite model. Cavity nucleation and growth are analysed in order to explain the intergranular fracture and to estimate the ductility.

Place, publisher, year, edition, pages
Stockholm: KTH, 2010. 48 p.
Keyword
creep rate modelling, particle hardening, microstructure evolution, dislocation climb, ferritic steels
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-12235 (URN)978-91-7415-590-7 (ISBN)
Public defence
2010-04-28, F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC20100616Available from: 2010-04-09 Created: 2010-03-30 Last updated: 2011-10-03Bibliographically approved

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