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Influence of aluminium on creep strength of 9–12% Cr steels
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.
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
2009 (English)In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 527, 118-125 p.Article in journal (Refereed) Published
Abstract [en]

The influence of aluminium on creep strength of 9% Cr steels is predicted by a fundamental model forcreep. Through thermodynamic modelling the particle structure is determined for a temperature andcomposition range. This shows how AlN is formed at the expense of MX carbonitrides of VN characterwhen the aluminium content is increased. The remaining MX particles are of NbC type and have approximatelyone fifth of the original phase fraction. The evolution in microstructure such as particle coarseningis included in the model as well as the recovery. Rupture time is predicted using a modified Norton equationincluding back-stresses calculated from microstructure. The predictions show correspondence tosome of the creep data for the steel P91 over a temperature and stress range. Furthermore, simulationwith high Al content verifies the observed early failure of Al rich components. Overall, the simulationsshow a decrease in rupture time by a factor 6 due to Al additions of up to 0.2%.

Place, publisher, year, edition, pages
Elsevier , 2009. Vol. 527, 118-125 p.
Keyword [en]
Creep rate modelling, Aluminium influence, Dislocation evolution, Particle strengthening, Ferritic steels
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-12232DOI: 10.1016/j.msea.2009.07.060ISI: 000271611500017Scopus ID: 2-s2.0-71749091664OAI: oai:DiVA.org:kth-12232DiVA: diva2:306587
Note
QC20100616Available from: 2010-03-30 Created: 2010-03-30 Last updated: 2017-12-12Bibliographically approved
In thesis
1. 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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Sandström, Rolf

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