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Growth of Creep Cavities in 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: Creep & Fracture in High Temperature Components – Design & Life Assessment, 2nd International ECCC Conference, Empa, Dübendorf, Switzerland, 21-23 April, 2009 / [ed] I A Shibli, S R Holdsworth, LANCASTER, PA: DESTECH PUBLICATIONS, INC , 2009, 950-963 p.Conference paper, Published paper (Refereed)
Abstract [en]

The nucleation and growth of creep cavities will eventually occupy a considerable fraction ofthe grain boundary. This will lead to microcracks and intergranular fracture thus controllingthe ductility of the component. The traditional approach to predicting this type of failure is tosimulate cavities with only one size. Assumptions of an instant nucleation with symmetricallyplaced cavities make all cavities equally sized. It has been observed, in 12% Cr steels as wellas in other commercial alloys that cavities nucleate during all stages of creep. Creep cavitiesget randomly placed mostly at grain boundaries directed transverse to the loading direction.With continuous nucleation a size distribution of cavities appears, which is compared toobserved average cavity size. Constraints on cavity growth are introduced, which reduces thegrowth rate. This is needed in order to explain the cavity growth of 12% Cr steels.Furthermore, creep rupture will be derived based on the area fraction of cavities, thus explaining the intergranular failure.

Place, publisher, year, edition, pages
LANCASTER, PA: DESTECH PUBLICATIONS, INC , 2009. 950-963 p.
Keyword [en]
Creep, 12% Cr steels, cavity, growth
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-12233ISI: 000266610600083ISBN: 978-1-60595-005-1 (print)OAI: oai:DiVA.org:kth-12233DiVA: diva2:306591
Conference
2nd International Creep Conference Zurich, SWITZERLAND, APR 21-23, 2009
Note
QC 20100616Available from: 2010-03-30 Created: 2010-03-30 Last updated: 2011-03-07Bibliographically 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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