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Creep modelling of particle strengthened 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.
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 [en]
creep rate modelling, particle hardening, microstructure evolution, dislocation climb, ferritic steels
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-12235ISBN: 978-91-7415-590-7 (print)OAI: oai:DiVA.org:kth-12235DiVA: diva2:306604
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
List of papers
1. The role of dislocation climb across particles at creep conditions in 9 to 12 pct Cr steels
Open this publication in new window or tab >>The role of dislocation climb across particles at creep conditions in 9 to 12 pct Cr steels
2007 (English)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 38A, no 10, 2428-2434 p.Article in journal (Refereed) Published
Abstract [en]

The influence of a distribution of particles on creep strength is analyzed. The time it takes for dislocations to climb across the particles is the basis for a model that can describe the effect of particle size distribution, particle area fraction, stress, and temperature on the creep rate. The degradation of microstructure through coarsening is taken into account. The particle size distributions for M23C6 carbides and MX carbonitrides in a 9 pct Cr steel are accurately represented by an exponential function. Coarsening coefficients and phase fractions for MX and M23C6 particles are predicted using thermodynamic modeling, and show good fit to experimental data. The size distributions are used to determine the amount of dislocations, which can either climb across particles or make Orowan loops. The dislocation climb model is integrated into a creep rate prediction model and is used to reproduce experimental creep data for P92 steel.

Keyword
9-percent chromium steel, stability, strength
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-7323 (URN)10.1007/s11661-007-9280-9 (DOI)000250150300008 ()2-s2.0-71749109379 (Scopus ID)
Note
Tidigare titel: Dislocation climb of particles at creep conditions in 9-12% Cr steels Uppdaterad från submitted till published(20101112) QC 20101112Available from: 2007-06-13 Created: 2007-06-13 Last updated: 2011-10-03Bibliographically approved
2. Creep strain modelling of 9-12 Pct Cr steels based on microstructure evolution
Open this publication in new window or tab >>Creep strain modelling of 9-12 Pct Cr steels based on microstructure evolution
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.

Keyword
ferritic steels, stability, behavior
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-7324 (URN)10.1007/s11661-007-9256-9 (DOI)000249538900020 ()2-s2.0-34548606418 (Scopus ID)
Note
QC20100616Available from: 2007-06-13 Created: 2007-06-13 Last updated: 2011-10-18Bibliographically approved
3. Modelling of the influence of Laves phase on the creep properties in 9% Cr steels
Open this publication in new window or tab >>Modelling of the influence of Laves phase on the creep properties in 9% Cr steels
2007 (English)In: PROCEEDINGS OF THE ASME PRESSURE VESSELS AND PIPING CONFERENCE / [ed] Jaske, CE; Jaske, CE, 2007, 519-526 p.Conference paper, Published paper (Refereed)
Abstract [en]

Nucleation and growth of Laves phase are calculated for a multi-component system. Coarsening Of MX, M23C6 and Laves are also determined. The influence on creep strength is discussed by analysing particle hardening and solid solution strengthening. A model for particle size distribution is presented in order to determine the amount of dislocations that can climb across particles or generate Orowan loops.

The model for solid solution hardening is based on a solution of Fick's second law with a moving frame of reference for the concentration profiles around a climbing dislocation. This is done in order to determine the slowdown in dislocations velocity due to solute drag. The results show a loss in creep strength as the Laves phase grows.

Keyword
creep, solid solution hardening, climb, Laves phase
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-7325 (URN)10.1115/CREEP2007-26417 (DOI)000253884500059 ()2-s2.0-43449094689 (Scopus ID)978-0-7918-4287-4 (ISBN)
Conference
Pressure Vessels and Piping Conference of the American-Society-of-Mechanical-Engineers Location: San Antonio, TX, JUL 22-26, 2007
Note
QC 20100616Available from: 2007-06-13 Created: 2007-06-13 Last updated: 2011-09-07Bibliographically approved
4. Influence of aluminium on creep strength of 9–12% Cr steels
Open this publication in new window or tab >>Influence of aluminium on creep strength of 9–12% Cr steels
2009 (English)In: Materials Science & Engineering: A, ISSN 0921-5093, 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
Keyword
Creep rate modelling, Aluminium influence, Dislocation evolution, Particle strengthening, Ferritic steels
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-12232 (URN)10.1016/j.msea.2009.07.060 (DOI)000271611500017 ()2-s2.0-71749091664 (Scopus ID)
Note
QC20100616Available from: 2010-03-30 Created: 2010-03-30 Last updated: 2012-03-21Bibliographically approved
5. Growth of Creep Cavities in 12% Cr Steels
Open this publication in new window or tab >>Growth of Creep Cavities in 12% Cr Steels
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
Keyword
Creep, 12% Cr steels, cavity, growth
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-12233 (URN)000266610600083 ()978-1-60595-005-1 (ISBN)
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
6. Modeling Creep Strength of Welded 9 to 12 Pct Cr Steels
Open this publication in new window or tab >>Modeling Creep Strength of Welded 9 to 12 Pct Cr Steels
2010 (English)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 41A, no 13, 3340-3347 p.Article in journal (Refereed) Published
Abstract [en]

The influence of weld simulated heat treatments of 9-12% steels is evaluated by a fundamental model for creep. The heat affected microstructure is predicted by considering particle coarsening, particle dissolution, and subgrain coarsening. Particle coarsening is predicted for a multi-component system, showing significant M23C6 coarsening in the BCC matrix. Dissolution simulations of MX and M23C6 are performed by considering a size distribution of particles, indicating that the smallest particles can be dissolved already at relatively low welding temperatures. Recovery in dislocation networks will take place due to the coarser particles. Creep rate modelling is performed based on the heat affected microstructure, showing strength reduction of weld simulated material by 12% at 850ºC and 26% at 900ºC. The main cause of this degradation is believed to be the loss of the smallest carbonitrides.

Keyword
Carbonitrides, Cr steel, Creep rates, Creep strengths, Dislocation networks, Distribution of particles, Fundamental models, matrix, Multi-component systems, Particle coarsening, Particle dissolution, Simulated materials, Strength reduction, Subgrain-coarsening, Welding temperatures, Carbon nitride, Chromium, Computer simulation, Creep, Dissolution, Welding, Welds
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-12234 (URN)10.1007/s11661-010-0449-2 (DOI)000283943900010 ()2-s2.0-78049442090 (Scopus ID)
Note
QC 2010616. Updated from submitted to published, 20120315Available from: 2010-03-30 Created: 2010-03-30 Last updated: 2012-03-15Bibliographically approved

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