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Creep modelling of particle strengthened steels
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
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 [en]
Particle strengthening, dislocation climb, ferritic steels, dislocation evolution, creep rate modelling
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-4436ISBN: 978-91-7178-720-0 (print)OAI: oai:DiVA.org:kth-4436DiVA: diva2:12304
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
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: 2017-12-14Bibliographically 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: 2017-12-14Bibliographically 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

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