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High temperature performance of materials for future power plants
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. (Materials Technology)ORCID iD: 0000-0002-8348-1633
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Increasing energy demand leads to two crucial problems for the whole society. One is the economic cost and the other is the pollution of the environment, especially CO2 emissions. Despite efforts to adopt renewable energy sources, fossil fuels will continue to dominate. The temperature and stress are planned to be raised to 700 °C and 35 MPa respectively in the advanced ultra-supercritical (AUSC) power plants to improve the operating efficiency. However, the life of the components is limited by the properties of the materials. The aim of this thesis is to investigate the high temperature properties of materials used for future power plants.

This thesis contains two parts. The first part is about developing creep rupture models for austenitic stainless steels. Grain boundary sliding (GBS) models have been proposed that can predict experimental results. Creep cavities are assumed to be generated at intersection of subboundaries with subboundary corners or particles on a sliding grain boundary, the so called double ledge model. For the first time a quantitative prediction of cavity nucleation for different types of commercial austenitic stainless steels has been made. For growth of creep cavities a new model for the interaction between the shape change of cavities and creep deformation has been proposed. In this constrained growth model, the affected zone around the cavities has been calculated with the help of FEM simulation. The new growth model can reproduce experimental cavity growth behavior quantitatively for different kinds of austenitic stainless steels. Based on the cavity nucleation models and the new growth models, the brittle creep rupture of austenitic stainless steels has been determined. By combing the brittle creep rupture with the ductile creep rupture models, the creep rupture strength of austenitic stainless steels has been predicted quantitatively. The accuracy of the creep rupture prediction can be improved significantly with combination of the two models.

The second part of the thesis is on the fatigue properties of austenitic stainless steels and nickel based superalloys. Firstly, creep, low cycle fatigue (LCF) and creep-fatigue tests have been conducted for a modified HR3C (25Cr20NiNbN) austenitic stainless steel. The modified HR3C shows good LCF properties, but lower creep and creep-fatigue properties which may due to the low ductility of the material. Secondly, LCF properties of a nickel based superalloy Haynes 282 have been studied. Tests have been performed for a large ingot. The LCF properties of the core and rim positions did not show evident differences. Better LCF properties were observed when compared with two other low γ’ volume fraction nickel based superalloys. Metallography study results demonstrated that the failure mode of the material was transgranular. Both the initiation and growth of the fatigue cracks were transgranular.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2016. , 55 p.
Keyword [en]
Austenitic stainless steels, Nickel based superalloys, Low cycle fatigue, Creep-fatigue, Creep cavitation, Grain boundary sliding, Cavity nucleation, Cavity growth, Creep rupture strength, Advanced ultra-supercritical power plants.
National Category
Materials Engineering
Research subject
Materials Science and Engineering
Identifiers
URN: urn:nbn:se:kth:diva-191547ISBN: 978-91-7729-100-8OAI: oai:DiVA.org:kth-191547DiVA: diva2:957326
Public defence
2016-10-07, Kollegiesalen, Brinellvägen 8, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20160905

Available from: 2016-09-05 Created: 2016-09-01 Last updated: 2016-09-06Bibliographically approved
List of papers
1. Modelling grain boundary sliding during creep of austenitic stainless steels
Open this publication in new window or tab >>Modelling grain boundary sliding during creep of austenitic stainless steels
2016 (English)In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 51, no 6, 2926-2934 p.Article in journal (Refereed) Published
Abstract [en]

Two models are presented for grain boundary sliding (GBS) displacement during creep. GBS is considered as crucial for the formation of creep cavities. In the first model, the shear sliding model, GBS is accommodated by grains freely sliding along the boundaries in a power-law creeping material. The GBS rate is proportional to the grain size. In the second model, the shear crack model, the sliding boundaries are represented by shear cracks. The GBS rate is controlled by particles in the boundaries. In both models, the GBS displacement rate is proportional to the creep strain rate. Both models are consistent with existing experimental observations for GBS during creep of austenitic stainless steels. For cavity nucleation at particles, Harris’ model (1965) for the relationship between GBS and a critical particle size has been analysed and found to be in agreement with observations.

Place, publisher, year, edition, pages
Springer-Verlag New York, 2016
Keyword
CAVITY NUCLEATION, POLYCRYSTALS, CAVITATION, PARTICLES, DUCTILITY, FRACTURE, CRACK, PHOSPHORUS, ADDITIONS, STRENGTH
National Category
Metallurgy and Metallic Materials Applied Mechanics
Identifiers
urn:nbn:se:kth:diva-180908 (URN)10.1007/s10853-015-9601-0 (DOI)000367681300014 ()2-s2.0-84953346122 (ScopusID)
Note

QC 20160129. QC 20160205

Available from: 2016-01-29 Created: 2016-01-25 Last updated: 2016-09-01Bibliographically approved
2. Formation of creep cavities in austenitic stainless steels
Open this publication in new window or tab >>Formation of creep cavities in austenitic stainless steels
2016 (English)In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 51, no 14, 6674-6685 p.Article in journal (Refereed) Published
Abstract [en]

The possibility of creep cavity formation at subboundaries in austenitic stainless steels is analysed. It is demonstrated that such nucleation is thermodynamically feasible. A minimum stress must be exceeded in order to create cavities. The nucleation is assumed to take place where subboundaries on one side of a sliding grain boundary meet subgrain corners on the other side (double ledge models). Alternative cavitation positions can be found where particles meet subboundaries. The nucleation model can quantitatively predict the observed nucleation rate. The model gives a nucleation rate that is proportional to the creep rate in agreement with many experiments

Place, publisher, year, edition, pages
Springer Science+Business Media B.V., 2016
Keyword
Austenite, Austenitic stainless steel, Creep, Grain boundaries, Grain boundary sliding, Stainless steel, Creep cavity, Creep rates, Minimum stress, Nucleation models, Nucleation rate, Subgrains
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-187070 (URN)10.1007/s10853-016-9954-z (DOI)000375317100011 ()2-s2.0-84963670581 (ScopusID)
Note

QC 20160517

Available from: 2016-05-17 Created: 2016-05-17 Last updated: 2016-09-01Bibliographically approved

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