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Strengthening and degradation mechanisms in austenitic stainless steels at elevated temperature
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

With rapid economic developments and rising living standards, the demand for electricity all over the world is greatly increased. Due to high fuel costs, the steam boilers with higher steam temperature and pressure are needed to decrease the cost of power generation throughout the world extensively. In recent years, human awareness of the gradual strengthening of environmental protection increases, therefore to reduce the CO2 emissions the power generation efficiency needs to be improved. The development of high temperature materials with improved creep rupture strength and oxidation resistance is critically needed. Materials for these demanding conditions are austenitic stainless steels such as 310, 310NbN and Sanicro 25.

Fundamental models have been developed for the precipitation of coarse particles during long time ageing of austenitic stainless steels and the influence of the particles on the mechanical properties. The models have been verified by ageing experiments. The austenitic stainless steel 310 was aged for up to 5000 h at 800 ºC. The precipitation models could satisfactorily describe the influence of ageing time on the radii and the volume fractions of particles. Models for the influence of the coarse precipitates on the tensile properties and the toughness were developed and reproduce the measured mechanical properties without the use of any fitting parameters. These developed models were utilised to investigate the influence of bands on ductility and toughness at room temperature. Up to 10 % σ-phase was observed to precipitate, which has a pronounced influence of the mechanical properties. Thermodynamic analysis demonstrated that the amount of precipitates due to ageing can significantly be reduced if the nitrogen or the carbon content is increased.

Microstructure investigations of austenitic stainless steel 310NbN and Sanicro 25 were carried out by light microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). The austenitic stainless steel 310NbN was aged for up to 10000 h at 650 and 750 ºC. The austenitic stainless steel Sanicro 25 was also aged for up to 10000 h at 650 and 700 ºC. Phase fractions and mean radii evolution of precipitates were calculated and compared to the experimental results. Size distributions of the precipitates in these steels were determined. Models for the different contributions to the creep strength have been applied: i) a recovery creep model for the dislocation hardening; ii) a climb controlled model for the precipitation hardening; iii) solid solution hardening from Cottrell clouds of solutes around the dislocations, and iv) A modified Dobes model for the effective stress. The total contributions can describe the experimental creep strength satisfactorily without the use of adjustable parameters.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. , ix, 56 p.
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:kth:diva-122158ISBN: 978-91-7501-777-8 (print)OAI: oai:DiVA.org:kth-122158DiVA: diva2:621120
Public defence
2013-06-04, Sal B1, Brinellvägen 23, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20130517

Available from: 2013-05-17 Created: 2013-05-13 Last updated: 2013-05-17Bibliographically approved
List of papers
1. Precipitation during long time ageing in the austenitic stainless steel 310
Open this publication in new window or tab >>Precipitation during long time ageing in the austenitic stainless steel 310
2012 (English)In: Materials at High Temperature, ISSN 0960-3409, E-ISSN 1878-6413, Vol. 29, no 1, 8-16 p.Article in journal (Refereed) Published
Abstract [en]

Extensive precipitation can occur in high temperature materials during service. This is known to significantly affect the mechanical properties. To be able to study this phenomenon, particle formation in the austenitic stainless steel 310 has been quantified. In particular, sigma-phase and M23C6-carbides have been analysed. Specimens have been aged up to 5000 h at 800 degrees C. The precipitation of the two phases has been modelled. The basis of the model is diffusion-controlled growth. sigma-phase shows a spherical growth and an M23C6 planar growth. The soft impingement of the diffusion zones with respect to both chromium and carbon is taken into account. The influence on mechanical properties is analysed in a parallel paper.

Keyword
austenitic stainless, long time ageing, elevated temperature, precipitation, thermodynamic modelling
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-95276 (URN)10.3184/096034012X13269748502792 (DOI)000303187900002 ()2-s2.0-84864353855 (Scopus ID)
Note
QC 20120521Available from: 2012-05-21 Created: 2012-05-21 Last updated: 2017-12-07Bibliographically approved
2. Influence of particle formation during long time ageing on mechanical properties in the austenitic stainless steel 310
Open this publication in new window or tab >>Influence of particle formation during long time ageing on mechanical properties in the austenitic stainless steel 310
2012 (English)In: Materials at High Temperature, ISSN 0960-3409, E-ISSN 1878-6413, Vol. 29, no 1, 1-7 p.Article in journal (Refereed) Published
Abstract [en]

Intense precipitation may occur in high-temperature materials during service. This precipitation significantly affects the mechanical properties. Coarse precipitates can reduce ductility and toughness dramatically. To investigate the phenomenon, precipitation in the austenitic stainless steel AISI 310 is quantified in a parallel paper. The samples were aged up to 5000 h at 800 degrees C. Especially, sigma-phase and M23C6-carbides were analysed. A model for the influence of coarse particles on ductility has been developed and successfully applied to the measured values. A model due to Chen and Mott has been used to compute the effect of particles on toughness. A satisfactory agreement with the observations was obtained.

Keyword
austenitic stainless, long time ageing, elevated temperature, precipitation, ductility, toughness
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-95275 (URN)10.3184/096034012X13269825057728 (DOI)000303187900001 ()2-s2.0-84864356264 (Scopus ID)
Note
QC 20120521Available from: 2012-05-21 Created: 2012-05-21 Last updated: 2017-12-07Bibliographically approved
3. Influence of long time ageing on ductility and toughness in the stainless steel 310 in the presence of banded microstructure
Open this publication in new window or tab >>Influence of long time ageing on ductility and toughness in the stainless steel 310 in the presence of banded microstructure
2012 (English)In: La Metallurgia Italiana, ISSN 0026-0843, Vol. 104, no 11-12, 33-38 p.Article in journal (Refereed) Published
Abstract [en]

During service at elevated temperatures extensive formation of particles can take place that can have a dramatic influence on mechanical properties. Precipitation of sigma-phase and M23C6-carbides have been studied both experimentally and with thermodynamic modelling for 25Cr20Ni austenitic stainless steels (AISI 310) at 800 degrees C for up to 5000 h. Previous work has demonstrated that the modelling could describe the nucleation and growth satisfactory. After long term ageing the particles form bands in the microstructure. In the present paper the influence of these bands on ductility and toughness at room temperature is analysed. For this purpose previously developed models for ductility and toughness are utilised. Model values for banded and non-banded microstructures have been generated for casts of 310 in fine and coarse grained conditions with 0.04 and 0.12%N The model values show that in the coarse grained condition, no reduction in ductility and toughness can be expected in the banded microstructure. In the fine grained condition a modest reduction is predicted.

Keyword
austenitic stainless, long time ageing, elevated temperature, precipitation, ductility, toughness
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-118264 (URN)000313596700004 ()2-s2.0-84874151727 (Scopus ID)
Conference
7th European Stainless Steel Conference, SEP 21-23, 2011, Como, Italy
Note

QC 20130214

Available from: 2013-02-14 Created: 2013-02-14 Last updated: 2017-12-06Bibliographically approved
4. Basic creep models for 25Cr20NiNbN austenitic stainless steels
Open this publication in new window or tab >>Basic creep models for 25Cr20NiNbN austenitic stainless steels
2013 (English)In: Materials research innovations (Print), ISSN 1432-8917, E-ISSN 1433-075X, Vol. 17, no 5, 355-359 p.Article in journal (Refereed) Published
Abstract [en]

Basic models for solid solution and precipitation hardening during creep are presented for the austenitic stainless steels 25Cr20NiNbN (TP310HNbN, HR3C, DMV310N). The solid solution hardening is a result of the formation of Cottrell clouds of solutes around the dislocations. In addition to slowing down the creep, the solutes increase the activation energy for creep. The increase in activation energy corresponds to the maximum binding energy between the solutes and the dislocations. The formation of fine niobium nitrides during service enhances the creep strength. It is found that the nitrides have an exponential size distribution. In the modelling the critical event is the time it takes for a dislocation to climb over a particle. The creep models can accurately describe the observed time and temperature dependence of the creep rupture strength.

Keyword
Austenitic stainless, Elevated temperature, Precipitation, Solid solution hardening
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-122155 (URN)10.1179/1433075X13Y.0000000120 (DOI)000322787400012 ()2-s2.0-84923125204 (Scopus ID)
Note

QC 20130905

Available from: 2013-05-13 Created: 2013-05-13 Last updated: 2017-12-06Bibliographically approved
5. Numerical modelling and validation of precipitation kinetics in advanced creep resistant austenitic steel
Open this publication in new window or tab >>Numerical modelling and validation of precipitation kinetics in advanced creep resistant austenitic steel
Show others...
2012 (English)In: Computer Methods in Materials Science, ISSN 1641-8581, Vol. 12, 175-182 p.Article in journal, Editorial material (Refereed) Published
Abstract [en]

The austenitic steel Sanicro 25 is one of the most promising austenitic steels for the application in superheater tubes in coal fired thermal power plants. In this work, the microstructural evolution of this material during heat treatment and thermal ageing has been investigated. The investigations were carried out by light microscopy (LIMI), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). Scheil calculations were carried out by thermo-kinetic software MatCalc to analyse the solidification process which indicates Nb(C,N), Cr2N and Laves phase in the melt. Long term precipitation calculations predict the formation of five precipitate types in Sanicro 25: M23C6, Z-phase, Nb(C,N), Laves and Cr2N. Phase fractions and mean radii evolution of precipitates were calculated and compared to the experimental results. Calculated precipitate evolution shows good compliance with experimental data.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-122156 (URN)
Note

QC 20130517

Available from: 2013-05-13 Created: 2013-05-13 Last updated: 2017-12-06Bibliographically approved
6. Precipitation hardening and other contributions to the creep strength of an 23Cr25NiWCuCo austenitic stainless steel
Open this publication in new window or tab >>Precipitation hardening and other contributions to the creep strength of an 23Cr25NiWCuCo austenitic stainless steel
(English)Manuscript (preprint) (Other academic)
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-122283 (URN)
Note

QS 2013

Available from: 2013-05-17 Created: 2013-05-17 Last updated: 2013-05-17Bibliographically approved

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PhD Thesis(1289 kB)5340 downloads
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