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On the low primary water stress corrosion cracking susceptibility of weld deformed Alloy 690
KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.). KTH Royal Institute of Technology.ORCID iD: 0000-0002-2641-7838
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

It has been shown in recent years that the nickel-base Alloy 690 can become susceptible to stress corrosion cracking (SCC) in the primary water of nuclear power plants with pressurized water reactors if it has been sufficiently deformed at room temperature. Although the material is not intentionally used in a deformed state, it can become deformed by various manufacturing processes. Welding is believed to be the process that is most likely to cause susceptibility, yet it does not seem quite that detrimental in experiments. The overall purpose of this work was to investigate why weld-induced deformation does not seem to cause the same degree of susceptibility as cold deformation.

The work started with a microstructural investigation, presented in Paper A, to assess if any of the changes caused by welding can explain the difference in behavior. While a beneficial change in the microstructure was observed, it was not enough to explain the differences.

The focus was then turned towards addressing knowledge gaps of the method used to assess weld-induced deformation. This method is based on measuring misorientations using electron backscatter diffraction (EBSD). It was shown in Paper B that kernel average misorientation (KAM) is closer related to the degree of hardening than the degree of deformation, and that it can be used to obtain a qualitative map of hardness at the micrometer scale. Improvements to the KAM-based method were presented in Paper C along with estimates near welds from component mockups.

The validity of using a misorientation-based method on warm deformation was tested in Paper D. It was shown that the method gives a rough estimate of the degree of strain hardening, although the data suggests it is a small overestimation. The overestimation would mean that weld deformation may have a lower hardness than the strain estimate implies, which is beneficial for SCC resistance.

Abstract [sv]

Under senare år har det visats att nickelbasen Alloy 690 kan bli känslig för spänningskorrosion (SCC) i primärvatten av kärnkraftverk med tryckvatten-reaktorer om det har blivit tillräckligt deformerat i rumstemperatur. Även om materialet inte medvetet används i ett deformerat tillstånd, så kan det bli deformerat av olika tillverkningsprocesser. Svetsning är den process som anses mest sannolik att orsaka känslighet, dock verkar den inte vara riktigt så skadlig i experiment. Det övergripande målet med det här arbetet var att undersöka varför svetsinducerad deformation inte verkar orsaka samma nivå av känslighet som kalldeformation.

Arbetet började med en mikrostrukturell undersökning, presenterad i Artikel A, för att bedöma om någon av förrändringarna orsakade av svetsning kan förklara skillnaderna i beteende. Även om en gynnsam förrändring i mikrostrukturen observerades så var det inte tillräckligt för att förklara skillnaderna.

Fokusen vändes istället mot att adressera kunskapsbristerna i metoden som användes för att uppskatta den svetsinducerade deformationen. Denna metod är baserad på att mäta misorienteringar med bakåtspridd elektrondiffraktion (EBSD). Det visades i Artikel B att kärn-medel-misorientering (KAM) är närmare besläktat med nivån av hårdnande än nivån av deformation, och att det kan användas för att erhålla en kvalitativ karta över hårdhet på mikrometerskalan. Förbättringar till en KAM-baserad metod presenterades i Artikel C tillsammans med uppskattningar nära svetsar från komponentattrapper.

Giltigheten i att använda en misorienteringsbaserad metod för varmdeformation testades i Artikel D. Det visades att metoden ger en grov uppskattning av nivån av hårdnande, dock antyder data att det är en liten överskattning. Överskattningen skulle betyda att svetsdeformerat material kan ha en lägre hårdhet än töjningsuppskattningen antyder, vilket är gynnsamt för SCC-resistens.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2017. , p. 42
Series
TRITA-HFL. Report / Royal Institute of Technology, Solid Mechanics, ISSN 1654-1472 ; 613
National Category
Metallurgy and Metallic Materials
Research subject
Solid Mechanics; Materials Science and Engineering
Identifiers
URN: urn:nbn:se:kth:diva-202573ISBN: 978-91-7729-294-4 (print)OAI: oai:DiVA.org:kth-202573DiVA, id: diva2:1077636
Public defence
2017-03-29, F3, Lindstedtsvägen 26, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20170301

Available from: 2017-03-01 Created: 2017-02-28 Last updated: 2017-03-01Bibliographically approved
List of papers
1. Experimental and theoretical investigation of three Alloy 690 mockup components: Base metal and welding induced changes
Open this publication in new window or tab >>Experimental and theoretical investigation of three Alloy 690 mockup components: Base metal and welding induced changes
2014 (English)In: International Journal of Nuclear Energy, ISSN 2314-6060, Vol. 2014Article in journal (Refereed) Published
Abstract [en]

The stress corrosion cracking (SCC) resistance of cold deformed thermally treated (TT) Alloy 690 has been questioned in recent years. As a step towards understanding its relevancy for weld deformed Alloy 690 in operating plants, Alloy 690 base metal and heat affected zone (HAZ) microstructures of three mockup components have been studied. All mockups were manufactured using commercial heats and welding procedures in order to attain results relevant to the materials in the field. Thermodynamic calculations were performed to add confidence in phase identification as well as understanding of the evolution of the microstructure with temperature. Ti(C,N) banding was found in all materials. Bands with few large Ti(C,N) precipitates had negligible effect on the microstructure, whereas bands consisting of numerous small precipitates were associated with locally finer grains and coarser M23C6 grain boundary carbides. The Ti(C,N) remained unaffected in the HAZ while the M23C6 carbides were fully dissolved close to the fusion line. Cold deformed solution annealed Alloy 690 is believed to be a better representation of this region than cold deformed TT Alloy 690.

Place, publisher, year, edition, pages
Hindawi Publishing Corporation, 2014
Keyword
nickel-base, banding, heat affected zone
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-152635 (URN)10.1155/2014/504927 (DOI)
Note

QC 20140930

Available from: 2014-09-29 Created: 2014-09-29 Last updated: 2017-03-01Bibliographically approved
2. Spatial correlation between local misorientations and nanoindentation hardness in nickel-base alloy 690
Open this publication in new window or tab >>Spatial correlation between local misorientations and nanoindentation hardness in nickel-base alloy 690
2016 (English)In: Journal of Materials Science and Engineering: A, ISSN 2161-6213, Vol. 674, p. 171-177Article in journal (Refereed) Published
Abstract [en]

Misorientation increases with plastic strain in metals, and this observation has been used as an empirical assessment of plastic strain in recent years. The method has been validated for a sample area corresponding to a 100 µm×100 µm square, but on the micrometer scale misorientations no longer seem to correlate with plastic strain. Misorientations are however not dependent on plastic strain but rather on dislocation density, which means it should also be related to hardness. Therefore, we have in this work compared maps of predicted hardness calculated from misorientation determination with maps of actual hardness measured by nanoindentation. It was shown that the predicted and measured hardness maps do indeed correlate spatially in nickel-base Alloy 690, although the measured values have a significantly smaller hardness variation. This is explained by a presumably high and uniform density of statistically stored dislocations, which contribute to hardness but do not affect the misorientation determination from electron backscatter diffraction. Thus local misorientation can be used to qualitatively map the local effective plastic strain distribution, for example to identify regions of increased hardness.

Place, publisher, year, edition, pages
Elsevier, 2016
Keyword
EBSD, Nanoindentation, Local Misorientation, Hardening, Plasticity, Nickel Based Superalloys
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-193478 (URN)10.1016/j.msea.2016.07.123 (DOI)000383292800022 ()2-s2.0-84982839184 (Scopus ID)
Note

QC 20161004

Available from: 2016-10-03 Created: 2016-10-03 Last updated: 2017-11-30Bibliographically approved
3. Plastic strain assessment of Alloy 690 heat affected zones from component mockups using KAM and GOS
Open this publication in new window or tab >>Plastic strain assessment of Alloy 690 heat affected zones from component mockups using KAM and GOS
2017 (English)Report (Other academic)
Abstract [en]

The plastic strain level in Alloy 690 heat affected zones (HAZs) of three component mockups have been assessed using misorientations quantified by electron backscatter diffraction (EBSD). Kernel average misorientation (KAM) and grain orientation spread (GOS) were used in this work, and both gave the same results. The plastic strain increased towards the weld interface in all mockups and reached around 0.05 logarithmic strain for two mockups, and 0.10 for the third.While GOS was straightforward to use, KAM was shown to be sensitive to measurement imprecision, and also dependent on EBSD step length, kernel design and average grain size. This work shows how these drawbacks of KAM can be overcome. The results suggest that KAM’s size dependencies can be interpreted as a dependency on the ratio between kernel length scale and the average grain size of the material.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2017. p. 12
Series
TRITA-HFL. Report / Royal Institute of Technology, Solid Mechanics, ISSN 1654-1472 ; 611
Keyword
nickel-base alloy, heat affected zone, plastic deformation, electron backscatter diffraction
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-202570 (URN)
Note

QC 20170301

Available from: 2017-02-28 Created: 2017-02-28 Last updated: 2017-03-03Bibliographically approved
4. Comparison of EBSD-based plastic strain estimation of Alloy 690 strained at 500–650 °C and at room temperature
Open this publication in new window or tab >>Comparison of EBSD-based plastic strain estimation of Alloy 690 strained at 500–650 °C and at room temperature
2017 (English)Report (Other academic)
Abstract [en]

The validity of plastic strain estimation using electron backscatter diffraction (EBSD) in a warm deformed material with cold deformed reference materials has been investigated. Nickel-base Alloy 690, recovery heat treated at 1050 °C, was used in this study and deformed at room temperature and at 500– 650 °C. Grain orientation spread (GOS) was used as misorientation metric.Both GOS and hardness of the warm deformed materials were lower than for cold deformed materials of comparable applied strain, and was attributed to dynamic recovery. The hardness of the warm deformed materials was nonetheless comparable to cold deformed materials with similar GOS, although being slightly lower on average. These results show that GOS does not give an accurate estimate of applied deformation for warm deformation. It still gives a rough estimate of the effective plastic strain, albeit results suggest it may be a slight overestimation.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2017. p. 9
Series
TRITA-HFL. Report / Royal Institute of Technology, Solid Mechanics, ISSN 1654-1472 ; 612
Keyword
electron backscatter diffraction, plastic deformation, warm deformation, hardness, nickel-base alloy
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering; Solid Mechanics
Identifiers
urn:nbn:se:kth:diva-202571 (URN)
Note

QC 20170301

Available from: 2017-02-28 Created: 2017-02-28 Last updated: 2017-03-03Bibliographically approved

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