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Mechanical Properties of Welds at Creep Activation Temperatures
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Welds in materials intended for service at temperatures above the creep activation temperature often develop damage before the base metal. The weld is a discontinuity in the material and stresses and strains often accumulate in the weld. Knowledge of the properties of the weld is essential to the safe operation of the component containing the weld. The work in this thesis has been aimed at the study of welds in service at high temperatures: The work is divided into two main chapters. The first chapter deals with welds in stainlesssteels and dissimilar metal welds and includes three papers, and the second chapter dealswith welds in copper intended for nuclear waste disposal, also including three papers. Common to both parts is that the temperature is high enough for most of the damage in the welds to result from creep.

In the first part the role of the weld microstructure on the creep crack propagation properties has been studied. Experiments using compact tension specimens have been performed on service exposed, low alloyed heat resistant steels. The results show good correlation with the crack tip parameter, C*, during steady state creep crack growth. The test methodology has also been reviewed and sensitive test parameters have been identified. The results from the creep crack propagation tests on service exposed material has been modeled using uniaxial creep data on both new and ex-service material. The development of the weld microstructure in a dissimilar metal weld between two heat resistant steels has also been investigated. A weld was made between one ferritic and one martensitic steel and the development of the microstructure during welding and post-weldheat treatments has been studied. The results show that the carbon depleted zone that develops near the weld metal in the lower alloyed steel depends on the formation and dissolution of the M23C6-carbide. Variations of the weld parameters and the post-weld heat treatment affect the size and shape of this zone. The process has been successfully modeled by computer simulation.

The second part focuses on oxygen free copper intended for nuclear waste disposal containers. The containers are made with an inner core of cast nodular iron and an outer core of copper for corrosion protection. The copper shell has to be welded and two weld methods has been tested, electron beam welding and friction stir welding. Creep specimens taken from both weld types have been tested as have base metal specimens. The technical specifications of the waste canisters demand that the creep ductility of both the copper shell and the welds has to be as high as possible. The creep test results show that base material doped with at least 30 ppm phosphorus has high creep ductility, and friction stir welds made from this material has almost as high creep strength and creep ductility. Copper without phosphorus does not exhibit the same ductility. The creep properties evaluated from testing has been modeled and extrapolated for the intended purpose

Place, publisher, year, edition, pages
Stockholm: KTH , 2010. , p. 69
Keyword [en]
creep, creep crack propagation, OFHC copper, welds, nuclear waste disposal, creep modeling
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-12077ISBN: 978-91-7415-567-9 (print)OAI: oai:DiVA.org:kth-12077DiVA, id: diva2:301009
Public defence
2010-03-05, Sal E3, Lindstedtsvägen 3, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC20100719Available from: 2010-03-02 Created: 2010-03-02 Last updated: 2010-07-19Bibliographically approved
List of papers
1. Creep crackgrowth in ex service weld metal of 0.5CrMoV
Open this publication in new window or tab >>Creep crackgrowth in ex service weld metal of 0.5CrMoV
1999 (English)In: Cape 99: Wilderness, Cape province, South Africa, 12-16 April (1999), 1999Conference paper, Published paper (Refereed)
Abstract [en]

Accurate assessment of the integrity of high temperature components will be of ever increasing importance. The reason for this is that many power plants have reached and exceeded their design life and the number of detected defects increases. This is accentuated by the improvement of the methods for non-destructive testing which means that more and smaller defects will be detected. The possibility to assess the influence of defects on the integrity of high temperature components, will be of vital importance to maintain safe and cost effective power plants.

The aim of the present work is to increase the understanding of the influence of service exposure on the remaining life of components in a high temperature plant. The investigation aims to creep test exserviceweld material, 14MoV 6 3, from a Swedish power plant. Thematerial has been in service for a period of about 80 000 hours at atemperature of 530-540 °C and with a nominal hoop stress of 52MPa.Both uniaxial and compact tension creep tests have been performedat a temperature of 550 °C. The stress range used was between 130MPa and 170 MPa for the uniaxial creep tests. For the creep crack growth tests the reference stress was ranging between 122 MPa and146 MPa.

A remaining life assessment according to the R5 procedure is included, where material data from the present experimental study is used. The analysis suggests that a defect or a crack with a depth of 2 mm and a length of 5 mm can be left unattended for a season of service under the condition that the service parameters are not changed. A comparison with the assessment of cracks, found in the same plant as the material for the experimental studies came from, and their known extension during service, is included. A parametric study where load level and type of initial defect/crack are varied is also included.

National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-12076 (URN)
Note
QC20100719Available from: 2010-03-02 Created: 2010-03-02 Last updated: 2010-07-19Bibliographically approved
2. Structural changes in 12-2.25% Cr weldments: an experimental and theoretical approach
Open this publication in new window or tab >>Structural changes in 12-2.25% Cr weldments: an experimental and theoretical approach
2000 (English)In: Materials at High Temperature, ISSN 0960-3409, E-ISSN 1878-6413, Vol. 17, no 3, p. 389-396Article in journal (Refereed) Published
Abstract [en]

In welds between low alloy and 12% Cr-steels, a depletion of carbon occurs in the material of lower carbon activity, the low alloy steel, during post weld heat treatment. A carbon depleted zone is formed along the fusion boundary. This zone consists of large ferritic grains and affects the creep strength of the material by promoting type IIIa cracking. In this work the absence of carbides in the carbon depleted zone is verified by transmission electron microscopy, and identified as the cause of the grain growth. A computer simulation of the diffusion processes with the DICTRA software shows good agreement with experimental microprobe carbon content measurements. The simulations could also predict the types of carbides appearing in the weld zone. Methods for reducing the carbon depletion by varying the weld and heat treatment parameters are identified.

Keyword
12-2.25% Cr weldments, structural changes, computer simulation, TEM, creep strength, post weld heat treatment, diffusion, FE-NI ALLOYS, SYSTEMS, CARBON
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-12071 (URN)000089735600003 ()
Note
QC20100719Available from: 2010-03-02 Created: 2010-03-02 Last updated: 2017-12-12Bibliographically approved
3. Creep crack growth in service-exposed weld metal of 2.25Cr1Mo
Open this publication in new window or tab >>Creep crack growth in service-exposed weld metal of 2.25Cr1Mo
2001 (English)In: International Journal of Pressure Vessels and Piping, ISSN 0308-0161, E-ISSN 1879-3541, Vol. 78, no 11-12, p. 749-755Article in journal (Refereed) Published
Abstract [en]

Creep crack growth (CCG) has been studied for ex-service weld metal of 2.25Cr1Mo (P22). The testing was conducted at a temperature of 550degreesC and prior to testing, the material had been exposed to high temperature service for 110 000 h at 530 C. The results show a marked effect of the service exposure on the CCG properties of the material when compared to similar testing performed on a new material. The CCG rate was higher by a factor 3.1 in the service-exposed material, which should be compared to the model value of 3.2 based on the relations between the elongation values. The consumed deformation capacity was also estimated with the omega model for tertiary creep. In this case, an enhanced growth rate of 2.4 was obtained. Within a distance of about 10 mm in front of the propagating cracks, the number of creep cavities was significantly higher than in the surrounding material. The variation of the density of cavities as a function of distance from the crack tip was successfully modelled. (C) 2002 Published by Elsevier Science Ltd.

Keyword
creep crack growth, CrMo-steel, P22, ex-service, creep cavity, STRAIN
Identifiers
urn:nbn:se:kth:diva-12072 (URN)10.1016/S0308-0161(01)00086-2 (DOI)000174338300004 ()
Note
QC20100719Available from: 2010-03-02 Created: 2010-03-02 Last updated: 2017-12-12Bibliographically approved
4. Creep testing of thick-wall copper electron beam and friction stir welds
Open this publication in new window or tab >>Creep testing of thick-wall copper electron beam and friction stir welds
2004 (English)In: Materials Research Society Symposium Proceedings, ISSN 0272-9172, E-ISSN 1946-4274, Vol. 824, p. 51-56Article in journal (Refereed) Published
Abstract [en]

Thick section copper canisters are planned to be used as a corrosion protection of nuclear waste disposal containers for long term underground deposal in Sweden. The copper canisters will have the top and possibly the bottom lid welded to the canister walls using electron beam or friction stir welding. Due to the high external hydrostatic pressure and the relatively high temperature of the waste during the first one hundred years the copper will creep. The creep process will close the manufacturing gap between the cast iron container and the copper canister. The creep ductility must be sufficient to avoid cracking of the weld. Specimens cut from the friction stir welds and the electron beam welds have been creep tested at temperatures ranging from 75 to 175 degreesC. Cross-weld specimens were used for both friction stir and electron beam welds. Weld metal, heat affected zone and base metal were also studied for friction stir welds. The results for the electron beam welds show that the main creep deformation is concentrated to the weld metal where the failure takes place. Weld metal and most cross-weld tests of friction stir weld material show similar creep lives and ductility as base metal tests. Ductility at rupture was found to exceed 30% for friction stir weld specimens, and the Norton power law exponent was determined to be between 30 and 50.

Keyword
Materials Science
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-12073 (URN)000224543500008 ()1-55899-774-1 (ISBN)
Note
QC20100719Available from: 2010-03-02 Created: 2010-03-02 Last updated: 2017-12-12Bibliographically approved
5. Influence of phosphorus, sulphur and grain size on creep in pure copper
Open this publication in new window or tab >>Influence of phosphorus, sulphur and grain size on creep in pure copper
2009 (English)In: Materials Science and Technology, ISSN 0267-0836, E-ISSN 1743-2847Article in journal (Other academic) Submitted
Abstract [en]

Uniaxial creep tests have been performed at 175 °C to study the influence of phosphorus, sulphur and grain size on the creep properties of oxygen free copper. Copper with no phosphorous content and copper with 2000 μm grain size showed lower creep strength and ductility than the reference material which contained 58 ppm phosphorous and had 350 μm average grain size. Phosphorous content of 29 and 106 ppm showed no difference in relation to the reference material, and neither did grain sizes of 100 and 800 μm average grain size. 6 or 12 ppm sulphur did not affect the creep properties at all. The main creep rupture mechanisms were found to be cavitation and microcracking at the grain boundaries. The observed influence of P on creep is consistent to previously published models both with respect to creep rate and creep ductility.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-12074 (URN)
Note
QS 20120315Available from: 2010-03-02 Created: 2010-03-02 Last updated: 2017-12-12Bibliographically approved
6. Testing and modelling of creep in copper friction stir welds
Open this publication in new window or tab >>Testing and modelling of creep in copper friction stir welds
2009 (English)In: Materials Science and Technology, ISSN 0267-0836, E-ISSN 1743-2847Article in journal (Other academic) Submitted
Abstract [en]

Specimens cut from friction stir welds in copper canisters for nuclear waste have been used for creep experiments at 75°C. The specimens were taken from a cross-weld position as well as heat affected zone and weld metal. The weld specimens exhibited shorter creep lives than the parent metal specimens by a factor of three in time. The cross weld and HAZ specimens were shorter by an order of magnitude when compared to the weld metal. The creep exponent was in the interval 50 to 69 implying that the material was well inside the power-law breakdown regime. The ductility properties expressed as reduction in area were not significantly different in the weld zones and all the rupture specimens demonstrated valu esexceeding 80%. The stationary creep rate for the parent metal was consistent with a previously developed model. The primary creep was successfully modelled. Weld reduction factors have been obtained by comparing the results from base metal tests and weld tests. Measured values at 75 °C for are about 6% for friction stir welds and 14% for electron beam welds.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-12075 (URN)
Note
QS 20120315Available from: 2010-03-02 Created: 2010-03-02 Last updated: 2017-12-12Bibliographically approved

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