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Structural changes in 12-2.25% Cr weldments: an experimental and theoretical approach
KTH, Superseded Departments, Materials Science and Engineering.
KTH, Superseded Departments, Materials Science and Engineering.
2000 (English)In: Materials at High Temperature, ISSN 0960-3409, E-ISSN 1878-6413, Vol. 17, no 3, 389-396 p.Article 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.

Place, publisher, year, edition, pages
2000. Vol. 17, no 3, 389-396 p.
Keyword [en]
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: urn:nbn:se:kth:diva-12071ISI: 000089735600003OAI: oai:DiVA.org:kth-12071DiVA: diva2:300969
Note
QC20100719Available from: 2010-03-02 Created: 2010-03-02 Last updated: 2017-12-12Bibliographically approved
In thesis
1. Mechanical Properties of Welds at Creep Activation Temperatures
Open this publication in new window or tab >>Mechanical Properties of Welds at Creep Activation Temperatures
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. 69 p.
Keyword
creep, creep crack propagation, OFHC copper, welds, nuclear waste disposal, creep modeling
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-12077 (URN)978-91-7415-567-9 (ISBN)
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

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