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Loss of constraint during fracture toughness testing of duplex stainless steels
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.ORCID iD: 0000-0002-8494-3983
2013 (English)In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 99, 239-250 p.Article in journal (Refereed) Published
Abstract [en]

Delamination of the fracture surfaces, so called splits, is an important phenomenon that occurs at sub-zero temperature for hot-rolled duplex stainless steels during impact and fracture toughness testing. To evaluate how the splits influence the fracture toughness, sub-zero temperature fracture toughness testing of 50, 30 and 10 mm thick plates of hot rolled 2205 duplex stainless steel was performed. The results show that the splits cause loss of constraint along the crack front. This can be observed as local difference in crack growth in the specimen. The initiation fracture toughness is not influenced by the specimen thickness. Furthermore, due to the delamination the material exhibits a stable fracture process despite the presence of cleavage fracture. This is interfering with the master curve method so for evaluating the fracture toughness at sub-zero temperatures an assessment of the fracture resistance curve is instead suggested. For assessing the brittle crack behaviour at sub-zero temperatures it is proposed to use the split initiation as a "failure" criteria. The splits are also the cause of the pop-in behaviour observed for the duplex stainless steels. The susceptibility for pop-in is influenced by the microstructure.

Place, publisher, year, edition, pages
2013. Vol. 99, 239-250 p.
Keyword [en]
Duplex stainless steel, Fracture toughness, Delamination, Splits, Pop-in, Normalization, Master curve
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-122353DOI: 10.1016/j.engfracmech.2013.01.002ISI: 000317325100019Scopus ID: 2-s2.0-84875241165OAI: oai:DiVA.org:kth-122353DiVA: diva2:622545
Note

QC 20130522

Available from: 2013-05-22 Created: 2013-05-20 Last updated: 2017-12-06Bibliographically approved
In thesis
1. The fracture mechanisms in duplex stainless steels at sub-zero temperatures
Open this publication in new window or tab >>The fracture mechanisms in duplex stainless steels at sub-zero temperatures
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The aim of the thesis was to study the susceptibility for brittle failures and the fracture process of duplex stainless steels at sub-zero temperatures (°C). In the first part of the thesis plates of hot-rolled duplex stainless steel with various thicknesses were used to study the influence of delamination (also known as splits) on the fracture toughness. The methods used were impact and fracture toughness testing. Light optical microscopy and scanning electron microscopy were used to investigate the microstructure and fracture surfaces. It was concluded that the delaminations caused a loss of constraint along the crack front which resulted in a stable fracture process despite the presence of cleavage cracks. These delaminations occurred when cleavage cracks are constrained by the elongated austenite lamellae. The pop-in phenomenon which is frequently observed in duplex stainless steels during fracture toughness testing was shown to occur due to these delaminations. The susceptibility for pop-in behaviour during testing increased with decreasing plate thickness. The toughness anisotropy was also explained by the delamination phenomenon.In the second part of the thesis duplex stainless steel weld metals from lean duplex and super duplex were investigated. For the lean duplex weldments with different nickel contents, tensile, impact and fracture toughness testing were conducted from room temperature to sub-zero temperatures. The result showed that increased nickel content decreased the susceptibility for critical cleavage initiation at sub-zero temperatures. The super duplex stainless steel weldment was post weld heat treated. The fracture sequence at low temperature was critical cleavage fracture initiation after minor crack-tip blunting and ductile fracture. Energy-dispersive X-ray spectroscopy investigation of the weld metals showed that substitutional element partitioning is small in the weld metal. However, for the post weld heat treated weldments element partitioning occurred which resulted in decreased nickel content in the ferrite.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. vii, 58 p.
Keyword
Duplex stainless steel, Weldments, Delamination, Fracture toughness, Impact toughness, Cleavage fracture, Nickel
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-133677 (URN)978-91-7501-931-4 (ISBN)
Public defence
2013-11-29, B3, Brinellvägen 23, KTH, Stockholm, 10:00 (Swedish)
Opponent
Supervisors
Note

QC 20131108

Available from: 2013-11-08 Created: 2013-11-08 Last updated: 2013-11-08Bibliographically approved

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Sandström, Rolf

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