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The fracture mechanisms in duplex stainless steels at sub-zero temperatures
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
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 [en]
Duplex stainless steel, Weldments, Delamination, Fracture toughness, Impact toughness, Cleavage fracture, Nickel
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-133677ISBN: 978-91-7501-931-4 (print)OAI: oai:DiVA.org:kth-133677DiVA: diva2:662735
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
List of papers
1. Loss of constraint during fracture toughness testing of duplex stainless steels
Open this publication in new window or tab >>Loss of constraint during fracture toughness testing of duplex stainless steels
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.

Keyword
Duplex stainless steel, Fracture toughness, Delamination, Splits, Pop-in, Normalization, Master curve
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-122353 (URN)10.1016/j.engfracmech.2013.01.002 (DOI)000317325100019 ()2-s2.0-84875241165 (Scopus ID)
Note

QC 20130522

Available from: 2013-05-22 Created: 2013-05-20 Last updated: 2017-12-06Bibliographically approved
2. Delaminations by Cleavage Cracking in Duplex Stainless Steels at Sub-zero Temperatures
Open this publication in new window or tab >>Delaminations by Cleavage Cracking in Duplex Stainless Steels at Sub-zero Temperatures
2014 (English)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 45A, no 3, 1327-1337 p.Article in journal (Refereed) Published
Abstract [en]

Impact toughness testing was conducted on 10 and 30 mm plates of 2205 together with a 30 mm plate of LDX 2101(A (R)) duplex stainless steel (DSS). The testing temperatures were between 153 K (-120 A degrees C) and room temperature. Interrupted fracture toughness tests of the 10 mm plate and a 50 mm plate of 2205 were also performed. The conclusion from the fractographic investigation was that the delaminations that occur in hot-rolled DSSs were cleavage fractures. The toughness anisotropy can be explained by the cleavage fracture and the appearance of the microstructure. The result from the interrupted fracture toughness test revealed that the delaminations initiated prior to the maximum force plateau and propagated ahead of the stable crack growth during testing. Estimated upper limit for the fracture delamination initiation toughness at sub-zero temperatures for the 2205 base metal according to the crack-tip opening displacement method was 28 to 61 mu m for the 10 mm plate, 70 to 106 mu m for the 30 mm plate and below 100 mu m for the 50 mm plate.

Keyword
Duplex stainless steel, Cleavage fracture, Impact toughness, Fracture toughness, Delamination, Splits
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-133673 (URN)10.1007/s11661-013-2055-6 (DOI)000331094900025 ()2-s2.0-84896761832 (Scopus ID)
Note

QC 20140321

Available from: 2013-11-08 Created: 2013-11-08 Last updated: 2017-12-06Bibliographically approved
3. Influence of lattice orientation on the fracture toughness of duplex stainless steel 2205
Open this publication in new window or tab >>Influence of lattice orientation on the fracture toughness of duplex stainless steel 2205
2010 (English)In: Duplex World 2010, Beaune, France, 13-15 October 2010, 2010Conference paper, Oral presentation only (Other academic)
Abstract [en]

Fracture toughness measurements on single-edge bend T-L specimens at low temperatures forduplex stainless steels show that 10 mm plate is prone to exhibit pop-in failure. These pop-in have been relatedto the initiation of splits that is commonly seen after fracture toughness testing of this type of material.Microstructural and texture characterisations have been conducted with light optical microscope and electronbackscatter diffraction to evaluate the cause behind split initiation and why these splits cause pop-in behaviourin thinner plates. The results are that 10 mm plates have a strong cube-on-edge texture in the ferritic phase whilethe 50 mm plate has a weak texture. This indicates that the initiation of splits may be due to the interactionbetween ferrite and austenite crystallographic orientation and on the stress and strain constraints enforced by thelamellar microstructure. This uniform lattice orientation for the ferritic phase in the 10 mm plate can explainwhy thinner plates of 2205 are prone to exhibit pop-in behaviour during fracture toughness testing.

Keyword
Duplex stainless steels, Pop-in, Texture, Delamination, Splits
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-133674 (URN)
Conference
Duplex World 2010
Note

QC 20131108

Available from: 2013-11-08 Created: 2013-11-08 Last updated: 2013-11-08Bibliographically approved
4. Influence of nickel on the toughness of lean duplex stainless steel welds
Open this publication in new window or tab >>Influence of nickel on the toughness of lean duplex stainless steel welds
2014 (English)In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 602, 49-57 p.Article in journal (Refereed) Published
Abstract [en]

Three weldments with the nickel contents 13, 4.9 and 6.0 wt% were made from 30 mm LDX 2101((R)) plates. The weldments were subjected to tensile, impact and fracture toughness testing. The aim was to evaluate the susceptibility for brittle failure in the weld metal at sub-zero temperatures (degrees C). The amount of ferrite was higher for the 1.3 wt% nickel weldment compared to the other two which had similar phase composition and mean free ferrite distance. The result from the tensile testing showed that for the weldment with the highest nickel content the ductility remained unchanged with decreasing temperature while the other two weldments became less ductile with decreasing temperature. J-integral based fracture toughness testing showed a significant difference in the susceptibility for brittle failure with higher values for the weldment with 6 wt% nickel than for the others with lower nickel content.

Keyword
Duplex stainless steel, Weld metal, Impact toughness, Fracture toughness, Nickel
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-133675 (URN)10.1016/j.msea.2014.01.093 (DOI)000335098500007 ()2-s2.0-84897808896 (Scopus ID)
Note

QC 20140604. Updated from submitted to published.

Available from: 2013-11-08 Created: 2013-11-08 Last updated: 2017-12-06Bibliographically approved
5. Fracture toughness of a welded super duplex stainless steel
Open this publication in new window or tab >>Fracture toughness of a welded super duplex stainless steel
2014 (English)In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 606, 40-45 p.Article in journal (Refereed) Published
Abstract [en]

Fracture toughness testing was conducted on standard single-edge notched bend bar specimens of base and weld metal. The material was the SAF 2906 super duplex stainless steel. The aim was to evaluate the susceptibility for brittle failure at sub-zero temperatures for the base and weld metal. The base metal was tested between -103 and -60. °C and was evaluated according to the crack-tip opening displacement method. The fracture event at and below -80. °C can be described as ductile until critical cleavage initiation occurs, which caused unstable failure of the specimen. The welding method used was submerged arc welding with a 7. wt% nickel filler metal. The welded specimens were post-weld heat treated (PWHT) at 1100. °C for 20. min and then quenched. Energy-dispersive X-ray spectroscopy analysis showed that during PWHT substitutional element partitioning occurred which resulted in decreased nickel content in the ferrite. The PWHT weld metal specimens were tested at -72. °C. The fracture sequence was critical cleavage fracture initiation after minor crack-tip blunting and ductile fracture.

Keyword
Duplex stainless steel, Super duplex, Fracture toughness, Weldments, Cleavage fracture, Post weld heat treatment
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-133676 (URN)10.1016/j.msea.2014.03.049 (DOI)000337775800005 ()2-s2.0-84897951163 (Scopus ID)
Note

QC 20140519. Updated from submitted to published.

Available from: 2013-11-08 Created: 2013-11-08 Last updated: 2017-12-06Bibliographically approved

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