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Effect of sigma phase morphology on the degradation of properties in a super duplex stainless steel
University West, Department of Engineering Science, Division of Welding Technology. (PTW)ORCID iD: 0000-0001-6242-3517
University West, Department of Engineering Science, Division of Welding Technology. (PTW)ORCID iD: 0000-0001-8822-2705
University West, Department of Engineering Science, Division of Welding Technology. Swerea KIMAB AB, P.O. Box 7047, Kista, Sweden. (PTW)ORCID iD: 0000-0001-5110-449X
Swerea KIMAB AB, P.O. Box 7047, Kista, Sweden.
2018 (English)In: Materials, ISSN 1996-1944, E-ISSN 1996-1944, Vol. 11, no 6, article id 933Article in journal (Refereed) Published
Abstract [en]

Sigma phase is commonly considered to be the most deleterious secondary phase precipitating in duplex stainless steels, as it results in an extreme reduction of corrosion resistance and toughness. Previous studies have mainly focused on the kinetics of sigma phase precipitation and influences on properties and only a few works have studied the morphology of sigma phase and its influences on material properties. Therefore, the influence of sigma phase morphology on the degradation of corrosion resistance and mechanical properties of 2507 super duplex stainless steel (SDSS) was studied after 10 h of arc heat treatment using optical and scanning electron microscopy, electron backscattered diffraction analysis, corrosion testing, and thermodynamic calculations. A stationary arc was applied on the 2507 SDSS disc mounted on a water-cooled chamber, producing a steady-state temperature gradient covering the entire temperature range from room temperature to the melting point. Sigma phase was the major intermetallic precipitating between 630 °C and 1010 °C and its morphology changed from blocky to fine coral-shaped with decreasing aging temperature. At the same time, the average thickness of the precipitates decreased from 2.9 Όm to 0.5 Όm. The chemical composition of sigma was similar to that predicted by thermodynamic calculations when formed at 800-900 °C, but deviated at higher and lower temperatures. The formation of blocky sigma phase introduced local strain in the bulk of the primary austenite grains. However, the local strain was most pronounced in the secondary austenite grains next to the coral-shaped sigma phase precipitating at lower temperatures. Microstructures with blocky and coral-shaped sigma phase particles were prone to develop microscale cracks and local corrosion, respectively. Local corrosion occurred primarily in ferrite and in secondary austenite, which was predicted by thermodynamic calculations to have a low pitting resistance equivalent. To conclude, the influence of sigma phase morphology on the degradation of properties was summarized in two diagrams as functions of the level of static load and the severity of the corrosive environment. © 2018 by the authors.

Place, publisher, year, edition, pages
2018. Vol. 11, no 6, article id 933
Keywords [en]
Austenite; Corrosion resistance; Heat resistance; Localized corrosion; Microscopic examination; Morphology; Scanning electron microscopy; Stainless steel; Steel testing; Temperature, Chemical compositions; Duplex stainless steel; Electron backscattered diffraction analysis; Sigma-phase precipitation; Steady-state temperature; Super duplex stainless steel; Thermodynamic calculations; Water-cooled chambers, Steel corrosion
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
URN: urn:nbn:se:hv:diva-12703DOI: 10.3390/ma11060933ISI: 000436500300066Scopus ID: 2-s2.0-85047834414OAI: oai:DiVA.org:hv-12703DiVA, id: diva2:1231133
Funder
Knowledge Foundation, 20140130Available from: 2018-07-05 Created: 2018-07-05 Last updated: 2019-05-27Bibliographically approved
In thesis
1. Super duplex stainless steels: Microstructure and propertiesof physically simulated base and weld metal
Open this publication in new window or tab >>Super duplex stainless steels: Microstructure and propertiesof physically simulated base and weld metal
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

High-temperature processing and application of super duplex stainless steel(SDSS) are associated with the risk of changes in the ferrite/austenite balance and precipitation of secondary phases. This study was therefore aimed at improving knowledge about effects of thermal cycles on the microstructure and properties of SDSS base and weld metal. Controlled and repeatable thermal cycles were physically simulated using the innovative multiple TIG reheating/remelting and the arc heat treatment techniques. In the first technique, one to four autogenous TIG-remelting passes were applied. During arc heat treatment, a stationary arc was applied on a disc mounted on a water-cooled chamber thereby subjecting the material to a steady state temperature gradient from 0.5 minute to 600 minutes. Microstructures and properties were assessed and linked to thermal history through thermal cycle analysis, thermodynamic calculations and temperature field modelling, Remelting studies showed that nitrogen loss from the melt pool was a function of arc energy and initial nitrogen content and could cause highly ferritic microstructures. Heat affected zones were sensitized by nitride formation next to the fusion boundary and sigma phase precipitation in regions subjected to peak temperatures of 828-1028°C. Accumulated time in the critical temperature range, peak temperature and the number of thermal cycles are the most relevant criteria when evaluating the risk of sigma phase precipitation. Arc heat treatment produced graded microstructures in SDSS base and weld metal with the formation of a ferritic region at high temperature due to solid-state nitrogen loss, precipitation of sigma, chi, nitrides, and R-phase with different morphologies at 550-1010°C and spinodal decomposition below 500°C. This caused sensitization and/or increased hardness and embrittlement. Results were summarized as time-temperature-precipitation and properties diagrams for base and weld metal together with guidelines for processing and welding of SDSS.

Place, publisher, year, edition, pages
Trollhättan: University West, 2018. p. 102
Series
PhD Thesis: University West ; 24
Keywords
Super duplex stainless steel; weld metal; physical simulation; phase balance; precipitation; secondary phases; corrosion; nitrogen loss; arc heat treatment; production technology
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology; ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-12850 (URN)978-91-87531-98-9 (ISBN)978-91-87531-97-2 (ISBN)
Public defence
2018-09-28, F127, lhättan, 09:00 (English)
Supervisors
Available from: 2018-09-04 Created: 2018-08-30

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Hosseini, VahidKarlsson, LeifWessman, Sten

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