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Effect of multipass TIG welding on the corrosion resistance and microstructure of a super duplex stainless steel
Högskolan Väst, Institutionen för ingenjörsvetenskap, Avdelningen för svetsteknologi (SV). Hogskolan Vast.ORCID-id: 0000-0001-6242-3517
Högskolan Väst, Institutionen för ingenjörsvetenskap, Avdelningen för svetsteknologi (SV). (PTW)ORCID-id: 0000-0002-0234-3168
Högskolan Väst, Institutionen för ingenjörsvetenskap, Avdelningen för svetsteknologi (SV). (PTW)ORCID-id: 0000-0001-8822-2705
2017 (Engelska)Ingår i: Materials and corrosion - Werkstoffe und Korrosion, ISSN 0947-5117, E-ISSN 1521-4176, Vol. 68, nr 4, s. 405-415Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

This is a study of the effect of repetitive TIG (tungsten inert gas) welding passes, melting and remelting the same material volume on microstructure and corrosion resistance of 2507 (EN 1.4410) super duplex stainless steel. One to four weld passes were autogenously (no filler added) applied on a plate using two different arc energies and with pure argon shielding gas. Sensitization testing showed that multipass remelting resulted in significant loss of corrosion resistance of the weld metal, in base material next to the fusion boundary, and in a zone 1 to 4 mm from the fusion boundary. Metallography revealed the main reasons for sensitization to be a ferrite-rich weld metal and precipitation of nitrides in the weld metal, and adjacent heat affected zone together with sigma phase formation at some distance from the fusion boundary. Corrosion properties cannot be significantly restored by a post weld heat treatment. Using filler metals with higher nickel contents and nitrogen containing shielding gases, are therefore, recommended. Welding with a higher heat input and fewer passes, in some cases, can also decrease the risk of formation of secondary phases and possible corrosion attack.

Ort, förlag, år, upplaga, sidor
2017. Vol. 68, nr 4, s. 405-415
Nyckelord [en]
Welding, corrosion, resistance
Nationell ämneskategori
Metallurgi och metalliska material
Forskningsämne
TEKNIK, Produktions- och materialteknik
Identifikatorer
URN: urn:nbn:se:hv:diva-9885DOI: 10.1002/maco.201609102ISI: 000398581800002Scopus ID: 2-s2.0-84986268706OAI: oai:DiVA.org:hv-9885DiVA, id: diva2:968858
Forskningsfinansiär
Vinnova, 2014‐01910Tillgänglig från: 2016-09-13 Skapad: 2016-09-13 Senast uppdaterad: 2019-05-23Bibliografiskt granskad
Ingår i avhandling
1. Influence of multiple welding cycles on microstructure and corrosion resistance of a super duplex stainless steel
Öppna denna publikation i ny flik eller fönster >>Influence of multiple welding cycles on microstructure and corrosion resistance of a super duplex stainless steel
2016 (Engelska)Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
Abstract [en]

Super duplex stainless steel (SDSS) has found a wide use in demanding applications such as offshore, chemical and petrochemical industries thanks to its excellent combination of mechanical properties and corrosion resistance. Welding of SDSS, however, is associated with the risk of precipitation of secondary phases and formation of excessive amounts of ferrite in the weld metal and heat affected zone. The present study was therefore aimed at gaining knowledge about the effect of multiple welding thermal cycles on the microstructure and possible sensitization to corrosion of welds in SDSS.Controlled and repeatable thermal cycles were produced by robotic welding. Oneto four autogenous TIG-remelting passes were applied on 2507 type SDSS plates using low or high heat inputs with pure argon as shielding gas. Thermal cycles were recorded using several thermocouples attached to the plates. Thermodynamic calculations and temperature field modelling were performed in order to understand the microstructural development and to predict the pitting corrosion resistance. Etching revealed the formation of different zones with characteristic microstructures: the fused weld zone (WZ) and the heat affected zone composed of the fusion boundary zone (FBZ), next to the fusion boundary, and further out Zone 1 (Z1) and Zone 2 (Z2). The WZ had a high content of ferrite and often nitrides which increased with increasing number of passes and decreasing heati nput. Nitrogen content of the WZ decreased from 0.28 wt.% to 0.17 wt.% after four passes of low heat input and to 0.10 wt.% after four passes of high heatinput. The FBZ was reheated to high peak temperatures (near melting point) and contained equiaxed ferrite grains with austenite and nitrides. Zone 1 was free from precipitates and the ferrite content was similar to that of the unaffected base material. Sigma phase precipitated only in zone 2, which was heated to peak temperatures in the range of approximately 828°C to 1028°C. The content of sigma phase increased with the number of passes and increasing heat input. 

All locations, except Z1, were susceptible to local corrosion after multiplere heating. Thermodynamic calculations predicted that a post weld heat treatment could restore the corrosion resistance of the FBZ and Z2. However, the pitting resistance of the WZ cannot be improved significantly due to the nitrogen loss. Steady state and linear fitting approaches were therefore employed to predict nitrogen loss in autogenous TIG welding with argon as shielding gas. Two practical formulas were derived giving nitrogen loss as functions of initial nitrogen content and arc energy both predicting a larger loss for higher heat input and higher base material nitrogen content. A practical recommendation based on the present study is that it is beneficial to perform welding with a minimum number of passes even if this results in a higherheat input as multiple reheating strongly promotes formation of deleterious phases.

Ort, förlag, år, upplaga, sidor
Trollhättan: University West, 2016. s. 84
Serie
Licentiate Thesis: University West ; 12
Nyckelord
Super duplex stainless steel, autogenous TIG welding, multiple welding thermal cycles, sensitization, nitrogen loss, sigma phase, ferrite content, production technology
Nationell ämneskategori
Bearbetnings-, yt- och fogningsteknik
Forskningsämne
Produktionsteknik
Identifikatorer
urn:nbn:se:hv:diva-10151 (URN)978-91-87531-41-5 (ISBN)978-91-87531-40-8 (ISBN)
Presentation
2016-12-02, 13:36 (Engelska)
Handledare
Tillgänglig från: 2016-11-25 Skapad: 2016-11-17 Senast uppdaterad: 2019-03-13Bibliografiskt granskad
2. Super duplex stainless steels: Microstructure and propertiesof physically simulated base and weld metal
Öppna denna publikation i ny flik eller fönster >>Super duplex stainless steels: Microstructure and propertiesof physically simulated base and weld metal
2018 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
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.

Ort, förlag, år, upplaga, sidor
Trollhättan: University West, 2018. s. 102
Serie
PhD Thesis: University West ; 24
Nyckelord
Super duplex stainless steel; weld metal; physical simulation; phase balance; precipitation; secondary phases; corrosion; nitrogen loss; arc heat treatment; production technology
Nationell ämneskategori
Bearbetnings-, yt- och fogningsteknik
Forskningsämne
Produktionsteknik; TEKNIK, Produktions- och materialteknik
Identifikatorer
urn:nbn:se:hv:diva-12850 (URN)978-91-87531-98-9 (ISBN)978-91-87531-97-2 (ISBN)
Disputation
2018-09-28, F127, lhättan, 09:00 (Engelska)
Handledare
Tillgänglig från: 2018-09-04 Skapad: 2018-08-30

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Hosseini, VahidHurtig, KjellKarlsson, Leif

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