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Nitrogen loss and effects on microstructure in multipass TIG welding of a super duplex stainless steel
University West, Department of Engineering Science, Division of Manufacturing Processes. (PTW)ORCID iD: 0000-0001-6242-3517
University West, Department of Engineering Science, Division of Manufacturing Processes. Swerea KIMAB AB, Kista, Sweden. (PTW)ORCID iD: 0000-0001-5110-449X
University West, Department of Engineering Science, Division of Manufacturing Processes. (PTW)ORCID iD: 0000-0002-0234-3168
University West, Department of Engineering Science, Division of Manufacturing Processes. (PTW)ORCID iD: 0000-0001-8822-2705
2016 (English)In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 98, no May, p. 88-97Article in journal (Refereed) Published
Abstract [en]

Nitrogen loss is an important phenomenon in welding of super duplex stainless steels. In this study, a super duplex stainless steel was autogenously TIG-welded with one to four bead-on-plate passes with low or high heat inputs using pure argon shielding gas. The goal was to monitor nitrogen content and microstructure for each weld pass. Nitrogen content, measured by wavelength dispersive X-ray spectrometry, was after four passes reduced from 0.28 wt% in the base metal to 0.17 wt% and 0.10 wt% in low and high heat input samples, respectively. Nitrogen loss resulted in a more ferritic structure with larger grains and nitride precipitates. The ferrite grain width markedly increased with increasing number of passes and heat input. Ferrite content increased from 55% in base metal to 75% at low and 79% at high heat inputs after four passes. An increasing amount of nitrides were seen with increasing number of weld passes. An equation was suggested for calculation of the final nitrogen content of the weld metal as functions of initial nitrogen content and arc energy. Acceptable ferrite contents were seen for one or two passes. The recommendation is to use nitrogen in shielding gas and proper filler metals.

Place, publisher, year, edition, pages
Reigate, Surrey: Scientific and technical P. , 2016. Vol. 98, no May, p. 88-97
Keywords [en]
Nitrogen loss, TIG welding, multipass welding, ferrite content, thermodynamical calculation, super duplex stainless steel
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology; ENGINEERING, Manufacturing and materials engineering
Identifiers
URN: urn:nbn:se:hv:diva-9189DOI: 10.1016/j.matdes.2016.03.011ISI: 000373273000010Scopus ID: 2-s2.0-84963604761OAI: oai:DiVA.org:hv-9189DiVA, id: diva2:909034
Available from: 2016-03-04 Created: 2016-03-04 Last updated: 2018-08-30Bibliographically approved
In thesis
1. Influence of multiple welding cycles on microstructure and corrosion resistance of a super duplex stainless steel
Open this publication in new window or tab >>Influence of multiple welding cycles on microstructure and corrosion resistance of a super duplex stainless steel
2016 (English)Licentiate thesis, comprehensive summary (Other academic)
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.

Place, publisher, year, edition, pages
Trollhättan: University West, 2016. p. 84
Series
Licentiate Thesis: University West ; 12
Keywords
Super duplex stainless steel, autogenous TIG welding, multiple welding thermal cycles, sensitization, nitrogen loss, sigma phase, ferrite content, production technology
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology
Identifiers
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 (English)
Supervisors
Available from: 2016-11-25 Created: 2016-11-17 Last updated: 2016-11-17Bibliographically approved
2. 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, Vahid A.Wessman, StenHurtig, KjellKarlsson, Leif

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