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Influence of multiple thermal cycles on microstructure of heat-affected zone in TIG-welded super duplex stainless steel
University West, Department of Engineering Science, Division of Manufacturing Processes. Innovatum AB, Trollhättan, Sweden. (PTW)ORCID iD: 0000-0001-6242-3517
University West, Department of Engineering Science, Division of Manufacturing Processes. (PTW)ORCID iD: 0000-0003-3374-6282
Swerea KIMAB AB, Kista, Sweden.
University West, Department of Engineering Science, Division of Manufacturing Processes. (PTW)ORCID iD: 0000-0002-0234-3168
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2016 (English)In: Welding in the World, ISSN 0043-2288, E-ISSN 1878-6669, Vol. 60, no 2, 233-245 p.Article in journal (Refereed) Published
Abstract [en]

The influence of heat input and multiple welding cycles on the microstructure of the heat-affected zone in autogenously TIG-welded 6 mm 2507 type super duplex stainless steel plates was investigated. In order to produce multiple thermal cycles, one to four pass bead-on-plate welds were made with arc energies of 0.47 and 1.08 kJ/mm, corresponding to heat inputs of 0.37 and 0.87 kJ/mm. Several thermocouples were attached to record thermal cycles on the front and back sides of the plates. Finite element modelling was successfully done to map and correlate measured and calculated peak temperatures. Only minor changes were seen in the ferrite content at 1 and 2 mm from the fusion boundary. Nitrides formed in all passes of the low heat input samples in a region next to the fusion boundary, but only after the third and fourth passes of the high heat input samples. Sigma phase precipitated only in a zone heated to a peak temperature in the range of approximately 828 to 1028 °C. Multiple reheating was found to promote precipitation of sigma phase relatively more than slower cooling. A precipitation free zone was observed between the nitride and sigma phase bands. The precipitation behaviour could be understood from equilibrium phase diagrams, evaluation of local thermal cycles and by correlating results from the modelling and measurements of peak temperatures. It is suggested that the peak temperature, the accumulated time in the critical temperature range between approximately 828 and 1028 °C, and the number of thermal cycles are the most relevant criteria when evaluating the risk of sigma phase formation.

Place, publisher, year, edition, pages
Springer Berlin/Heidelberg, 2016. Vol. 60, no 2, 233-245 p.
Keyword [en]
Duplex stainless steels, GTA welding, multirun welding, heat affected zone, microstructure, mathematical models, energy input
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology; ENGINEERING, Manufacturing and materials engineering
Identifiers
URN: urn:nbn:se:hv:diva-9001DOI: 10.1007/s40194-016-0300-5ISI: 000374785100007Scopus ID: 2-s2.0-84958787089OAI: oai:DiVA.org:hv-9001DiVA: diva2:900761
Available from: 2016-02-05 Created: 2016-02-05 Last updated: 2017-11-30Bibliographically 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. 84 p.
Series
Licentiate Thesis: University West, 12
Keyword
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

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Hosseini, Vahid A.Valiente Bermejo, María AsunciónHurtig, KjellKarlsson, Leif

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