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Influence of multiple welding cycles on microstructure and corrosion resistance of a super duplex stainless steel
University West, Department of Engineering Science, Division of Welding Technology. (PTW)ORCID iD: 0000-0001-6242-3517
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 [en]
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: urn:nbn:se:hv:diva-10151ISBN: 978-91-87531-41-5 (Printed)ISBN: 978-91-87531-40-8 (Electronic)OAI: oai:DiVA.org:hv-10151DiVA: diva2:1047442
Presentation
2016-12-02, 13:36 (English)
Supervisors
Available from: 2016-11-25 Created: 2016-11-17 Last updated: 2016-11-17Bibliographically approved
List of papers
1. Influence of multiple thermal cycles on microstructure of heat-affected zone in TIG-welded super duplex stainless steel
Open this publication in new window or tab >>Influence of multiple thermal cycles on microstructure of heat-affected zone in TIG-welded super duplex stainless steel
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2016 (English)In: Welding in the World, ISSN 0043-2288, E-ISSN 1878-6669, 1-13 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
Keyword
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:nbn:se:hv:diva-9001 (URN)10.1007/s40194-016-0300-5 (DOI)
Available from: 2016-02-05 Created: 2016-02-05 Last updated: 2016-11-17Bibliographically approved
2. Nitrogen loss and effects on microstructure in multipass TIG welding of a super duplex stainless steel
Open this publication in new window or tab >>Nitrogen loss and effects on microstructure in multipass TIG welding of a super duplex stainless steel
2016 (English)In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 98, no May, 88-97 p.Article 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
Keyword
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:nbn:se:hv:diva-9189 (URN)10.1016/j.matdes.2016.03.011 (DOI)000373273000010 ()2-s2.0-84963604761 (ScopusID)
Available from: 2016-03-04 Created: 2016-03-04 Last updated: 2016-11-17Bibliographically approved
3. Effect of multipass TIG welding on the corrosion resistance and microstructure of a super duplex stainless steel
Open this publication in new window or tab >>Effect of multipass TIG welding on the corrosion resistance and microstructure of a super duplex stainless steel
2016 (English)In: Materials and corrosion - Werkstoffe und Korrosion, ISSN 0947-5117, E-ISSN 1521-4176Article in journal (Refereed) Epub ahead of print
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.

Place, publisher, year, edition, pages
Wiley-Blackwell, 2016
Keyword
Welding, corrosion, resistance
National Category
Metallurgy and Metallic Materials
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-9885 (URN)10.1002/maco.201609102 (DOI)
External cooperation:
Available from: 2016-09-13 Created: 2016-09-13 Last updated: 2016-11-17Bibliographically approved

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