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Computational thermodynamics study of the influence of tungsten in superduplex stainless weld metal
2012 (English)In: Welding in the World, ISSN 0043-2288, E-ISSN 1878-6669, Vol. 56, no 11-12, p. 79-87Article in journal (Refereed) Published
Abstract [en]

Tungsten is used by some duplex stainless steel producers for partial substitution of molybdenum, both elements enhancing the corrosion resistance of duplex stainless steel. The negative aspect of both molybdenum and tungsten alloying is that they increase the tendency to precipitate intermetallic phases, which may have a detrimental effect on corrosion and mechanical properties. The temperature region for intermetallic phase precipitation is about 700-1000 degrees C, depending on alloy composition, and the time for precipitation is within minutes for superduplex steels. There has been scientific discussion on the relative effects of Mo alone or Mo-W on intermetallic precipitation behaviour in superduplex steels for the past two decades. While the base material response to ageing and precipitation of intermetallic phases has been satisfactorily assessed, weld metal has proved more of a challenge. The main reason for this is that welding is a very complex process introducing many parameters to the assessment which not have to be considered in studies of base material. For example superduplex weld metal typically solidify fully ferritic but may in case excessive nitrogen is added solidify as a mixture of ferrite and austenite. The solidification mode may vary also between weld passes as a consequence of minor variations in composition. Ferritic solidification is the preferred mode, giving the well known Widmanstatten austenite, which forms in the solid state during cooling. Mixed mode solidification gives a vermicular appearance, which is known to increases the tendency to intermetallic formation. A comprehensive study using computational thermodynamics was done to investigate this matter. This study included equilibrium calculations, Scheil-Gulliver solidification simulations, and calculations of the driving force for intermetallic phase precipitation and further a study of diffusion of Mo and W in these alloy systems. The different approaches were applied on model superduplex weld metals with nominal compositions matching commercial superduplex fillers available today. The principal conclusion is that all thermodynamic calculations clearly indicates the W containing filler to show a more pronounced sensitivity to heat treatments by precipitation of intermetallic phases.

Place, publisher, year, edition, pages
2012. Vol. 56, no 11-12, p. 79-87
Keywords [en]
Computation, Thermodynamics, Calculation, Duplex stainless steel, Weld metal
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:kth:diva-121332DOI: 10.1007/BF03321398ISI: 000311195500009OAI: oai:DiVA.org:kth-121332DiVA, id: diva2:618455
Note

QC 20130502

Available from: 2013-04-28 Created: 2013-04-28 Last updated: 2017-12-06Bibliographically approved
In thesis
1. Applications of Computational Thermodynamics and Kinetics on Transformations in Stainless Steels
Open this publication in new window or tab >>Applications of Computational Thermodynamics and Kinetics on Transformations in Stainless Steels
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Stainless steels are high-alloyed, usually with multiple components and often also dual matrix phases, as for duplex stainless steels. This make predictions and calculations of alloying effects on equilibria and transformations complicated. Computational thermodynamics has emerged as an indispensable tool for calculations within these complex systems with predictions of equilibria and precipitation of phases. This thesis offers examples illustrating how computational methods can be applied both to thermodynamics, kinetics and coarsening of stainless steels in order to predict microstructure and, to some extent, also properties. The performance of a current state-of-the-art commercial thermodynamic database was also explored and strengths and weaknesses highlighted.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2013. p. 60
Keywords
Stainless Steels, Duplex, Ferrite, Austenite, Computational Thermodynamics, Thermo-Calc, Dictra
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-121337 (URN)978-91-7501-714-3 (ISBN)
Public defence
2013-05-16, B2, Brinellvägen 23, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20130429

Available from: 2013-04-29 Created: 2013-04-28 Last updated: 2013-04-29Bibliographically approved

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