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Weldability aspects of a newly developed duplex stainless steel LDX 2101
Outokumpu Stainless.
SSAB Tunnplat.
Outokumpu Stainless Res Fdn.
2008 (English)In: STEEL RES INT, ISSN 1611-3683, Vol. 79, no 6, 473-481 p.Article in journal (Refereed) Published
Abstract [en]

Duplex grades have, due to balanced chemical compositions of both filler and base metals, a weldability that allows for successful welding using a majority of the technically relevant techniques of today. In order to fulfil the performance requirements several aspects must be considered. In the heat affected zone (HAZ) the austenite reformation must be reasonably high and in the weld metal the microstructure must be stable so that e.g. high productivity welding and multi-pass welding are possible, without precipitation of detrimental phases in previous passes. This paper addresses the effect of alloying elements and thermal cycles on phase balance in the high temperature HAZ (HTHAZ) of the newly developed lean duplex grade LDX 2101 (EN 1.4162, UNS S32101). Bead-on-plate welds and simulated weld structures have been produced and investigated using metallography, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results are analysed using the thermodynamic database Thermo-Calc and a model for phase transformation based on a paraequilibrium assumption for ferrite-austenite transformation. In the temperature region outside the paraequilibrium domain, growth controlled by diffusion of substitutional elements was considered. The analysis follows a model by Cahn regarding grain boundary nucleated growth and the Hillert-Engberg model on kinetics of spherical and planar growth.

Place, publisher, year, edition, pages
2008. Vol. 79, no 6, 473-481 p.
Keyword [en]
duplex stainless steel, welding, weld simulation, heat affected zone, HAZ, microstructure, thermodynamic modelling, impact energy, impact toughness, phase transformation, kinetics
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-27217DOI: 10.2374/SRI08SP046-79-2008-473ISI: 000257357500011OAI: oai:DiVA.org:kth-27217DiVA: diva2:375983
Note
QC 20101209Available from: 2010-12-09 Created: 2010-12-09 Last updated: 2010-12-13Bibliographically approved
In thesis
1. Microstructure and properties of welds in the lean duplex stainless steel LDX 2101
Open this publication in new window or tab >>Microstructure and properties of welds in the lean duplex stainless steel LDX 2101
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Duplex stainless steels can be very attractive alternatives to austenitic grades due to their almost double strength at equal pitting corrosion resistance. When welding, the duplex alloys normally require addition of filler metal, while the commodity austenitic grades can often be welded autogenously. Over-alloyed consumables are used to counteract segregation of important alloying elements and to balance the two phases, ferrite and austenite, in the duplex weld metal. This work focuses on the weldability of the recently-developed lean duplex stainless steel LDX 2101® (EN 1.4162, UNS S32101). The pitting corrosion resistance of this grade is better than that of austenitic AISI 304 (EN 1.4307) and can reach the level of AISI 316L (EN 1.4404). The austenite formation is rapid in LDX 2101 compared to older duplex grades. Pitting resistance tests performed show that 1-2.5 mm thick laser and gas tungsten arc (GTA) welded LDX 2101 can have good corrosion properties even when welding autogenously. Additions of filler metal, nitrogen in the shielding gas, nitrogen-based backing gas and use of laser hybrid welding methods, however, increase the austenite formation. The pitting resistance may also be increased by suppressing formation of chromium nitrides in the weld metal and heat affected zone (HAZ). After thorough post-weld cleaning (pickling), pitting primarily occurred 1-3 mm from the fusion line, in the parent metal rather than in the HAZ. Neither the chromium nitride precipitates found in the HAZ, nor the element depletion along the fusion line that was revealed by electron probe microanalysis (EPMA) were found to locally decrease the pitting resistance. The preferential pitting location is suggested to be controlled by the residual weld oxide composition that varies over the surface. The composition and thickness of weld oxide formed on LDX 2101 and 2304 (EN 1.4362, UNS S32304) were determined using X-ray photoelectron spectroscopy (XPS). The heat tint on these lean duplex grades proved to contain significantly more manganese than what has been reported for standard austenitic stainless steels in the AISI 300 series. A new approach to heat tint formation is presented; whereby evaporation of material from the weld metal and subsequent deposition on the already-formed weld oxide are suggested to contribute to weld oxide formation. This is consistent with manganese loss from the weld metal, and nitrogen additions to the GTA shielding gas enhance the evaporation. The segregation of all elements apart from nitrogen is low in autogenously welded LDX 2101. This means that filler wire additions may not be required as for other duplex grades assuming that there is no large nitrogen loss that could cause excessive ferrite contents. As the nitrogen appears to be controlling the austenite formation, it becomes essential to avoid losing nitrogen during welding by choosing nitrogen-containing shielding and backing gas.

Place, publisher, year, edition, pages
Stockholm: KTH, 2010. vii, 65 p.
Keyword
Duplex stainless steel, welding, HAZ, nitrogen, manganese, microstructure, austenite formation, phase balance, precipitates, element distribution, segregation, depletion, solidification, pitting corrosion resistance, solidification, element loss, evaporation, deposition, weld oxide, thermo-mechanical simulation, thermodynamic modelling, EPMA, XPS, post-weld cleaning, pickling
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-27387 (URN)978-91-7415-801-4 (ISBN)
Public defence
2010-12-17, Sal Q2, Osquldasväg 10 NB, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20101213Available from: 2010-12-13 Created: 2010-12-13 Last updated: 2010-12-13Bibliographically approved

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