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HENRY GRANJON PRIZE COMPETITION 2009 Winner Category B: "Materials Behaviour and Weldability" PITTING CORROSION RESISTANCE OF GTA WELDED LEAN DUPLEX STAINLESS STEEL
Outokumpu Stainless, Avesta Research Centre.
2010 (English)In: WELD WORLD, ISSN 0043-2288, Vol. 54, no 11-12, R308-R321 p.Article in journal (Refereed) Published
Abstract [en]

The use of lean duplex stainless steel increases steadily worldwide and the fluctuations in nickel price in recent years have contributed to further growth. The weldability of these grades is different from that of the substituted austenitic grades and this paper focuses on the effect of weld metal microstructure and weld oxide on the pitting corrosion resistance. The influence of chromium nitride precipitates on the corrosion performance is illustrated and it is shown that if the weld metal has sufficient nitrogen content and hence adequate austenite formation, the weld oxide controls the location of pitting. Diffusion and oxidation are normally the discussed processes affecting the composition and distribution of the heat tint, but here a new approach to weld oxide formation is proposed. It is shown that manganese is evaporated from the weld metal and contributes to forming thicker weld oxide by subsequent deposition. Addition of nitrogen to the shielding gas further enhanced the manganese evaporation and also formed a significant fraction of oxynitrides in the weld oxide.

Place, publisher, year, edition, pages
2010. Vol. 54, no 11-12, R308-R321 p.
Keyword [en]
Austenite, Corrosion, Duplex stainless steels, Ferrite, GTA welding, Heat affected zone, Pitting corrosion, Reference lists, Weld metal, Weldability, Welding, Welding without filler
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-27350ISI: 000283679400001OAI: oai:DiVA.org:kth-27350DiVA: diva2:376687
Note
QC 20101213Available from: 2010-12-13 Created: 2010-12-13 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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