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Microstructure and properties of welds in the lean duplex stainless steel LDX 2101
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
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 [en]
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: urn:nbn:se:kth:diva-27387ISBN: 978-91-7415-801-4 (print)OAI: oai:DiVA.org:kth-27387DiVA: diva2:376827
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
List of papers
1. Weldability aspects of a newly developed duplex stainless steel LDX 2101
Open this publication in new window or tab >>Weldability aspects of a newly developed duplex stainless steel LDX 2101
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.

Keyword
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:nbn:se:kth:diva-27217 (URN)10.2374/SRI08SP046-79-2008-473 (DOI)000257357500011 ()
Note
QC 20101209Available from: 2010-12-09 Created: 2010-12-09 Last updated: 2010-12-13Bibliographically approved
2. Weld oxide formation on lean duplex stainless steel
Open this publication in new window or tab >>Weld oxide formation on lean duplex stainless steel
2008 (English)In: Corrosion Science, ISSN 0010-938X, E-ISSN 1879-0496, Vol. 50, no 9, 2620-2634 p.Article in journal (Refereed) Published
Abstract [en]

Weld oxides have a strong influence on corrosion resistance, but have hitherto only been studied to a limited extent for duplex stainless steels. X-ray photoelectron spectroscopy (XPS) has here been used to study heat tint formed on gas tungsten arc (GTA) welds on the commercial duplex grades LDX 2101 (EN 1.4162/UNS S32101) and 2304 (EN 1.4362/UNS S32304) welded with and without nitrogen additions to the shielding gas. The process of heat tint formation is discussed in terms of transport phenomena to explain the effect of atmosphere, temperature and composition. The oxides formed were found to be enriched in manganese and corrosion testing shows that nitrogen has a strong influence on the weld oxide. A mechanism is proposed including evaporation from the weld pool and subsequent redeposition.

Keyword
Stainless steel, EPMA, XPS, Oxidation, Pitting corrosion
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-9294 (URN)10.1016/j.corsci.2008.06.024 (DOI)000260358900023 ()
Note
QC 20101126 Uppdaterad från submitted till published (20101126).Available from: 2008-10-17 Created: 2008-10-17 Last updated: 2017-12-07Bibliographically approved
3. HENRY GRANJON PRIZE COMPETITION 2009 Winner Category B: "Materials Behaviour and Weldability" PITTING CORROSION RESISTANCE OF GTA WELDED LEAN DUPLEX STAINLESS STEEL
Open this publication in new window or tab >>HENRY GRANJON PRIZE COMPETITION 2009 Winner Category B: "Materials Behaviour and Weldability" PITTING CORROSION RESISTANCE OF GTA WELDED LEAN DUPLEX STAINLESS STEEL
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.

Keyword
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:nbn:se:kth:diva-27350 (URN)000283679400001 ()
Note
QC 20101213Available from: 2010-12-13 Created: 2010-12-13 Last updated: 2010-12-13Bibliographically approved
4. Effect of laser and laser hybrid welding on the corrosion performance of a leanduplex stainless steel
Open this publication in new window or tab >>Effect of laser and laser hybrid welding on the corrosion performance of a leanduplex stainless steel
(English)In: Journal of laser applications, ISSN 1042-346X, E-ISSN 1938-1387Article in journal (Refereed) In press
Identifiers
urn:nbn:se:kth:diva-27351 (URN)
Note
QC 20101213Available from: 2010-12-13 Created: 2010-12-13 Last updated: 2017-12-11Bibliographically approved
5. Element distribution in lean duplex stainless steel welds
Open this publication in new window or tab >>Element distribution in lean duplex stainless steel welds
(English)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940Article in journal (Other academic) Submitted
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-27386 (URN)
Note
QS 20120328Available from: 2010-12-13 Created: 2010-12-13 Last updated: 2017-12-11Bibliographically approved

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