Laser beam-submerged arc hybrid welding of thick duplex steels for industry

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Laser Zentrum Hannover’s Rabi Lahdo and Stefan Kaierle describe a process that promises big benefits for heavy industry

Co-authored by Sarah Nothdurft and Jörg Hermsdorf of Laser Zentrum Hannover

Duplex steels are used in many applications that place high demands on strength, toughness and corrosion resistance. After they were introduced for industrial use in the early 1980s, duplex steels were primarily used in applications in the oil and gas industry1, where corrosive materials are stored, transported or processed.

Duplex steels were continuously developed and became established in multiple industries, for example in the building of cargo tanks for ships2, or the construction of bridges3,4.

In combination, duplex steels achieve excellent properties. For example, a structure consisting of 40 per cent delta ferrite and 60 per cent austenite exhibits high strength and toughness, as well as good resistance to corrosion and stress corrosion cracking5,6,7.

When welding duplex steels, the temperature regime has to be especially considered to maintain the delta ferrite-austenite structure. On the one hand, excessive cooling time leads to precipitation of, for example, sigma- and chi-phases, as well as to the formation of a brittle coarse grain zone in the delta ferrite region. Both phases reduce corrosion resistance and toughness.

Insufficient cooling time, on the other hand, results in a low austenite formation in the weld, whereby the corrosion resistance is strongly limited8.

Arc or beam welding?

Duplex steels can either be arc welded using tungsten inert gas welding (TIG), gas metal arc welding (GMAW) and submerged-arc welding (SAW), or beam welded using electron/laser beam welding (EBW/LBW).

The arc-welded joints have good mechanical-technological properties and good corrosion resistance. But these joints are characterised by a multi-layer weld, combined with complex edge preparation and a high consumption of filler material. The consequence is a high production time, as well as high production costs. In addition, the high-performance arc welding processes are limited by high heat input during welding.

Duplex steels are used in building cargo containers for ships, as well as in the construction of bridges. (Image: Shutterstock/Avigator Fortuner)

In the case of beam-welded joints, the production time and production costs are lower, but the structure is characterised by an unfavourable delta ferrite-austenite ratio, with a high delta ferrite proportion. The consequence is reduced corrosion resistance and notch impact resistance, so much so that beam welding processes have not yet been established in practice. Standards such as ISO 1516-3 for oil and gas, ISO 17781 for petrochemicals, M-601 for piping or EN 13445-4 for pressure vessels require at least 30 per cent austenite in the weld metal and in the heat-affected zone.

Why not both?

At Laser Zentrum Hannover we have developed a laser beam-submerged arc (LB-SA) hybrid welding process – where the laser beam and submerged arc act in a common process/melting zone – to combine the advantages of SAW and LBW. The process offers high productivity due to achieving a high welding speed and a high penetration depth for a small number of layers. At the same time, high seam quality regarding the delta ferrite-austenite ratio is achieved due to controlled heat input and alloying with the filler material – which itself is consumed at a lower rate9.

Our investigation of the LB-SA hybrid welding process was carried out using a TruDisk 16002 disc laser beam source from Trumpf Laser- und Systemtechnik, with a maximum power of 16kW. An LAF 1001 welding current source from ESAB Welding & Cutting, with a maximum current of 1,000A, was also used. For the process development we used 1.4462 duplex steel from Outokumpu Nirosta with a thickness of 16mm. S 22 9 N L / ER2209 filler wire with a diameter of 2.4mm was also used in combination with basic non-alloying agglomerated SAW-flux (10.93).

LB-SA hybrid welding was carried out on y-seams in a butt configuration with an opening angle of 30° and a bevel height of 5mm in the flat position. Using 5kW laser beam power, a welding speed of 0.6m/min, a current of 500A and a voltage of 30V, the resultant seams had a good appearance: a homogeneous top and roots without undercuts, spatter, high root reinforcement or root dropping.

The top view of the top of the LB-SA hybrid welded seam is illustrated in figure 1a and the root illustrated in figure 1b.

Figure 1: Appearance of top (a) and root (b) of laser beam-submerged arc hybrid welded seams; cross-section (c); the austenite content along the vertical centre line of the weld metal within the penetration depth of the filler material in the submerged arc-dominated zone (d) and laser beam-dominated zone (e), as well as below the penetration depth of the filler material (f).

The quality and typical geometry of hybrid welded seams can also be observed in cross-sections such as that in figure 1c, which shows a large SA-dominated zone and a narrow LB-dominated zone. Due to the use of filler material, the entire SA-dominated zone and part of the LB-dominated zone shows a higher nickel content, which may lead to the formation of austenite. To confirm the minimum 30 per cent austenite requirement highlighted earlier, we used metallographic analysis to investigate our results.

Figures 1d ,1e and 1f show the austenite content along the vertical centre line of the weld metal within the penetration depth of the filler material in the SA-dominated zone and LB-dominated zone, as well as below the penetration depth of the filler material. The austenite content within the penetration depth of the filler material is comparatively higher than below its penetration depth. This can be explained by the fact that the filler material has a higher proportion of austenite-forming nickel compared to the base material.

On the other hand, the penetration depth range of the filler material is primarily characterised by the SA-dominated area, which demonstrated a lower cooling rate and longer cooling time compared to the laser-dominated area. At a welding speed of 0.6m/min, an average austenite content of at least 30 per cent can be achieved even below the penetration depth of the filler metal. Comparatively, autogenous laser beam welding at a comparable speed results in an austenite content of about 13 per cent.

Outlook

In view of our findings, it can be concluded that LB-SA hybrid welding leads to an extension of the cooling time and thus also to a higher austenite content. Using higher welding speeds, the required minimum 30 per cent austenite content below the penetration depth of the filler material cannot be met.

It is therefore obvious that LB-SA hybrid welding is an efficient single-layer welding process for joining thick duplex steels to increase the welding productivity of duplex steels for industry. For joining duplex steel with a thickness of 16mm using conventional SAW, a double-side welding process with at least three layers is necessary.

In addition, a lower consumption of filler material compared to SAW is achieved using LB-SA hybrid welding, due to a hybrid-specific y-seam preparation with a low opening angle and high bevel height. The hybrid-welded seams show high quality regarding imperfections and the required ferrite-austenite ratio of at least 30 per cent austenite, at a welding speed of 0.6m/min.

Further investigations will be carried out with a laser beam that is wobbled transversal to the welding direction, to increase the penetration depth of the filler material, using laser beam powers up to 16kW to increase the austenite content as a result of longer cooling time at high welding speeds.

Rabi Lahdo works for the Materials and Processes Department of the Joining and Cutting of Metals Group at LZH.

Stefan Kaierle is the executive director of LZH and professor for laser additive processing in the Faculty of Mechanical Engineering at the Leibniz University Hannover.

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References

  1. Charles, J., Chemelle, P., 2010. The history of duplex developments, nowadays DSS properties and duplex market future trends, 8. Duplex stainless Steels conference, Beaune.
  2. Karlsson, L., Strömberg, J., Rigdal, S., Lake, F., 2000. Developments in welding of duplex stainless cargo tanks for chemical carriers, Duplex America 2000, pp. 273 - 280, Houston.
  3. Sorrentino, S., Fersini, M., Zilli, G., 2009. Comparison between SAW and laser welding processes applied to duplex structures for bridges, Welding International, pp. 687 - 698, Roma.
  4. Sorrentino, S., Fersini, M., Zilli, G., 2010. Duplex stainless steel for bridges construction: comparison between saw and laser-gma hybrid welding, Welding in the World Vol. 54, pp. R123 - R133, Roma.
  5. Mateo, A., Llanes, L., Akdut, N., Anglada, M., 2001. High cycle fatigue behaviour of a standard duplex stainless steel plate and bar, Mater. Sci. Eng. A, pp. 319 - 321.
  6. Roberti, R., Nicodemi, W., La Vecchia, G.M., Basha, Sh., 1993. J-R curve dependence on specimen geometry and microstructure in two austenitic-ferritic stainless steels, Int. J. Pres. Ves. & Piping 55, pp. 343 -352.
  7. Charles, J., 1995. Composition and properties of duplex stainless steels, Weld World (UK) 36, pp. 43-55.
  8. Lippold, J.C., Kotecki, D.J., 2005. Welding Metallurgy and Weldability of Stainless Steels, John Wiley & Sons, Hoboken, p. 238, New Jersey.
  9. Lienert, T., Siewert, T., Babu, S., Acoff, V., 2011. Hybrid Laser Arc Welding, ASM Handbook 6A, Welding Fundamentals and Processes, p. 32.

The research project IFG 20736 BG / P 1227 ‘Joining of Duplex Stainless Steels using the Laser-submerged arc hybrid welding’ from the Research Association for steel Application (FOSTA), Düsseldorf, is supported by the Federal Ministry of Economic Affairs and Energy through the German Federation of Industrial Research Associations as part of a programme for promoting industrial cooperative research, on the basis of a decision by the German Bundestag. The project is carried out at Laser Zentrum Hannover e.V. and Fraunhofer Institute for Machine Tools and Forming Technology IWU.

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