ANALYSIS: STEEL WELDING
Laser beam-submerged arc hybrid welding of thick duplex steels for industry
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
bridges3,4
, or the construction of .
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
20 LASER SYSTEMS EUROPE AUTUMN 2021
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. 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
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)
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
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