African Fusion March 2015

Welding super duplex stainless steels

Welding of super duplex stainless steels Omar Abou Samra Filho, voestalpine Böhler Welding

S uper duplex stainless steel (SDSS) has been increasingly applied in the upstreamoil and gas industry segment in recent years. This steel grade is normally used in the manufacture of subsea components and in the construc- tion of topside equipment and pipes on oil and gas platforms. The extraordinary growth on SDSS use for manufacturing such components and equipment can be mainly explained due to two factors. • First, SDSS has excellent mechani- cal properties when compared to austenitic stainless steels of the 3xx series. The yield strength of UNS S 32750 super duplex, for example, is close to twice that of austenitic stainless steels types 304L and 316L This strength advantage enables significant weight, thick- ness and lifecycle cost reductions. In addition to higher tensile strength, excellent toughness at low tempera- tures (down to -46 ºC) is available from SDSS. Second, super duplex offers high resistance to pitting corrosion and stress corrosion cracking in media containing hydrochloric acid, sul- phuric acid, acetic acid and chloride ions, among others. Despite the noted advantages described above, inadequate welding procedures of SDSS can result in metallurgical changes in its microstructure, resulting in loss of the desired properties of this grade of stainless steel. Figures 1 and 2 show one of the typical problems that can occur in the heat affected zone (HAZ) of SDSS when welders and or welding procedures are not properly qualified, consumables and welding processes are wrongly selected, and the specific requirements, suchas heat input control, are not properly followed. To avoid such problems, technical welding expertise when using these material grades is necessary. Voestalpine BöhlerWelding develops, manufactures, and supplies SDSSwelding consumables for all of the major welding processes. They meet all main global requirements •

for this alloy type and are approved by oil and gas companies such as Petrobras, Norsok and NACE, for example. Welding consumables for each of the recom- mended welding process are listed in Table 1. Process voestalpine Böhler Welding consumable GTAW THERMANIT 25/09 CuT SMAW THERMANIT 25/09 CuT GMAW THERMANIT 25/09 CuT FCAW Avesta2507/P100-PW NOR SAW wire THERMANIT 25/09 CuT SAW flux Marathon 431 Table 1: Böhler Welding consumables suitable for welding of super duplex stainless steel UNS S 32750 and UNS N 32760.

Figure 2: A super duplex UNS S 32750 pipe removed from an oil platformmodule. Corrosion of HAZ can be observed due to a HAZ microstructure imbalance presented in Figure 1.

Figure 3: Welding procedure qualification of an SDSS pipe in the 6G position.

Figure 1: The microstructure of welded UNS S 32750 super duplex stainless steel. Region A: base metal containing an optimum balance of ferrite and austenite phases (approximately 50% each). Region B: the HAZ with unbalanced ferrite and austenite phases due to unsuitable welding procedure (approximately 75% and 25% respectively). Region C: The weld metal region that contains the correct phase balance due to the use of a welding consumable with a well-balanced chemical composition. Voestalpine Böhler Welding has regional teams of highly trained and qualified professional engineers – sup- ported by central R&D and Applications in Europe – to assist when selecting the best welding processes and welding consumables; trainingwelders; develop- ing welding procedure qualifications; interpreting technical standards; and for all the choices necessary to ensure

Figure 4: Welding of an SDSS pipe for a desulphurisation system on an oil platform.

anoptimumsolution fromSDSSwelding. Voestalpine Bohler Welding’s SDSS solutions and consumables for the oil and gas industry comply fully with all national and international industry seg- ment standards.

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March 2015

AFRICAN FUSION