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How to Weld Duplex Stainless Steels

How to Weld Duplex Stainless Steels - How to weld duplex...

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HOW TO WELD duplex stainless steels
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How to weld duplex stainless steels Austenitic-ferritic stainless steels, usually referred to as duplex steels, combine many of the good properties of ferritic and austenitic stainless steels. The high chromium content in combination with nitrogen, and often also molybdenum, gives duplex steels their superior resistance to both pitting and cre- vice corrosion. The duplex structure gives very good strength and, allied with the high corrosion resistance, very good resistance to stress corrosion. Thanks to this exceptional combination of strength and corrosion resistance, duplex steels are widely used in everything from tanks for corrosive media to structural components, chemical tankers and offshore applications. Duplex steels are primarily intended for applications where the working temperature is from –40 to +250°C. The weldability of duplex steels is good and all com- mon welding methods can be used. Uses • Heat exchangers • Water heaters • Pressure vessels • Storage tanks • Rotors, impellers and shafts • Digesters and other equipment in pulp and paper production • Cargo tanks in chemical tankers • Desalination plants • Waste gas purifiers • Sea water systems Chemical compositions Table 1 shows the chemical compositions (parent and filler metals) of some duplex steels. Matching fillers are used for welding. Fillers that are more highly alloyed can also be used. For example, LDX 2101, 2304 and 2205 can be welded with 2507/P100. Outokumpu EN ASTM LDX 2101 ® 1.4162 S32101 2304 1.4362 S32304 2205 1.4462 S32205/31803 SAF 2507 ® 1.4410 S32750 Table 1: Chemical compositions – parent and filler metals EN ASTM/AWS C N Cr Ni Mo Other Plate* LDX 2101 ® 2304 2205 SAF 2507 1.4162 1.4362 1.4462 1.4410 S32101 S32304 S32205 S32750 0.03 0.02 0.02 0.02 0.22 0.10 0.17 0.27 21.5 23 22 25 1.4 4.8 5.7 7.0 0.3 0.3 3.1 4.0 5 Mn MMA LDX 2101 2304 2205 2507/P100 EN 1600 22 9 3 N L R 25 9 4 N L R A5.4 E2209 E2594 0.04 0.02 0.02 0.03 0.14 0.12 0.15 0.23 23.5 24.5 23.0 25.5 7.0 9.0 9.5 0.4 <0.3 3.0 3.6 Wire** LDX 2101 2304 2205 2507/P100 EN 12072 22 9 3 N L 25 9 4 N L A5.9 ER2209 ER2594 0.02 0.02 0.02 0.02 0.15 0.15 0.17 0.25 23.5 23.5 23.0 25.0 7.5 7.5 8.5 9.5 0.2 0.2 3.1 4.0 FCW LDX 2101 2304 2205 EN 12073 22 9 3 N L A5.22 E2209 0.02 0.02 0.03 0.14 0.14 0.13 24.0 24.0 22.5 9.0 9.0 9.0 0.6 0.6 3.2 * Hot rolled plate, cold rolled plate, bars, pipes, pipe fittings and flanges ** MIG, TIG and SAW wire 2 SAF 2507 is a trademark owed by Sandvik AB 10.0
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4 Microstructure The chemical composition of duplex steels is balanced to ensure that, in their solution-annealed states, they have a structure with approximately equal amounts of ferrite and austenite. Duplex steels initially solidify with a completely ferritic structure. They then undergo a phase trans- formation in which primary and secondary austenite grows at the ferrite’s grain boundaries. The amount of austenite is strongly dependent on composition and cooling rate. In the production of plates, coils, pipes, etc., controlled heat treatment can be used to give a 50-50 balance of austenite and ferrite. However, cooling conditions when welding are not as good. Cooling is often very rapid here and, consequently, there is little time for austenite to form. Thus, to give a balanced structure, filler metals are always over-alloyed with nickel. This is strongly austenite stabilising. Nitrogen
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