How to Weld Duplex Stainless Steels

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

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Unformatted text preview: How to weld duplex stainless steels 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 crevice 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 +250C. The weldability of duplex steels is good and all common welding methods can be used. Outokumpu LDX 2101 2304 2205 SAF 2507 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. EN 1.4162 1.4362 1.4462 1.4410 ASTM S32101 S32304 S32205/31803 S32750 SAF 2507 is a trademark owed by Sandvik AB table 1: Chemical compositions parent and filler metals EN Plate* LDX 2101 2304 2205 SAF 2507TM LDX 2101 2304 2205 2507/P100 Wire** LDX 2101 2304 2205 2507/P100 FCW LDX 2101 2304 2205 1.4162 1.4362 1.4462 1.4410 eN 1600 22 9 3 N L R 25 9 4 N L R eN 12072 22 9 3 N L 25 9 4 N L eN 12073 22 9 3 N L ASTM/AWS S32101 S32304 S32205 S32750 A5.4 E2209 E2594 A5.9 ER2209 ER2594 A5.22 E2209 C 0.03 0.02 0.02 0.02 0.04 0.02 0.02 0.03 0.02 0.02 0.02 0.02 0.02 0.02 0.03 N 0.22 0.10 0.17 0.27 0.14 0.12 0.15 0.23 0.15 0.15 0.17 0.25 0.14 0.14 0.13 Cr 21.5 23 22 25 23.5 24.5 23.0 25.5 23.5 23.5 23.0 25.0 24.0 24.0 22.5 Ni 1.4 4.8 5.7 7.0 7.0 9.0 9.5 10.0 7.5 7.5 8.5 9.5 9.0 9.0 9.0 Mo 0.3 0.3 3.1 4.0 0.4 <0.3 3.0 3.6 0.2 0.2 3.1 4.0 0.6 0.6 3.2 Other 5 Mn MMA * Hot rolled plate, cold rolled plate, bars, pipes, pipe fittings and flanges ** MIG, TIG and SAW wire 2 Figure 1: Microstructure of a weld in 2205 transition between plate and weld 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 transformation 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 is another austenite stabilising element and is of great importance in the re-forming of austenite. However, variations of between 20 and 70% ferrite are normal. Welds with this ferrite content have good corrosion and mechanical properties. Figure 1 shows the fusion line in a 2205 joint. Welding with the "wrong" filler metal (e.g. "plate analysis"), or with no or too little filler metal (e.g. narrow groove/no root gap), can give a ferrite content of over 70%. This entails a risk of lower ductility and reduced corrosion resistance. When duplex steels are subjected to temperatures from 350C up to around 950C, secondary precipitates are formed. Intermetallic phases, e.g. sigma phase, are formed in the 600950C temperature range. Ferrite is re-formed at 350525C (embrittlement at 475C). Ferrite re-formation can have an embrittling effect and a negative impact on corrosion resistance. Hence, unnecessary exposure to these temperatures must be avoided. In normal welding, the hold time at these temperatures is relatively short. However, there is an evident risk if the metal has to undergo subsequent heat treatment. Table 3 sets out the recommended heat treatments. At any other temperatures than those given in the table, stress-relieving annealing results in lower ductility and reduced corrosion resistance. Consequently, it is to be avoided. Mechanical properties Duplex steels are characterised by high strength. Table 2 shows typical mechanical properties of parent and weld metals (pure weld metal). The high tensile strength also means that the fatigue properties are very good. However, fatigue strength is highly dependent on the component's shape. The fatigue properties of welded joints are also clearly inferior. Welding method and joint type are of great significance. For example, a TIG welded joint has considerably better properties than one made with covered electrodes. Because their ductility is lower than that of austenitic steels, duplex steels are not suitable for use at low temperatures (< 40C). 4 Corrosion properties Duplex steels offer a very wide range of corrosion properties. Thanks to the high chromium content, corrosion resistance is generally very good in most environments. This applies to both pitting and crevice corrosion. The high strength also means that the resistance to stress corrosion is very good. Because of the low carbon content, intergranular corrosion is rarely a problem. Generally speaking, corrosion resistance increases with increased nickel, chromium and nitrogen content. This is reflected in the "resistance ranking" of the duplex steels: LDX 2101; 2304; 2205 ; SAF 2507. The pitting corrosion resistance is shown in diagram 1. For the most part, the corrosion resistance of a welded joint is slightly lower than that of the parent metal. This is primarily due to: the temperature cycle undergone by the weld and the heat-affected zone (HAZ); the shape of the weld surface; and, the contaminants and defects generated in welding. To achieve the best possible corrosion resistance, the surfaces of the weld and the plate must be clean and even. After welding, the weld metal and HAZ must be pickled. Refer also to the "Pre-weld cleaning" and "Post-weld cleaning" sections. Detailed information on the corrosion properties of duplex steels is given in the corrosion handbook published by Outokumpu. CPT (C) 100 100 80 60 40 20 4404 LDX 2101 2304 2205 SAF 2507 254 SMO Welded joint 80 60 40 20 0 0 Parent metal Diagram 1: Typical critical pitting temperatures (CPT) as per ASTM G48 parent metal and weld, brushed and pickled TIG joint table 2: Mechanical properties Steel grades ldX 2101 Rp0.2 (MPa) Rm (MPa) Elongation A5 (%) Impact strength (J) +20C 40C 2304 Rp0.2 (MPa) Rm (MPa) Elongation A5 (%) Impact strength (J) +20C 40C 2205 Rp0.2 (MPa) Rm (MPa) Elongation A5 (%) Impact strength (J) +20C 40C SAF 2507 Rp0.2 (MPa) Rm (MPa) Elongation A5 (%) Impact strength (J) +20C 40C Min.-value1) P H 450 650 30 60 400 630 25 100 80 460 640 25 100 80 530 730 20 100 80 480 680 30 60 400 600 20 460 660 25 530 750 15 C 530 700 30 420 600 20 480 660 20 530 750 15 Typical values (pure weld metal) MMA MIG TIG SAW 640 800 25 45 28 640 780 23 40 25 620 810 25 45 35 695 895 27 80 55 520 710 30 150 110 520 710 30 150 110 550 770 30 150 110 570 830 29 140 550 730 30 180 180 550 730 30 180 180 610 805 31 200 170 660 860 28 190 170 570 750 25 140 60 570 750 25 140 60 590 800 29 100 70 650 870 25 80 FCAW 580 760 25 50 40 580 760 25 50 40 590 810 29 55 40 1) P = hot rolled plate, H = hot rolled coil, C = cold rolled coil 5 table 3: Recommended heat treatments LDX 2101 Hot forming (C) Solution heat treatment (C) Stress-relieving annealing (C) 9001100 10201080 10201100 2304 9001100 9501050 9501050 2205 9501150 10201100 10201100 SAF 2507TM 10251200 10401120 10401120 Shaping Hot forming, if required, must be performed at the temperatures given in table 3. Duplex steels are prone to precipitation when they are subjected to temperatures under approximately 900C. Precipitation entails a lowering of both ductility and corrosion resistance. To reduce the quantity of precipitates, the workpiece should undergo solution heat treatment after hot forming. Duplex steels soften considerably at high temperatures. This must be borne in mind during handling and when tooling up/positioning prior to heat treatment. Cold forming of duplex steels can be accomplished using conventional methods. However, because of the high strength, operations such as deep drawing, stretch forming and spinning are more difficult than they are with austenitic steels. Machining of duplex steels (LDX 2101 excepted) is, owing to their hardness, slightly more difficult than it is for austenitic steels. Tools made of high-speed steel are usually more effective than ceramic tools. With all products, direct current (DC+) gives the best welding results. Nonetheless, all rutile-acid electrodes can also be used with alternating current. However, weldability is clearly inferior than it is with direct current. A short arc is to be used for welding. This gives the best stability and reduces the risk of nitrogen pickup. The latter can lead to pore formation and increase surface oxidation. MIG welding (really MAG welding is often carried out with an active component in the shielding gas) is a particularly good method for welding sheet metal up to around 6 mm thick. Welding is usually from two sides, but sheet metal (< 4 mm) can be welded single-sided with a root backing. A spray arc or pulsed current is normally used for welding. The advantage of spray-arc welding is the higher deposition rate. However, because the weld pool is relatively large, position welding possibilities are limited. Drop transfer is considerably more sedate and more controlled with a pulsed arc. The opportunity for position welding, especially vertical-down, is thus very great. As the stability of a spray arc is relatively poor, a pulsed arc is particularly important when welding the super duplex steel, SAF 2507. The MIG method is especially suited to robot or automatic welding in all positions. TIG welding is normally used for thin (up to around 4 mm) workpieces. It is especially common in the welding of pipe joints. The method is also highly suitable for welding single-sided root beads (both with and without root backing). Subsequent beads can then be welded using a method with a higher deposition rate. SAW is widely used with duplex steels. Its high productivity and beautiful weld finishes are a big plus. Furthermore, the SAW work environment is considerably better than that of other methods. Both fume generation and radiation are minimal. The disadvantages of SAW are that it is restricted to the horizontal position and that the heat input is relatively large. Consequently, small objects present problems. A basic agglomerated flux, e.g. Avesta 805, must be used for SAW. welding methods All conventional welding methods such as MMA (covered electrodes), MIG/MAG, TIG, SAW, FCAW, plasma and laser can be used to weld duplex steels. Welding without filler metals is only permitted where subsequent heat treatment (solution heat treatment) is possible. If heat treatment is not carried out, there is a great risk that the ferrite content in the weld metal will be too high. As a result, ductility and corrosion resistance will be lower. Property requirements, positional weldability and productivity usually determine the choice of welding method. MMA welding is particularly excellent for position welding, single-sided welding and where access is limited. Avesta Welding has a very wide range of covered electrodes for duplex steels: LDX 2101 AC/DC 2304 AC/DC 2205-3D 2205-4D 2205-2D 2205 Basic 2507/P100 Rutile 2507/P100-4D all positions all positions all positions position welding high metal recovery high impact strength requirements all positions position welding 6 Figure 2: Welding with FCW 2205 FCAW is suitable for material thicknesses above approximately 2.5 mm. Thanks to the slag that is formed, positional weldability is very good. When FCW is used, the arc and weld pool are protected by both the slag and the shielding gas. Drop transfer is even and finishes are extremely smooth and fine. FCAW can advantageously be used for single-sided welding against a ceramic backing. This is fast and efficient. At the same time, the surface properties on the root side are very good. For the best results, the root bead should here be welded using a slightly lower current intensity. Flux cored wire is available as LDX 2101, 2304 and 2205 in the following variants: FCW-2D LDX 2101 welding in the flat and horizontalvertical positions FCW-2D 2304 welding in the flat and horizontalvertical positions FCW-2D 2205 welding in the flat and horizontalvertical positions as well as against a ceramic backing in all positions FCW 2205-PW position welding table 4: example welding parameters for different types of joints Method MMA MMA MIG TIG TIG FCAW SAW SAW FCAW FCAW FCAW Filler 2205 2507/P100 2205 2205 2205 2205 2507/P100 2205 2205-PW LDX 2101 Diam. (mm) 2.50 3.25 4.00 1.20 1.60 2.40 1.20 3.20 2.40 1.20 1.20 1.20 Position EN/ASTM PF (3G) PA (1G) PA (1G) H-L 045 (6G) PA (1G) PA (1G) PA (1G) PA (1G) PF (3G) PA (1G) Root* Cap Root Cap Root Cap Root Root Cap Bead Root* Cap Current (A) 50 60 80 95 125135 180200 45 50 100120 190210 400450 350400 135145 200220 140150 160180 170190 200220 Voltage (V) 2022 2325 2426 2830 910 1618 2830 3032 2830 2426 2830 2325 2426 2628 2729 Speed (cm/min) 4 6 7 9 1525 3040 3 5 5 8 1722 4050 4050 2025 3045 812 913 3040 3045 * Single-sided 7 Shielding gases Laser, laser hybrid and plasma welding are high productivity methods that are very suitable for duplex steels. However, as previously stated, if a filler metal is not used, the workpiece should be heat treated after welding. Laser hybrid is a particularly interesting method. It combines keyhole welding (laser) with arc welding (MIG/MAG, TIG or plasma). The method ensures a high productivity process that, thanks to the filler metal and the low heat input, preserves metallurgical properties. Nowadays, laser hybrid welding is most often performed using a CO2 laser or a Nd:YAG laser. With the exception of the considerably better penetration, laser hybrid welding of thin sheets has much in common with ordinary MIG/MAG welding. Penetration depth is primarily determined by the laser beam's ability to create a keyhole. The width is dependent on the heat transferred by the arc. There are two variants of laser hybrid welding, namely, "leading" and "trailing" laser. Whichever is chosen, it is important that the arc and the beam are sufficiently close to each other for them to work in the same weld pool. For better process stability in "leading" laser hybrid welding, the angle of the MIG/MAG nozzle should be as slight as possible (i.e. nozzle in the upright position). Having the arc in the leading position allows material from the filler wire to fill any gaps. This means that the laser beam creates a keyhole in a stable weld pool. The result is an even weld with good penetration. In the laser-MIG/MAG process, the following parameters have proved to be important: torch angle, "offset", stick-out, working distance and focal length. The effect of torch angle is much the same as in conventional MIG/MAG welding. Spray and pulsed arcs can advantageously be used. However, because there is no stabilising of the arc, a short arc must not be used in laser-MIG/MAG welding. table 5: Shielding gases for MIG, tIG, FCAw, plasma and laser welding Method MIG Grades LDX 2101, 2304, 2205 Shielding gases 1. Ar+30%He+13%CO2 2. Ar+12%O2 or Ar+23%CO2 1. Ar+30%He+13%CO2 2. Ar 3. Ar+30%He+12%N2+12%CO2 1. Ar+2%N2 +1030%He 2. Ar 1. Ar+1625%CO2 2. 100% CO2 1. Ar* 2. Ar+2030%He+12%N2* 1. Ar MIG welding of duplex steels is possible using the conventional shielding gases used with stainless steels. Normally, argon is used with an addition of 2% O2 or 23% CO2. Both of these act as arc stabilisers. An addition of around 30% helium is advantageous. It increases arc energy which, in turn, increases weld pool fluidity and enables higher welding speeds. Using a pulsed arc, a four-component gas (Ar +30% He + 2.5% CO2 + 0.03% NO) has given very good results. Arc stability varies greatly between different arc types, different steel grades and even between different welding machines. Table 5 sets out general recommendations for the MIG welding of various duplex grades. TIG welding is usually performed with pure argon as the shielding gas. Resistance to, in particular, pitting corrosion can be considerably raised by the addition of up to 2% nitrogen. However, because the risk of pores increases with increased nitrogen content, the latter should not exceed 2%. The addition of around 30% helium markedly increases arc energy and thus enables a considerable increase (2030%) in welding speed. In the welding of duplex steels, the addition of hydrogen is not to be recommended. In combination with the high ferrite content (over 70%), this can lead to hydrogen embrittlement. Single-sided root beads must be welded with a backing gas. This is normally the same as the shielding gas. However, Formier gas (90% N2 + 10% H2) is a good alternative that also provides first-class root protection while also being cheaper than pure argon. Because only a negligible quantity of the hydrogen penetrates the weld metal, no negative effect has been demonstrated. A backing gas should also be used for tack welding all the way up until weld thickness is at least 8 mm. FCAW is most suitably performed using argon with an addition of 1625% carbon dioxide as the shielding gas. Welding with pure carbon dioxide is also possible, but arc stability and weld pool control are noticeably poorer. However, compared with a mixed gas, one advantage is that penetration is slightly better. Also compared with a mixed gas, the voltage should be increased by 23 volts when welding with pure carbon dioxide. This prevents the arc being too short. Plasma welding normally uses pure argon, or argon with an addition of 2030% helium, as both the plasma and the shielding gas. As with TIG welding, the addition of 23% nitrogen has a positive effect on corrosion resistance. The addition of hydrogen should be avoided. Laser welding can be carried out with pure argon, nitrogen, helium or mixtures of these gases. To ensure high-quality welds when using a CO2 laser or a 2507/P100 TIG FCAW Plasma Laser LDX 2101, 2304, 2205, 2507/P100 LDX 2101, 2304, 2205 LDX 2101, 2304, 2205, 2507/P100 LDX 2101, 2304, 2205, 2507/P100 * Also as plasma gas Nd:YAG-laser, a shielding gas is required. Because interaction between the beam and the shielding gas affects heat transfer to the workpiece, the choice of shielding gas in CO2 laser welding is critical. The normal shielding gases are pure argon or, where high laser powers (1.52.0 kW) are used, helium. As there is little or no interaction between shielding gases and the wavelength of the Nd:YAG laser, argon, which is relatively cheap, is normally used. Laser hybrid welding with a CO2 laser has demonstrated that the shielding gas need not be pure helium. It is sufficient that a minimum of 30% helium is added via the MIG/MAG nozzle. For Nd:YAG laser hybrid welding, a mixture of Ar + 3035% He + 25% CO2 can advantageously be used. The mixture is added via the MIG/MAG nozzle. The addition of helium improves process stability and gives even welds. particularly to duplex steels. Because of the weld pool's slightly poorer penetration and fluidity (compared with standard austenites), the joint must be correctly designed to give full penetration without the risk of burn-through. The groove angle must be sufficiently wide to allow the welder full control of the arc, weld pool and slag. A groove angle of around 35 (i.e. somewhat larger than for austenitic steels) is to be recommended for manual welding. General recommendations: An X-joint can advantageously be used for plate thicknesses above approximately 15 mm. For plate thicknesses above approximately 30 mm, a double U-joint is advantageous. In single-sided welding, a root gap of 23 mm and a straight edge of about 01 mm are recommended. For double-sided welding, the straight edge can be increased to 1.52 mm. A wider root gap, 46 mm, should be used when welding against a ceramic backing. edge preparation When welding stainless steels, meticulous edge preparation and the correct choice of joint type are important for good results. This applies even more Figure 3 shows a number of common joint types. Joint type 1 D = 1.0 2.0 mm I-joint, t < 2.5 mm D = 1.02.0 mm Single-sided, with or without root backing D = 2.0 2.5 mm I-joint, t < 4.0 mm D = 2.02.5 mm Double-sided without root backing but with root grinding D 1. I-joint for: single-sided MMA, TIG MIG and PAW; and, double-sided welding using the same methods plus MIG and MIG FCAW. Suitable root protection must be used with single-sided TIG and plasma welding. C Joint type 2 V-joint, t = 416 mm = 6070 C = 0.51.5 mm D = 2.04.0 mm (46 mm against abacking) Single-sided, with or without root backing V-joint, t = 416 mm = 6070 C = 2.02.5 mm D = 2.53.5 mm Double-sided without root backing but with root grinding D D C 2. V-joint (t > 4 mm) for: single and double-sided MMA and TIG welding as well as double-sided MIG and FCAW. Single-sided welding is also possible with FCAW, but a ceramic backing must then be used. D C Joint type 3 V-joint, t = 816 mm = 8090 C = 36 mm Double-sided welding without root gap, but with root grinding D C C 3. V-joint for SAW. So that full penetration is possible, the root bead must be ground precisely. Joint type 4 X-joint, t = 1430 mm = 8090 C = 38 mm (2507/2101 34 mm) Double-sided welding without root gap, but with root grinding C C 4. In SAW, an X-joint is to be recommended where plate thickness exceeds 16 mm. To achieve best penetration when welding 2205 and 2304, the straight edge can be increased up to 8 mm. The torch must then be slightly angled (around 15) in the direction of welding. In this way, thicknesses up to 20 mm can be welded with only two beads. However, for LDX 2101 and SAF 2507, the straight edge should not exceed 4 mm. 9 C D Joint type 5 V-joint, t = 416 mm = 50 C = 1.02.0 mm D = 2.03.0 mm Single-sided without root backing t2 A C D t1 5. Edge preparation for pipe joints. Welding is most suitably performed using TIG or MMA for the root bead. For in creased productivity, FCAW may then be used. C D t2 Joint type 6 Half V-joint, t = 1430mm = 50 C = 1.52.5 mm D = 2.03.0 mm (46 mm against a backing) Single-sided, with or without root backing D C D C t1 C t2 Joint type 7 U-joint, t > 20 mm = 10 R = 8 mm C = 2.02.5 mm D = 2.02.5 mm (46 mm against a backing) Double-sided without root backing but with root grinding 6. Half V-joint with full burn-through. Where grinding the root presents difficulties, the root should be welded as a single-sided TIG or MMA weld or, alternatively, as FCAW against a ceramic backing. In this type of joint, the distance between tacks should not exceed 150 mm. This is so that shrinkage does not prevent full burn-through. 7. Simple U-joint for the welding of thick sections (t > 30 mm). The joint can advantageously be made as a symmetrical or asymmetrical double U-joint. Root welding is most suitably carried out as a TIG or MMA weld followed by, for example, FCAW or SAW. C D t1 t R D C Pre-weld cleaning To ensure good weldability and reduce the need for R post-weld cleaning, all joint surfaces, and the surfaces adjoining these, must be thoroughly cleaned before welding. Dirt, oil and grease must be removed using, D for example, a cleaning agent such as Avesta Cleaner. All rough edges must be completely removed by gentle grinding. Oxides, paints and primers must be entirely removed not only in the joint but also in the 50 mm from the joint edges. C tack welding So that shrinkage during welding does not prevent full burn-through, precise tack welding is extremely important. For metal thicknesses up to 6 mm, tack length should be 1015 mm. This should be increased to 2025 mm for thicker workpieces. A suitable distance between tacks is 150200 mm. In single-sided welding, the entire tack must be ground away before welding. In double-sided welding, it is sufficient to grind away the beginning and the end of the tack. A common alternative in single-sided welding is the use of bridges or distance pieces (see figure 4). These must be made of, and tacked with, duplex steel. Note that gap width must be constant throughout the joint. Figure 4: Tack welding of thick-walled pipe using distance pieces 10 "Starts and stops" striking and extinguishing the arc It is very important to use the right technique when striking and extinguishing the arc. As regards metallurgical, mechanical and corrosion properties, each start and stop is a "critical" area. To avoid striking scars, the arc must always be struck down in the joint. If, despite this, striking scars occur, they must be carefully repaired by grinding and polishing or, in the worst cases, repair welding. In MMA welding, the arc must be extinguished carefully by first making several circular movements in the centre of the weld pool. The electrode is then to be moved slowly backwards 10 mm through the weld pool before being gently lifted. If this is done too quickly, crater cracks and slag inclusions may result. Modern power sources for MIG and TIG welding often have a so-called crater filling facility. This gives smooth and controlled stops. To remove any crater cracks and slag inclusions, each start and stop must be carefully ground with a suitable grinding disc. This is most simply done using MMA or TIG welding with electrode diameters of 2.50 mm and 1.602.40 mm respectively. As already stated, a backing gas must be used in TIG welding. Single-sided welding without root backing places the severest demands on even and thorough edge preparation. Figure 5 shows a correctly executed TIG root bead. Root beads must satisfy three important requirements: Correct metallurgy and structure (right root gap to ensure sufficient quantity of filler metal). Correct geometry (no concavity, undercutting or lack of fusion). Best possible productivity (always in relation to weldability). Filler beads must be deposited with the highest possible productivity. At the same time, structure and mechanical properties have to be maintained. In many cases, fill passes use the same filler metal as that used in root passes. High productivity welding methods may thus be economical for joint filling. Several common choices are: TIG root pass + MMA, MIG or SAW fill passes MMA root pass + SAW or FCAW fill passes Generally speaking, welding is carried out with the highest possible heat input that is still consistent with maintained properties and weldability. Visual inspection between the passes is important. Slag residues and severe welding oxide are removed before depositing the next layer. Otherwise, there is always the risk of slag inclusions being left behind. A suitable grinding disc must be used. To avoid damaging adjacent surfaces, grinding should be carried out with some care. Figure 6 shows deleterious grinding scars. Planning the welding sequence Because it makes burn-through unnecessary, doublesided welding is always to be preferred over singlesided welding. To ensure full burn-through on the last bead, the root side must be ground to clean metal. A grinding disc not exceeding 2 mm in width is a suitable tool. If it is difficult to decide whether grinding has reached the first bead, penetrant testing can be used. In double-sided MMA welding, electrodes with a diameter of 3.25 to 4.00 mm can be used from the very start. Single-sided welding is most simply carried out against a root backing. Single-sided root beads are suitably welded with a 2.50 mm diameter electrode. The joint is then filled using 3.25, 4.00 or 5.00 mm electrodes. The choice of electrode diameter is determined by welding position. In certain cases (e.g. pipe joints) single-sided welding without root backing is required. Figure 5: Single-sided TIG root bead Figure 6: Grinding scars 11 The cap bead is primarily intended to give the weld good corrosion protection. Besides structure, surface geometry can also play a critical role here. Undercutting, unevenness, high crowns, gaps, etc. can all have a negative impact on corrosion resistance. Aesthetic considerations are often also important. When using slag forming welding methods, weld reinforcements must be cleaned of all slag residues. Interpass temperature The recommended interpass temperature for LDX 2101 is 150C. Both 2304 and 2205 are slightly more tolerant, but should be welded below 200C. Super duplex steels such as SAF 2507 have a far more sensitive structure and, because the risk of deleterious precipitation rises sharply with increased interpass temperature, should not be welded above 100C. Thermal conductivity is of the same order as that of austenitic stainless steels, i.e. considerably lower than it is for low-alloy and carbon steels. This means that, compared to carbon steels, it takes longer to reach the correct interpass temperature. The cooling rate can be increased by using compressed air. This is most suitably directed at the back of the plate or the inside of the pipe. Compressed air directed straight into the welded joint presents the risk of contamination. Cooling can also be accelerated by intermittent welding or using a correctly planned welding sequence. The interpass temperature must be measured. Some form of thermometer or thermoelement is appropriate for this. Temperature crayons seldom give good results and must be avoided. welding techniques In the flat position, there should be no significant weaving. However, in the vertical-up position, weaving of up to 20 mm is advantageous. For the best control of arc and weld pool, welding is normally carried out with a torch or electrode angle of around 10 away from the welding direction, i.e. "backhand". In submerged arc welding, the torch is not normally angled. A torch angle of 1015 in the welding direction (i.e. "forehand") increases penetration. This allows the unbevelled edge to be increased to up to around 8 mm. However, because LDX 2101 and SAF 2507 are slightly more sensitive to the necessary high heat input, this increase must only be used for 2205 and 2304. Especially when using welding wire, backings are very often ceramic. Backing shape may vary with joint type. A root gap of 46 mm most often gives a nicely shaped root bead. Too wide a gap can result in a too thin root bead that, in the worst cases, may crack because of the degree of restraint. Ceramic backings are frequently used for welding stainless steel cargo tanks in chemical tankers. Here, welding is often in difficult positions with little access from both sides. Heat input Without negatively affecting microstructure and, consequently, properties, 2205 can be welded using a relatively high heat input. Heat inputs above 3 kJ/mm have been used with no negative effects. Welding method, radiation, distortion and weld pool size are most often the limiting factors (rather than heat input). LDX 2101, 2304 and, in particular, SAF 2507 must be welded with lower heat inputs. General recommendations: 2304 max. 2.0 kJ/mm 2205 max. 2.5 kJ/mm LDX 2101, SAF 2507 max. 1.5 kJ/mm Duplex steels should not be welded with a too low heat input. The cooling rate could then be very high, which might result in a high ferrite content (above 70%). This is particularly true in the welding of thick workpieces. Theoretical minimum heat inputs are 0.5 kJ/mm for 2304 and 2205 and 0.3 kJ/mm for LDX 2101 and SAF 2507. Especially in automatic welding, heat input is easy to control. Although it is always desirable to optimise productivity by increasing the welding parameters, heat input should never exceed the recommended value. Heat input = UxI V x 1,000 distortion Broadly speaking, the coefficient of expansion of duplex steels is lower than that of austenitic steels. It is only slightly higher than that of carbon steels. Consequently, distortion during the welding of duplex steels is somewhat less than it is with austenitic steels. However, this does not mean that tack welding can be simplified. Preheating On the whole, stainless steels (duplex steels included therein) must not be preheated before welding. Normally, welding takes place at room temperature. At lower temperatures, preheating to a maximum of 50C is advisable. This drives off any moisture that may otherwise lead to pore formation. When welding castings, or where the workpiece is thick or where restraint is high, preheating to a maximum of 150C may be advantageous. This is particularly true where the welding method has a low heat input (max. 0.5 kJ/mm). In these cases, a suitable preheating method is the use of electric blankets or similar. The use of soot-depositing flames can result in local carbon pick-up. This reduces resistance to intergranular corrosion. { } UxI = kJ/mm mm/s x 1,000 U = voltage I = current V = speed 12 Figure 7: Storage tanks are a major end use for duplex stainless steels. 13 Figure 8: Avesta BlueOneTM being used to spray pickle a stainless steel tank. Post-weld heat treatment Duplex stainless steels do not normally need post-weld heat treatment. However, in certain situations, it may be necessary to subject the workpiece to solution heat treatment or stress-relieving annealing. The spinning of dished ends is just such an example. Shaping is here carried out in stages with intermediate heat treatment. Table 3 gives the recommended temperatures. The heat treatment of duplex steels requires very precise control of both time and temperature. It must only be carried out by qualified personnel using suitable equipment. which also gives a weld metal that is highly resistant to cracking. Welding to other stainless steels such as EN 1.4301 or EN 1.4401 is also fully possible. It can be done with a duplex filler metal or with Avesta P5 or Avesta 309L (only stainless steels that are not alloyed with molybdenum). Welding to fully austenitic steels or nickel base alloys is suitably carried out using a filler metal that matches the other metal, for example, Avesta P12 when welding 2205 to 254 SMO. welding duplex steels to other metals Duplex or austenitic filler metals such as Avesta P5 (309MoL) or Avesta 309L are used to weld duplex steels to carbon or low-alloy steels. As austenitic metals demonstrate a somewhat greater toughness, Avesta P5 or 309L may be particularly suitable for welding workpieces where there is a high degree of restraint (t > 20 mm). A further alternative is to use Avesta P7, Post-weld cleaning Post-weld cleaning is critical in achieving fully satisfactory corrosion resistance. Clearly enough, it is thus an integral part of the entire stainless steel welding procedure. Despite this, post-weld cleaning is not always standard. The method and extent of cleaning is determined by the requirements imposed in respect of corrosion resistance, hygiene and appearance. 14 Generally speaking, one basic requirement is that defects, welding oxide, organic contaminants and carbon steel contamination must be removed from weld and parent metal surfaces. This can be done mechanically (grinding, brushing, polishing, blasting) or chemically (pickling). An important rule of thumb for grinding is to always finish with polishing. The risk of harmful grinding scars is otherwise very great. The demonstrably most reliable method is a combination of mechanical and chemical cleaning, i.e. brushing with a stainless steel brush followed by pickling. Avesta Finishing Chemicals has a complete product programme for the pickling of stainless steel welds. It comprises cleaning products, pickling pastes, pickling sprays, pickling fluids and various items of equipment. Duplex steels are generally more difficult to pickle than are austenitic steels such as 1.4401 (308L) and 1.4404 (316L). Thus, Avesta BlueOneTM and Avesta RedOneTM, which are comparatively strong pickling products, should be used for pickling duplex grades. Further details are available at www.avestafinishing.com or can be obtained directly from Avesta Finishing Chemicals. defects Broadly speaking, duplex steels are no more prone to defects than other stainless steels. However, several factors require special attention. The high nitrogen content of duplex steels means poorer penetration. Compared to austenitic steels, there is a slightly greater tendency to pore formation. Arc stability, fluidity and arc control are also somewhat poorer than they are for austenitic stainless steels. Consequently, to avoid incomplete penetration, slag inclusions and pores, the margins for welding parameters and root gaps are more restricted. Figure 9: Incomplete penetration, MIG 2205 Figure 10: Slag inclusions, SAW 2205 Figure 11: Pores, FCW LDX 2101 15 Repair welding All defects must be suitably repaired. Minor surface defects such as spatter, slag and oxide islands can easily be remedied by grinding followed by polishing using an at least 320 mesh disc. Note that a grinding disc intended for stainless steel must be used. After polishing, conventional pickling is to be carried out. Pickling paste is most often the simplest alternative. Defects must never be repaired by TIG dressing (remelting using a TIG electrode). This is because TIG dressing has the same effect as welding without filler metal, i.e. high ferrite content. Large defects and subsurface defects require heavier grinding with a coarser grinding disc. Once the entire defect has been removed (which can be checked by, for example, penetrant testing), the ground area is to be filled using a suitable method, most often MMA welding. A plasma arc can be used to remove deep subsurface defects in thick workpieces. Because of the resultant carbonisation, carbon arcs should not be used. The problem with both plasma and carbon arcs is the powerful spatter. If care is not taken, this can damage adjacent surfaces. The latter should be protected using, for example, Masonite or chalk paint. After gouging, the area must be ground before welding can start. Repair welding can be carried out at least 5 times with no negative impact on the parent metal. results, but is both time-consuming and costly. Hence, ferrite content is normally determined using a socalled "ferritescope" such as the Fischer Feritscope MP30 or by calculations based on the chemical composition. There are a number of calculation methods, e.g. DeLong and WRC-92. For duplex steels, calculation as per WRC-92 gives results that are closer to reality than those obtained using DeLong. Figure 12 shows a WRC92 diagram. When it is obtained by measurement, ferrite content is normally expressed as a percentage. Where it is obtained by calculation it is usually expressed as a ferrite number (FN). A normal range is 2070 (%/FN). overlay welding Duplex filler metals can be advantageously used for the overlay welding of carbon steels. The duplex overlay is resistant to corrosion and has good wear resistance. Although all welding methods can be used, those with a high deposition rate (i.e. SAW, FCAW and MIG) are normally preferred. Welding with 2205 can be direct onto carbon steel. However, filler metals such as 309L or P5 can also be used for the first layer. This is somewhat more cost-efficient, especially when welding with 2507/P100. In overlay welding, there should be as little mixing with the parent metal as possible. This can be a particular problem with SAW, FCAW and MIG welding. Welding parameters and technique are of great importance. Each run is built up on the preceding. The arc should never be directed towards the parent metal. Measuring ferrite content Ferrite content can be assessed in several ways. Point counting, which is a standardised method (ASTM E562), is one of these. This method gives very precise Figure 12: WRC-92 diagram 16 table 6. example chemical compositions of overlay weld metals: Method SAW MMA MMA FCW Final layer1 2205 2205 2207 2205 Filler P54 2205 P54 2205 P52 2507/P100 FCW-2D P5 FCW-2D 2205 Layer 1 2 1 2 1 2 1 2 Flux 805 805 Chemical composition, % by weight C Si Mn Cr Ni 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.7 0.7 0.8 0.8 0.8 0.6 0.6 0.7 1.2 1.2 1.1 0.7 1.1 1.3 1.4 1.1 Other Mo 2.3 Mo 2.8 Mo 2.4 Mo 2.8 Mo 2.4 Mo 3.5 Mo 2.1 Mo 3.1 Ferrite FN2 %3 5 35 8 25 8 25 15 30 6 45 8 35 8 35 15 40 21.0 22.5 21.5 22.5 21.5 24.5 22.0 22.5 13.0 9.0 13.0 9.5 13.0 10.5 12.0 9.5 1. Target analysis of the final layer 2. Ferrite as per Schaeffler-DeLong 3. Ferrite in % using a Fischer Feritscope MP30 4. Welding is also possible with 2205 or 2507/P100 How to weld duplex steels of similar compositions There are a number of steel grades that have compositions similar to those of the Outokumpu duplex steels described above. Some general recommendations are set out below. table 7. welding duplex steels of similar compositions Steel grades ASTM 329 AL 2003 (UNS S32003) 3RE60 (S31500) URANUS 35N, SAF 2304 SAF 2205, URANUS 45N, Remanit 4462,1903SC, AF22, VS22, Falc 223, SM 22Cr, NKCr22 SAF 2507, Zeron 100, DP-3W, S32760, URANUS 52N+ Filler metal Avesta 2205 Avesta 2205 Avesta 3RE60 or 2205 Avesta 2304 or 2205 Avesta 2205 Avesta 2507/P100 measurement using a "ferritescope". In ultrasound testing, it is important that surfaces are ground flat so that defects such as pores and cracks can be reliably detected. Handling of filler metals Stainless steel covered electrodes, flux cored wires and fluxes can be prone to moisture pickup. Avesta Welding's consumables are supplied in packages that have been designed to resist moisture. However, for the best possible results, the following storage and handling precautions are still recommended. Storage of unbroken packages: Covered electrodes, FCWs and fluxes must be stored in their unbroken, original packaging. Storage in opened packaging can considerably shorten the product's service life. Following the "first in, first out" principle, storage time must be kept as short as possible. Covered electrodes and fluxes should not be stored longer than 5 years. Products that are over 5 years old should be redried before use. Covered electrodes, FCWs and fluxes should not be stored in direct contact with floors or outer walls. Storeroom temperature must be kept as even as possible ( 5C) and should not fall below 15C. The relative air humidity should not exceed 50%. Handling of opened packages: Electrodes that remain unused at the end of a shift should be replaced in their packaging and resealed. Alternatively, they can be put in a warm heating cabinet at 6070C. The relative air humidity should not exceed 50%. Flux that has not been used should be stored in a heating cabinet at 6070C. Handling during welding: It is an advantage if welding can be carried out at room temperature and low relative air humidity. Covered electrodes, Inspection and quality assurance The rules that apply to structural steels apply also to stainless steels (duplex included therein). The following are some of the relevant international standards: ISO 5817, which gives guidelines on acceptance levels for various defects in welded joints. EN 288 and ASME IX, which describe the approval of welding procedures. However, duplex steels are used in applications where the strength and corrosion requirements are very severe. There is thus every reason to be extra careful from beginning to end. Welding oxide, spatter, striking scars and grinding scars must be removed to achieve the correct corrosion resistance. For the best fatigue resistance, the weld surface must be even with no sharp edges. Nondestructive testing is an integral part of the examination of welded joints. Suitable methods are visual inspection, penetrant testing (PT), radiographic testing (RT), ultrasound testing (UT) and ferrite content 17 Health and safety FCWs and fluxes should be used at the same rate as they are unpacked preferably within 24 hours. During shifts, electrodes must be kept as dry as possible. If the climate so demands, they should be kept warm in a portable heat-retaining container or similar. One alternative is to use smaller packs, e.g. half or quarter capsules. Rebaking: Electrodes and flux cored wires that have sustained slight moisture damage can be rebaked for around 3 hours at, respectively, 250280C and 150C. Heating and cooling must both be gradual. Items should not be rebaked any more than three times. Fluxes can be rebaked for 2 hours at 250300C. Procedures that have been approved for carbon steel electrodes are also completely satisfactory for stainless steel electrodes. This is because the latter are not as prone to moisture pick-up. Recycling: Because they can be reused, leftover products and scrap are valuable. Wherever possible, products and packaging must be recycled in accordance with local regulations. The fumes and radiation given off during welding can be hazardous to health. Spatter, molten metal and arcs can cause burns and fires. Furthermore, electrical equipment is used. If it is not handled correctly, there is the risk of electrical shock. Thus, it is of the greatest importance that welders and supervisors are aware of all the potential dangers. Ensure that ventilation is adequate and that the welding site has an extractor system that removes fumes and gases from the welder's "breathing zone". When welding in confined spaces, use respiratory protective equipment or a compressed air line breathing apparatus. Use safety equipment for hands, eyes and body, e.g.: gloves; helmet or face mask with filter glass; safety boots; apron; and arm and shoulder guards. Keep the workplace and equipment clean and dry. Regularly check that safety clothing and equipment are in good condition. As far as possible, insulate all conducting elements. Further information on each product group is contained in Avesta Welding's material safety data sheets. These can be downloaded from Avesta Welding's website, www.avestawelding.com, or ordered from Avesta Welding's distributors and retailers. Figure 13: Order and tidiness are essential for a good work environment. Photo: The Karl Kremsmller Welding Academy, Austria 1 Figure 14: Storage tanks in chemical tankers are often made of duplex stainless steels. All rights reserved. Contents subject to change without warning or notification. Great care has been taken to ensure that the contents of this publication are correct. However, Avesta Welding and its subsidiaries cannot accept responsibility for errors or for information that is found to be misleading. Suggestions for, or descriptions of, working methods or of the use, treatment or machining of products are for information only and Avesta Welding and its subsidiaries can accept no liability in respect thereof. Before using products supplied or manufactured by the company, customers should satisfy themselves of product suitability. 19 Avesta Welding AB P.O. Box 501, Koppardalen SE- 774 27 Avesta, Sweden Tel: +46 (0) 226 815 00 Fax: +46 (0) 226 815 75 info@avestawelding.com www.avestawelding.com 10601EN-GB, Centrum Tryck, Avesta 2006. ...
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