MB

How to weldduplex stainless steels

How to weld duplex stainless steelsAustenitic-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 compositionsTable 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®

23042205SAF 2507™

1.41621.43621.44621.4410

S32101S32304S32205S32750

0.030.020.020.02

0.220.100.170.27

21.5232225

1.4 4.8 5.7 7.0

0.30.33.14.0

5 Mn

MMALDX 2101230422052507/P100

eN 1600––22 9 3 N L R25 9 4 N L R

A5.4––E2209E2594

0.040.020.020.03

0.140.120.150.23

23.524.523.025.5

7.0 9.0 9.5

0.4<0.33.03.6

Wire**LDX 2101230422052507/P100

eN 12072––22 9 3 N L25 9 4 N L

A5.9––ER2209ER2594

0.020.020.020.02

0.150.150.170.25

23.523.523.025.0

7.5 7.5 8.5 9.5

0.20.23.14.0

FCWLDX 210123042205

eN 12073––22 9 3 N L

A5.22––E2209

0.020.020.03

0.140.140.13

24.024.022.5

9.0 9.0 9.0

0.60.63.2

* 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

4 54 5

MicrostructureThe 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 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 350°C up to around 950°C, secondary precipitates

are formed. Intermetallic phases, e.g. sigma phase, are formed in the 600–950°C temperature range. Ferrite is re-formed at 350–525°C (embrittlement at 475°C). 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 propertiesDuplex 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 (< –40°C).

Figure 1: Microstructure of a weld in 2205 – transition between plate and weld

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Min.-value1) Typical values (pure weld metal)P H C MMA MIG TIG SAW FCAW

ldX 2101Rp0.2 (MPa)Rm (MPa)Elongation A5 (%)Impact strength (J)+20°C–40°C

450650 30

60 –

480680 30 60 –

530700 30 – –

640800 25 45 28

520710 30

150110

550730 30

180180

570750 25

140 60

580760 25

50 40

2304Rp0.2 (MPa)Rm (MPa)Elongation A5 (%)Impact strength (J)+20°C–40°C

400630 25

100 80

400600 20

– –

420600 20

– –

640780 23

40 25

520 710 30

150 110

550730 30

180180

570750 25

140 60

580760 25

50 40

2205Rp0.2 (MPa)Rm (MPa)Elongation A5 (%)Impact strength (J)+20°C–40°C

460640 25

100 80

460660 25 – –

480660 20

– –

620810 25

45 35

550770 30

150110

610805 31

200170

590800 29

100 70

590810 29

55 40

SAF 2507Rp0.2 (MPa)Rm (MPa)Elongation A5 (%)Impact strength (J)+20°C–40°C

530730 20

100 80

530750 15

– –

530750 15

– –

695895 27

80 55

570830 29

140 –

660860 28

190170

650870 25

80 –

– – –

– –

table 2: Mechanical properties

1) P = hot rolled plate, H = hot rolled coil, C = cold rolled coil

Corrosion propertiesDuplex steels offer a very wide range of corrosion pro-perties. 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 cor-rosion 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 cor-rosion 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.

0

20

40

60

80

100

0

20

40

60

80

100

4404 LDX 2101 2304 2205 SAF 2507 254 SMO

Parent metal Welded joint

) C ° ( T P C

Steel grades

Diagram 1: Typical critical pitting temperatures (CPT) as per ASTM G48 – parent metal and weld, brushed and pickled TIG joint

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ShapingHot forming, if required, must be performed at the temperatures given in table 3. Duplex steels are prone to precipitation when they are subjected to tempera- tures under approximately 900°C. 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 for-ming. Duplex steels soften considerably at high tempe-ratures. This must be borne in mind during handling and when tooling up/positioning prior to heat treat-ment.

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.

welding methodsAll 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 resis-tance 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 limi-ted. Avesta Welding has a very wide range of covered electrodes for duplex steels:

LDX 2101 AC/DC all positions2304 AC/DC all positions2205-3D all positions2205-4D position welding2205-2D high metal recovery2205 Basic high impact strength requirements2507/P100 Rutile all positions2507/P100-4D position welding

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 pick-up. 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 trans-fer is considerably more sedate and more controlled with a pulsed arc. The opportunity for position weld-ing, 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 weld-ing 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 pro-ductivity and beautiful weld finishes are a big plus. Furthermore, the SAW work environment is consider-ably better than that of other methods. Both fume ge-neration 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.

LDX 2101® 2304 2205 SAF 2507™

Hot forming (C°) 900–1100 900–1100 950–1150 1025–1200

Solution heat treatment (C°) 1020–1080 950–1050 1020–1100 1040–1120

Stress-relieving annealing (C°) 1020–1100 950–1050 1020–1100 1040–1120

table 3: Recommended heat treatments

6 76 7

FCAW is suitable for material thicknesses above ap- proximately 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.

table 4: example welding parameters for different types of joints

Method Filler Diam. (mm) Position EN/ASTM Bead Current (A) Voltage (V) Speed (cm/min)

MMA 2205 2.503.25

PF (3G) Root*Cap

50– 60 80– 95

20–2223–25

4– 6 7– 9

MMA 2507/P100 4.00 PA (1G) 125–135 24–26 15–25

MIG 2205 1.20 PA (1G) 180–200 28–30 30–40

TIG 2205 1.60 H-L 045 (6G) Root 45– 50 9–10 3– 5

TIGFCAW

2205 2.401.20

PA (1G) RootCap

100–120190–210

16–1828–30

5– 817–22

SAW 2205 3.20 PA (1G) 400–450 30–32 40–50

SAW 2507/P100 2.40 PA (1G) 350–400 28–30 40–50

FCAW 2205 1.20 PA (1G) Root*Cap

135–145200–220

24–2628–30

20–2530–45

FCAW 2205-PW 1.20 PF (3G) RootCap

140–150160–180

23–2524–26

8–12 9–13

FCAW LDX 2101 1.20 PA (1G) RootCap

170–190200–220

26–2827–29

30–4030–45

* Single-sided

Figure 2: Welding with FCW 2205

Flux cored wire is available as LDX 2101, 2304 and 2205 in the following variants:

FCW-2D LDX 2101 welding in the flat and horizontal- vertical positionsFCW-2D 2304 welding in the flat and horizontal- vertical positionsFCW-2D 2205 welding in the flat and horizontal- vertical positions as well as against a ceramic backing in all positionsFCW 2205-PW position welding

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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 proper-ties.

Nowadays, laser hybrid welding is most often per-formed 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, name-ly, “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 con-ventional 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.

Shielding gasesMIG 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 2–3% CO2. Both of these act as arc stabilisers. An addi-tion 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 recommen-dations 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 (20–30%) 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 embritt-lement.

Single-sided root beads must be welded with a backing gas. This is normally the same as the shiel-ding 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 16–25% 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 2–3 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 20–30% helium, as both the plasma and the shielding gas. As with TIG welding, the addition of 2–3% nitrogen has a positive effect on corrosion resis-tance. 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

Method Grades Shielding gases

MIG LDX 2101, 2304, 2205

2507/P100

1. Ar+30%He+1–3%CO2 2. Ar+1–2%O2 or Ar+2–3%CO2

1. Ar+30%He+1–3%CO2

2. Ar 3. Ar+30%He+1–2%N2+1–2%CO2

TIG LDX 2101, 2304, 2205, 2507/P100

1. Ar+2%N2 +10–30%He 2. Ar

FCAW LDX 2101, 2304, 2205 1. Ar+16–25%CO2 2. 100% CO2

Plasma LDX 2101, 2304, 2205, 2507/P100

1. Ar* 2. Ar+20–30%He+1–2%N2*

Laser LDX 2101, 2304, 2205, 2507/P100

1. Ar

table 5: Shielding gases for MIG, tIG, FCAw, plasma and laser welding

* Also as plasma gas

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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 (1.5–2.0 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 demon-strated 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 + 30–35% He + 2–5% 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.

edge preparationWhen welding stainless steels, meticulous edge preparation and the correct choice of joint type are important for good results. This applies even more

particularly to duplex steels.Because of the weld pool’s slightly poorer penetra-

tion and fluidity (compared with standard austenites), the joint must be correctly designed to give full pene-tration 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 2–3 mm and a straight edge of about 0–1 mm are recommended. For double-sided welding, the straight edge can be increased to 1.5–2 mm.

• A wider root gap, 4–6 mm, should be used when welding against a ceramic backing.

Figure 3 shows a number of common joint types.

DC

DC

C

1. I-joint for: single-sided MMA, TIG and PAW; and, double-sided welding using the same methods plus MIG and FCAW. Suitable root protection must be used with single-sided TIG and plasma welding.

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.

3. V-joint for SAW. So that full penetra- tion is possible, the root bead must be ground precisely.

4. In SAW, an X-joint is to be recommended where plate thickness exceeds 16 mm.

To achieve best penetration when weld-ing 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.

D = 1.0 – 2.0 mm MIG

D = 2.0 – 2.5 mm MIGD

DC

DC

C

C

Joint type 1I-joint, t < 2.5 mmD = 1.0–2.0 mmSingle-sided, with or without root backing

I-joint, t < 4.0 mmD = 2.0–2.5 mmDouble-sided without root backing but with root grinding

Joint type 2V-joint, t = 4–16 mmα = 60°–70°C = 0.5–1.5 mmD = 2.0–4.0 mm (4–6 mm against abacking)Single-sided, with or without root backing

V-joint, t = 4–16 mmα = 60°–70°C = 2.0–2.5 mmD = 2.5–3.5 mmDouble-sided without root backing but with root grinding

Joint type 3V-joint, t = 8–16 mmα = 80°–90°C = 3–6 mmDouble-sided welding without root gap, but with root grinding

Joint type 4X-joint, t = 14–30 mmα = 80°–90°C = 3–8 mm (2507/2101 3–4 mm)Double-sided welding without root gap, but with root grinding

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Pre-weld cleaningTo ensure good weldability and reduce the need for 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, 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.

tack weldingSo that shrinkage during welding does not prevent full burn-through, precise tack welding is extremely im-portant. For metal thicknesses up to 6 mm, tack length should be 10–15 mm. This should be increased to 20–25 mm for thicker workpieces. A suitable distance between tacks is 150–200 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 weld-ing 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

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.

6. Half V-joint with full burn-through. Where grinding the root presents dif- ficulties, the root should be welded as a single-sided TIG or MMA weld or, alternatively, as FCAW against a cera- mic 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 symmet-

rical 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.

D

C

C

D

CD

CD

Joint type 5V-joint, t = 4–16 mmα = 50°C = 1.0–2.0 mmD = 2.0–3.0 mmSingle-sided without root backing

Joint type 6Half V-joint, t = 14–30mmα = 50° C = 1.5–2.5 mmD = 2.0–3.0 mm (4–6 mm against a backing)Single-sided, with or without root backing

Joint type 7U-joint, t > 20 mmα = 10°R = 8 mmC = 2.0–2.5 mmD = 2.0–2.5 mm (4–6 mm against a backing)Double-sided without root backing but with root grinding

t2

C

Dt1

A

t1

t2

D

t2

t1

C

D

C

D

DC

Rt

C

R

C

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“Starts and stops”– striking and extinguishing the arcIt is very important to use the right technique when striking and extinguishing the arc. As regards metal-lurgical, 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 polish-ing or, in the worst cases, repair welding.

In MMA welding, the arc must be extinguished care-fully 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.

Planning the welding sequenceBecause it makes burn-through unnecessary, double- sided welding is always to be preferred over single-sided 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 suit-able 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 elec-trodes. 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.

This is most simply done using MMA or TIG welding with electrode diameters of 2.50 mm and 1.60–2.40 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 pos-sible 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 al-ways the risk of slag inclusions being left behind. A suit-able grinding disc must be used. To avoid damaging adjacent surfaces, grinding should be carried out with some care. Figure 6 shows deleterious grinding scars.

Figure 5: Single-sided TIG root bead Figure 6: Grinding scars

12 1312 13

The cap bead is primarily intended to give the weld good corrosion protection. Besides structure, surface geometry can also play a critical role here. Undercut-ting, 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.

welding techniquesIn 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 10–15° in the welding direction (i.e. “forehand”) in-creases 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 wel-ding wire, backings are very often ceramic. Backing shape may vary with joint type. A root gap of 4–6 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.

distortionBroadly 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 some-what less than it is with austenitic steels. However, this does not mean that tack welding can be simplified.

PreheatingOn the whole, stainless steels (duplex steels inclu-

ded therein) must not be preheated before welding. Normally, welding takes place at room temperature. At lower temperatures, preheating to a maximum of 50°C 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 150°C 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 inter-granular corrosion.

Interpass temperatureThe recommended interpass temperature for LDX 2101 is 150°C. Both 2304 and 2205 are slightly more tolerant, but should be welded below 200°C. Super duplex steels such as SAF 2507 have a far more sen-sitive structure and, because the risk of deleterious precipitation rises sharply with increased interpass temperature, should not be welded above 100°C.

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 suit-ably 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.

Heat inputWithout 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/mm2205 max. 2.5 kJ/mmLDX 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 produc-tivity by increasing the welding parameters, heat input should never exceed the recommended value.

{ }Heat input =U x I

––––––––––V x 1,000

U x I––––––––––

mm/s x 1,000= kJ/mm

U = voltageI = currentV = speed

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Figure 7: Storage tanks are a major end use for duplex stainless steels.

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Post-weld heat treatmentDuplex 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 pre-cise control of both time and temperature. It must only be carried out by qualified personnel using suitable equipment.

welding duplex steels to other metalsDuplex 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,

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 molyb-denum).

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.

Post-weld cleaningPost-weld cleaning is critical in achieving fully satis-factory 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 resist-ance, hygiene and appearance.

Figure 8: Avesta BlueOneTM being used to spray pickle a stainless steel tank.

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Generally speaking, one basic requirement is that de-fects, 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 combina-tion 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.

defectsBroadly speaking, duplex steels are no more prone to de-fects 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 parame-ters and root gaps are more restricted.

Figure 10: Slag inclusions, SAW 2205 Figure 11: Pores, FCW LDX 2101

Figure 9: Incomplete penetration, MIG 2205

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Repair weldingAll 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 in-tended 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 fil-led 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.

Measuring ferrite contentFerrite 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

results, but is both time-consuming and costly. Hence, ferrite content is normally determined using a so- called “ferritescope” such as the Fischer Feritscope® MP30 or by calculations based on the chemical compo-sition. 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 WRC-92 diagram.

When it is obtained by measurement, ferrite content is normally expressed as a percentage. Where it is ob-tained by calculation it is usually expressed as a ferrite number (FN). A normal range is 20–70 (%/FN).

overlay weldingDuplex 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 some-what 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 par-ticular problem with SAW, FCAW and MIG welding. Welding parameters and technique are of great im-portance. Each run is built up on the preceding. The arc should never be directed towards the parent metal.

Figure 12: WRC-92 diagram

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table 7. welding duplex steels of similar compositions

Steel grades Filler metal

ASTM 329 Avesta 2205

AL 2003 (UNS S32003)

3RE60 (S31500)

Avesta 2205

Avesta 3RE60 or 2205

URANUS 35N, SAF 2304 Avesta 2304 or 2205

SAF 2205, URANUS 45N, Remanit 4462,1903SC, AF22, VS22, Falc 223, SM 22Cr, NKCr22

Avesta 2205

SAF 2507, Zeron 100, DP-3W, S32760, URANUS 52N+

Avesta 2507/P100

table 6. example chemical compositions of overlay weld metals:

SAW 2205 P54

22051 2

805 805

0.03 0.03

0.7 0.7

1.2 1.2

21.0 22.5

13.0 9.0

Mo 2.3 Mo 2.8

5 35

6 45

MMA 2205 P54 1 – 0.03 0.8 1.1 21.5 13.0 Mo 2.4 8 8

2205 2 – 0.03 0.8 0.7 22.5 9.5 Mo 2.8 25 35

MMA 2207 P52 1 – 0.03 0.8 1.1 21.5 13.0 Mo 2.4 8 8

2507/P100 2 – 0.03 0.6 1.3 24.5 10.5 Mo 3.5 25 35

FCW 2205 FCW-2D P5 1 – 0.03 0.6 1.4 22.0 12.0 Mo 2.1 15 15

FCW-2D 2205 2 - 0.03 0.7 1.1 22.5 9.5 Mo 3.1 30 40

1. Target analysis of the final layer2. Ferrite as per Schaeffler-DeLong

Inspection and quality assuranceThe rules that apply to structural steels apply also to stainless steels (duplex included therein). The follow-ing 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

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 metalsStainless steel covered electrodes, flux cored wires and fluxes can be prone to moisture pick-up. 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 elec-trodes, 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” princi-ple, 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 (± 5°C) and should not fall below 15°C. 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. Alterna-tively, they can be put in a warm heating cabinet at 60–70°C. The relative air humidity should not exceed 50%.

Flux that has not been used should be stored in a heating cabinet at 60–70°C.

Handling during welding: It is an advantage if welding can be carried out at room temperature and low relative air humidity. Covered electrodes,

Method Final layer1 Filler Layer Flux Chemical composition, % by weight Ferrite C Si Mn Cr Ni Other FN2 %3

3. Ferrite in % using a Fischer Feritscope® MP304. Welding is also possible with 2205 or 2507/P100

How to weld duplex steels of similar compositionsThere are a number of steel grades that have composi-tions similar to those of the Outokumpu duplex steels described above. Some general recommendations are set out below.

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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 pos-sible. 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, 250–280°C and 150°C. 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 250–300°C.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 pro-ducts and scrap are valuable. Wherever possible, pro-ducts and packaging must be recycled in accordance with local regulations.

Health and safetyThe 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 elect- rical shock. Thus, it is of the greatest importance that wel-ders 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 Kremsmüller Welding Academy, Austria

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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.

Avesta Welding ABP.O. Box 501, KoppardalenSE- 774 27 Avesta, Sweden

Tel: +46 (0) 226 815 00Fax: +46 (0) 226 815 75

info@avestawelding.comwww.avestawelding.com

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