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Twin Submerged Arc Welding

13BME08113BME08213BME08313BME08413BME086

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Submerged Arc Welding The modern SAW is an arc welding process, in which one or more arcs formed between one or more bare wire electrodes and the work piece provides the heat coalescence.

Are is completely submerged under a blanket of granular, fusible flux.

Fully automatic or semi automatic process

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Submerged Arc Welding Fully Automatic

◦ Flux fed mechanically ahead of the arc◦ Wire fed automatically◦ Arc length controlled automatically

Semi automatic ◦ Wire feed and arc length control automatically◦ Welder moves the welding gun◦ Flux feed may be by gravity flow

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Submerged Arc Welding Methods1. • Single-wire welding

2. • Twin-arc welding

3. • Tandem welding

4. • Tandem Twin welding

5. • Strip welding

6. • Narrow gap welding

7. • Cold wire addition

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Basic Equipment A wire feeder to drive the electrode to the work through the contact tube of welding gun or welding head

A welding power source to supply electric current to the electrode at the contact tube

An arrangement for holding the flux and feeding it ahead of the arc

A means of traversing the weld joint

Twin torch

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Twin Submerged Arc Welding Twin arc welding involves feeding two wires in parallel through the same contact tip. It differs from tandem welding in using only one power unit and one wire feeder. In comparison with the use of a single wire, twin arc welding results in a higher rate of melt production and improved stability. A twin-arc welding machine can be easily produced by fitting a single-wire machine with feed rollers and contact tips for two wires. Without very much higher capital costs, it is possible to increase the deposition rate by 30-40 % in comparison with that of a single-wire machine. Wire sizes normally used for butt welding are 2.0,2.5 and 3.0 mm, with wire separations of about 8 mm. Depending on the desired result, the wires may be arranged side by side or one behind the other.

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Twin Submerged Arc Welding For twin-wire welding, two wires are connected to the same power source.

A standard SAW machine is equipped with double drive rolls and contact tips suitable for feeding two wires simultaneously.

It produces considerably higher deposition rates than the conventional single-wire process using large diameter wires.

Very high welding speeds can be achieved in fillet welding, but are also used successfully for butt welding.

http://www.esab.com/automation/en/process/subarc-general/SAW-Twin-wire.cfm

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Schematic of Twin SAW

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Tandem twin SAW For higher deposition rates, it can be obtained when tandem welding is combined with twin wires.

The tandem twin process is simply a combination of tandem and twin-wire welding.

It can use a combination of DC(+)/AC or AC/AC for greater deposition rate.

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A tandem twin welding head is shown in Figure. With the use of 4x2.5 mm diameter, wires deposition rates of up to 38 kg/h can be achieved.

The process can be used in joints that allows accessibility for the equipment, e.g. circumferential welding in wind tower fabrication

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Parallel twin wire Series twin wire

Twin- wire SAW having two versions: I) Twin-wire parallel Power II) Twin-wire series power

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Twin-wire parallel power Two electrodes are fed at the same rate through a common tip

The current from the single power source being split between them

The electrode also share the drive motor and control of equipment and therefore carry identical arc voltage.

Electrode dia: 1.6 to 3.2 mm

Spacing between them: 8 to 16 mm

DC power source with constant voltage type used

Fig: Twin arcs of same polarity in action

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Twin-wire series power Main advantage of this system is high deposition rate and minimum dilution with base metal

Two welding heads are used with a single DC or AC power source AC used for ferrous metals; DC used for non-ferrous metals

The output power cable is connected to one welding head and return power cable is connected to other welding head.

Fig: Two-wire SA series connection

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Process Parameters In SAW, the weld deposit quality is determined by the type of flux, grade of wire and the following parameters:

1) Welding current

2) Arc voltage

3) Speed of arc travel

4) Size of electrode

5) Electrode stick-out

6) Heat input rate

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Welding current It controls:

The Melting rate of the electrode – Deposition rate The Depth of penetration – The extent of dilution of the weld metal by the base metal

High current causes excessive weld reinforcement and high narrow bead and undercut

Low current gives an unstable arc, inadequate penetration and overlapping

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Arc voltage Increasing voltage:

Produces a flatter and wider bead Increases flux consumption Increases resistance to porosity caused by rust or scale Increases pickup of alloy from the flux

Excessively high voltage: Produces a hat shaped bead that is subject to cracking Produces poor slag removal in groove welds Produces a concave fillet weld that may be subjected to cracking Increases undercut on fillet welds.

Lowering the voltage produces a high narrow bead with poor slag removal

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Speed of arc travel Increasing the welding speed

Lesser penetration Lesser weld reinforcement Lower heat input per unit length of weld

Excessively high speed Decrease fusion between the weld deposit and the parent metal Increase tendencies for undercut, arc blow, porosity, irregular bead shape

For slow speed result in poor penetration.

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Electrode stick-out It is also termed electrode extension – Length of electrode, between the end of contact tube and the arc.

The longer the stick-out: The greater the amount of heating and Higher deposition rate Decreased penetration rate

75 mm for 2.0, 2.4 and 3.2 mm wire diameters

125 mm for 4.0, 4.8 and 5.6 mm wire diameters

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Heat input rate Also termed as arc energy:

HIR Where

HIR = heat input rate in kilojoules per mmV = arc voltageA = welding currentS = arc travel speed in mm/min

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Wire angles and positions: advantages and drawbacks • By varying the angle of the contact tip, the wire angle relative to the joint can be varied.

• With the wires in line with the joint, penetration will be highest and risk of undercutting will be least. This position ensures the least risk of porosity, as the molten weld metal has longer to cool, allowing more time for gas to escape from the weld.

• With the wires perpendicular to the joint, penetration is minimum. This arrangement is preferred in welds in which ordinary root faces for submerged arc welding cannot be used, e.g. corner/fillet welds, and also where wide joint widths need to be covered with one pass or where the edges of the joint are uneven. There is some risk of undercutting at high welding speeds. As, with the wires in this position, very little of the parent metal is melted relative to the amount normally melted in the submerged arc process, resulting in an improved form factor of the weld. This arrangement is also used for welding materials in which there is a risk of thermal cracking.

• A pair of wires arranged diagonally to the weld can be used as a compromise position to obtain the benefits of the two basic positions described above.

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Flux ClassificationFlux Type

Mn – Silicate Type

F

Ca – Silicate Type

High Silica F

Medium SilicaF/A

Low Silica A

Aluminate Type

Aluminate TiO2 Type A/F

Aluminate Basic Type A/F

Flouride basic Type A

F =Fused

A = Agglomerated

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Comparison between different SAW

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http://www.thefabricator.com/article/arcwelding/improving-productivity-with-submerged-arc-welding

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Comparison between single-wire and twin-wire welding The performance parameters shown in the table below are based on the performance of the wire feed motor, and not on basic welding characteristics.

TYPE OF WIRE DIAMETER(mm)

AREA(mm2)

WELDING CURRENT(A) max.

DEPOSTION RATE (kg/h)

SINGLE WIRE

3.0 7.06 650 8.0

4.0 12.56 850 11.5

5.0 19.62 1100 14.5

TWIN WIRE

2.0 6.28 1000 14.0

2.5 9.81 1200 17.0

3.0 14.13 1500 21.0

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WELD SYSTEM G or L(mm)

WIRE SIZE(mm) Amps. Volts STICKOUT

(mm)TRAVEL SPEED

(mm/sec)

DEPOSITE RATE

(kg/hr)

Single Electrode 4.8 4.8 1000+ 35 32 7 13.2

TwinElectrode 5.5 2 *2 1250- 44 32 13 30

Single Electrode 8 4 575- 34 25.4 9 9.5

TwinElectrode 8 2 * 2 850- 32 25.4 17 17

Single Electrode 12.5 4.8 950+ 36 32 5 13

TwinElectrode 12.5 2 * 2 1000- 42 32 11 22

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WELD SYSTEM WIRE SIZE (mm) Amps. Volts STICKOUT

(mm) TRAVEL

Single Electrode 3.2 550+ 27 19 28

TwinElectrode 1.6 * 2 850+ 27 19 63

Single Electrode 4 600+ 29 19 18

TwinElectrode 2 * 2 950+ 29 19 30

Single Electrode 3.2 575+ 30 19 30

TwinElectrode 1.6 * 2 925+ 26 16 55

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Arc Start With still wool or iron powder

Sharp wire start

Scratch start

Molten flux start

Wire retract start

High frequency start

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Applications The field of technology where submerged arc welding is commonly used is the offshore and energy sector. Offshore applications require extremely high quality welds, such as toughness of the material. Firstly the

deposition rate is sought to be increased to fill the joints as fast as possible without damaging the material of the object being welded.

In the energy sector, where wind towers and nuclear reactor container tanks need to be welded, SAW is often the chosen process due to the high quality and high productivity.

Next generation high productivity submerged arc welding by MARKUS LANGENOJA VINCENT ÖHRVALL KARLSSON

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Application of twin arc system

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Thank You..

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http://www.thefabricator.com/article/arcwelding/improving-productivity-with-submerged-arc-welding