Influence of welding parameters on bead geometry in SAW
BookRix GmbH & Co. KG
Welding is a process for joining two similar or dissimilar metals by fusion. It joins different metals/alloys, with or without the application of pressure and with or without the use of filler metal. The fusion of metal takes place by means of heat. The heat may be generated either from combustion of gases, electric arc, electric resistance or by chemical reaction. During some type of welding processes, pressure may also be employed, but this is not an essential requirement for all welding processes. Welding provides a permanent joint but it normally affects the metallurgy of the components. It is therefore usually accompanied by post weld heat treatment for most of the critical components. The welding is widely used as a fabrication and repairing process in industries. Some of the typical applications of welding include the fabrication of ships, pressure vessels, automobile bodies, off-shore platform, bridges, welded pipes, sealing of nuclear fuel and explosives, etc. (Singh, 2006)
1.2 Submerged Arc Welding
During 1930’s, having recognized the potential advantages of mechanized welding. Several attempts were made to mechanize the arc welding process. Developing a continuous coated electrode as an extension of the manual metal arc welding, electrode was ruled out of the following reasons (a) since the coating is non conducting, arranging electrical contact with the electrode is not practicable, (b) the coating is likely to be peel off when the electrode is coiled, and(c) the coating is also likely to be crushed when fed through the feed rolls.
In 1932, the idea of placing a thick layer of dry granular flux on the joint ahead of the carbon electrode was conceived and successfully developed in the U.S.A. and later applied to the welding of penstocks and water conduicts in California. Submerged arc welding is the next logical step and the process become a commercially success by the middle and late 1930’s.
The modern submerged arc welding is a welding process, in which one or more arc formed between one or more bare wired electrodes and the work piece provides the heat for coalescence. The arc is completely submerged under a blanket of granular, fusible flux, which adequately shields the arc from atmospheric contamination. Fig.1.1 shows mechanism of submerged arc welding. The process can be fully automatic or semi automatic.
Fig. 1.1 Submerged arc welding process
In fully automatic welding, the wire is fed automatically to the welding head, the flux is fed mechanically to the joint ahead of the arc, the arc length is automatically controlled and the traverse of the arc or the work piece is also mechanized.
In the semi automatic version, the wire feed and arc control are automatic, while the welder moves the welding gun, usually equipped with flux-feeding device, along the joint at the controlled rate of travel. Flux feed may be gravity flow, through a nozzle concentric with the electrode from a small hopper atop the gun, or it may be through a concentric nozzle connected to air pressurized flux tank. Flux may be applied in advance of the welding operation or ahead of the arc from a hopper run along the joint.
During welding, the intense heat of the arc simultaneously melts the tip of the bare wire electrode and the part of flux. The electrode tip and the welding zone are always surrounded and protected by molten flux, while all of them are covered by the top layer of unfused flux. As the arc progress along the joint, the molten metal settles down while the lighter molten metal rises from the puddle in the form of slag. The weld metal, having a higher melting point, solidifies first while the slag above it takes so time to freeze. The solidified slag continues to protect the weld metal while it is still hot, and is capable of reacting with atmospheric oxygen and nitrogen. (Nadkarni 1988)
1.3 Advantages of Submerged Arc Welding:
- High deposition rates (over (45 kg/h) have been reported).
- High operating factors in mechanized applications.
- Deep weld penetration.
- Sound welds are readily made (with good process design and control).
- High speed welding of thin sheet steels up to 5 m/min (16 ft/min) is possible.
- Minimal welding fume or arc light is emitted.
- Practically no edge preparation is necessary.
- The process is suitable for both indoor and outdoor works.
- Distortion is much less.
- Welds produced are sound, uniform, ductile, and corrosion resistant and have good impact value.
- Single pass welds can be made in thick plates with normal equipment.
- The arc is always covered under a blanket of flux, thus there is no chance of spatter of weld.
- 50% to 90% of the flux is recoverable.(http://en.wikipedia.org/wiki/submerged arc welding)
1.4 Limitations of Submerged Arc Welding:
- Limited to ferrous (steel or stainless steels) and some nickel based alloys.
- Normally limited to the 1F, 1G, and 2F positions.
- Normally limited to long straight seams or rotated pipes or vessels.
- Requires relatively troublesome flux handling systems.
- Flux and slag residue can present a health & safety concern.
- Requires inter-pass and post weld slag removal.(http://en.wikipedia.org/wiki/submerged arc welding)
1.5 Applications of Submerged Arc Welding:
- Submerged arc welding is mainly used for welding low carbon and low alloy steels, though with the development of suitable flux it can be used successfully for welding stainless steels, copper, aluminium and titanium base alloys.
- It is used mainly in downhand welding position for plate thickness between 5 to 50mm, particularly where the welds are straight and long.
- SAW is widely used for butt and fillet welds in heavy industries like shipbuilding, pressure vessel fabrications, structural engineering, rail road tank cars, pipe welding and for storage tanks.
- It is also used in hardfacing for steel mills, earthmoving equipments, mining etc.
- SAW is also capable of welding heat resistant steels, corrosion resistant steel and high strength steels. (Parmar, 2011)
1.6 Basic Equipment
The SAW equipment essentially consists of:
- A wire feeder to drive the electrode to the work through the contact tube of a 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.
Fig. 1.2 Equipment for single wire submerged arc welding
Typical Welding Outfits
Fig.1.2 shows equipment for single wire submerged arc welding and consists of
- Power source
- Wire feeder
- Torch with flux hopper
- Set of cables for connecting wire feeder to power source, comprising control
cable, welding cable with end lugs, welding cable with end lug and ground
- Set of accessories for wire feeder.
a) Power source: It is a DC welding rectifier giving maximum welding current of 600 amps at 60% duty cycle and 500 amps at 100% duty cycle. It has constant potential characteristics, giving a constant arc voltage, while the current is determined by the wire feed rate. The exceptionally high short circuit current makes arc striking very simple. The rectifier has drip proof casing with the following built in components, transformer, silicon rectifier, reactance coil, fan, safety device against ventilation failure.
b) Wire feeder: It consists of a DC wire drive motor reduction gearbox; four roll drive mechanism and a wire spool holder.Fig.1.3 showing a wire feeder. Wire-feed rate is continuously variable in the range of 0.5-2m/min. For infinitely stepless variation of the wire feed speed, a special solid state thyristorised circuitry is provided in the wire feeder. (Nadkarni, 1988)
Fig.1.3 Wire feeder
1.7 Power Sources
The welding power source can be either of the following:(a)an AC transformer of constant current type;(b)a DC generator of the constant voltage or constant current type;(c)a DC rectifier of the constant voltage or constant current type.
AC power calls for more elaborate and complicated controls and the arc starting is not easy.