Carbon electrode welding
The process utilizes a carbon
electrode in place of a metal electrode and an arc is struck between the carbon
electrode and the work piece. Usually straight polarity is used while welding.
The filler metal, if required is added separately in rod form. Direct current
is used as a source. The carbon arc is very easy to start, as there is no
tendency for the electrode to stick to the metal.
Most carbon arc welding of this
type is done with the use of automatic welding equipment where the arc voltage
and current rate of travel and rate of feeding the filler rod are all properly
controlled. Protection of metal can be obtained by introducing either an inert
gas or slag forming fluxes into the arc.
Carbon arc welding is used for
welding cast iron, steel, copper, bronze and aluminium.
Submerged arc welding
The submerged arc is a process
used with automatic welding equipment. It uses A.C. or D.C. supply with
straight or reverse polarity.
This process is so named
because the metal arc is shielded by a blanket of granular, fusible flux during
the welding operation. The electrode can melt rapidly and fuse with the parent
metal under a protective atmosphere. No arc flash is seen and the flux after
cooling is easily removable. This process gives 1) high current densities 2)
high rate of metal deposition 3) high quality weld and 4) deep penetration and
high welding speeds.
This process uses high currents
ranging from 600 A to 2000 A. This process is limited to flat welding although
welds can be made on a slight slope or on circumferential joints. The backing
plates are used to avoid loss of molten metal.
Most submerged arc welding is
done on low carbon steels and alloy steels and less frequently on nonferrous
metals.
Fig.
Insert gas shielded arc welding
The general arrangement of an
inert gas shielded arc welding using non-consumable tungsten electrode and
argon gas as shielding medium. Filler metal is supplied externally if required.
In other processes instead of tungsten electrode, consumable metal electrode is
fed at suitable rate, which also serves the purpose of filler metal.
Fig.
Other inert gases like helium
and carbon-dioxide can also be used. The inert gas prevents the parent molten
metal from oxidation. Both these methods are adoptable to either manual or
automatic machine welding and no flux or wire coating is required for
protection of weld. Both alternating or direct current source can be used,
depending upon the kind of metal to be welded. Direct current with ‘ straight
polarity’ is required for welding copper alloys and stainless steel, whereas
‘reverse polarity’ is required for welding magnesium. Alternating Current is
more versatile in its application and is used for steel, cast iron, aluminium,
nickel and magnesium.
Advantages of arc welding
- High quality welds.
- Great flexibility of the process.
- High deposition rates.
- Low welding costs.
Because of these advantages,
this process has been adopted for the fabrication of a great number of
different products and structures than any of the other welding methods.
Among the application of arc
welding arc work on tanks, bridges, boilers, piping, structural machinery,
steel furniture and ships.
Precautions to be taken while arc welding
1. The operator should never look directly at the
arc unless his eyes are shielded with protective glass. The same is also
applicable to anyone else observing arc welding.Direct exposure of the naked
eye to the rays of the arc will result in a painful burning of the eyes and
hands. The application of boric acid will give relief to the eyes.
2. A shield should be used to protect the face and
neck, and leather gloves and sleeves should be worn to protect the hands and
arms. A leather apron would also protect the clothing.To eliminate the
possibility of hot metal falling into trouser cuffs or into shoes, it is
important to use trousers without cuffs and to hold the legs of the trousers
over the shoe tops.
3. Before starting to weld, the operator should
always check whether the equipment is in proper working condition, the cable
connections should be right. The ground cable and work metal should make good
electric contact with the welding bench. For best results, the work metal
should be free of scale, rust, greases, oil, paint and other foreign matter.
4. The welding machine should be properly adjusted
and the welding circuit should be set up for the right polarity.
5. If possible, the operator should sit comfortably
on a stool, directly in front of the work.
6. A right handed welder should hold the electrode
holder in his right hand, just tight enough to control its movements.
7. To decrease the drag of the heavy cable and thus
to make it possible to manipulate the electrode holder more easily, the
electrode cable should be folded neatly over the operator’s lap or shoulder.
