Electric arc welding:
An electric arc is formed by bringing two conductors connected to a suitable source of electric current momentarily in contact and then separating by a small distance.
The current continues to flow across the small gap and gives intense heat. The heat developed is utilized to melt the part of the workpiece and filler metal and thus forms the joint.
The arc welded joint is a union of metal parts made by localized heating without any pressure.
The arc is struck when electrodes are separated, the contact area of the electrodes decreases causing an increase in resistance (R ∝ 1/a).
The heat is developed and the electrons from electrodes are accelerated due to the voltage gradient developed between them and these electrons collide with atoms and molecules of air.
The air gap between electrodes is now ionized i.e. acts as a conductor. Thus, the arc is initiated. A.C. and D.C. supply can be used in arc welding. For A.C. supply requirement of voltage is 50 to 60 volts and for D.C. it is 70 to 100 volts.
When the arc is struck the voltage required to maintain the arc is only 20 to 30 volts.
The arc has negative resistance characteristics i.e. its resistance decreases with an increase in current. Fig. shows volt-ampere characteristics of an electric arc.
When arc is struck, the current increases because the ionized air gap between electrodes acts as a conductor.
The arc diameter increases and arc resistance decreases, therefore current again increases.
The electric circuit with constant voltage supply and an arc load is basically unstable.
Fig. shows electric arc welding circuit which consists of a welding machine having two leads.
The positive lead is for an electrode and negative lead is for the workpiece.
The welding machine may be a generator or an A.C. transformer.
Carbon Arc Welding:
In this process, a carbon or graphite rod is used as negative electrode and the work being welded is used as a positive electrode.
Graphite electrodes posses longer life and have low resistance. Therefore, they are more suitable for conducting more current.
The arc produced between the two electrodes heats the metal to the melting temperature of about 3200 0C on the negative electrode and 3900 0C on the positive electrode.
There are two methods of carbon arc welding. In one method, no flux is used and this method is suitable for non-ferrous metals.
In other method, flux either in the form of powder or paste is used to prevent the weld from oxidation and is usually employed for ferrous metals.
Applications of carbon arc welding Include welding Of sheet steel, copper alloys, brass, bronze, and aluminum. But, it is not suitable for vertical and overhead welding.
The reason for using a carbon rod as a negative electrode that heat Will be generated at the electrode tip than that at the workpiece and carbon from the electrode will not fuse and mix up with the job.
If this happens the resultant weld will be rich in carbon and consequently very much brittle and unsound.
For this type of welding only D.C. supply can be used.
The current continues to flow across the small gap and gives intense heat.
Heat developed is utilized to melt the pan of the workpiece and the filler metal and thus form the joint.
No pressure is required in case of arc welding.
The voltage necessary to strike the arc is higher than that required to maintain it.
The temperature is of the order of 3600 0C. At which mechanical pressure is not required for joining two metals.
Metal Arc Welding:
As shown in figure, a metal rod of the same metal as being welded forms one of the electrode and also serves as a filler material.
In this type of welding no separate filler rod is required.
The arc is struck between the work being welded and the electrode which causes the melted rod to flow across the arc into the metal pool of parent metal.
Since, in the case of bare metal arc welding, molten electrode wire and parent material are exposed to oxygen and nitrogen of the atmosphere and forms the oxide.
This reduces the ductility of weld and produces imbrittlement in the weld.
Therefore. bare metal arc welding is preferred for unimportant works like metal deposition on railway wagons, wheels, and traveling crane wheels.
For this type of welding, D.C. and A.C. supplies can be used.
For D.C. supply, 50 – 60 volts are used for welding.
Advantages Of Metal Arc Welding:
- External pressure is not required.
- A filler electrode of suitable metal and strength is required.
- Due to the arc formed between electrode and workpiece, heat is produced.
- A.C. or D.C. supply may be used.
- The temperature achieved is (36000C to 60000C).
- It is suitable for repairing work.
Disadvantages Of Metal Arc Welding:
- Striking voltage is high and hence voltage control is required.
- The power factor (p.f.) is very poor.
- The arc forms the flexible conductor and produces the magnetic field. There are two effects of this magnetic field:
- Blow effect of the edges.
- Blow effect due to ground current through the workpiece.
All these effects will result in poor welds.
Applications Of Metal Arc Welding:
- Carbon arc welding is used for the welding sheet steel, copper alloys, brass, bronze, and aluminum.
- Metal arc welding is preferred for unimportant works like metal deposition on railway wagon wheels and traveling crane wheels.
Submerged Arc Welding:
It is an arc welding process that uses an arc between a bare metal electrode and the weld pool.
The arc and molten metal are shielded by a blanket of granular flux on the work piece.
The process is used without pressure and with the filler metal from an electrode and sometimes from a supplementary source such as a welding rod, flux, or flux with metal granules.
It is an automatic process developed primarily for the production of high-quality butt welds in thicker steel plates than is normally suited to other manual arc welding processes.
As in open arc welding, the source of heat in the submerged arc welding process is an electric arc or arcs between a metal electrode or electrodes and the work.
The welding zone is shielded by a blanket of flux, so that the arc is not visible. Hence, the name “submerged arc welding“.
The arc melts the patent metal, the electrode and the flux.
The fused flux produces a blanket of liquid slag which forms a protective envelope both around the arc and the welding zone.
Right beneath the electrode tip a weld pool is formed, holding an amount of molten metal.
On melting, the electrode metal forms globules which go over into the puddle and mix with the molten parent metal.
The pressure of the arc forces the mix out of the puddle so that it forms the weld.
This action continues as long as the arc advances.
The shape and size of the puddle have a strong effect on those of the weld.
Advantages Of Submerged Arc Welding:
- Partly because it is often automated, it is much faster than regular arc welding. Speeds up to 3,800 mm/minute are possible on 3 mm thick steel at 100 % efficiency.
- Deep penetration with a high-quality weld is possible.
- Less ‘distortion occurs from high speed and uniform heat input, especially when automated.
- No edge preparation is required.
- The operator is not exposed to the usual spatter and can work more easily without a helmet and other safety equipment.
- The welds obtained have good ductility, impact strength, uniformity, low nitrogen content, and high corrosion resistance.
- Manipulative skills are normally not involved.
- High utilization of electrode wire.
- Easily automated for high operator factor.
- Little or no smoke.