Welding Equipment (MMAW) :
1. – (a) AC or DC welding supply, electrode holder and welding cables.
(b) Welding electrodes,
AC transformers and DC generators or rectifiers can be employed welding with covered electrodes. Both AC and DC power sources produce good quality welds, but depending upon welding situation one may be preferred over the other.
For example, at higher arc currents AC gives a smooth arc, there is no arc blow, once established the arc can be easily maintained and controlled, it is suitable for welding thicker sections and AC is easily available.
As compared to DC welding equipment, AC welding power sources do not contain any rotating part, do not produce noise, occupy less space, are less costly to purchase and maintain, possess high efficiency (0.8), consume less energy per unit weight of deposited metal and have high no load voltage.
In AC, of course, the melting rate of the electrode cannot be controlled, because equal heat generates at electrode and job.
DC arc is more stable, arc heat can be regulated (i.c., through DCRP and DCSP), it is preferred for welding certain nonferrous metals and alloys, it has lower open circuit voltage and therefore is safer. A DC equipment can be a self-contained unit.
The most commonly used power source for AC welding is a trans former. A Transformer may be operated from the mains on single phase, two phase or three phase. Single phase transformer supply lower cur rents, and are therefore used for welding thin sections with small diameter electrodes.
Two phase and three phase supply transformers operate at higher currents and are used for the welding of thicker jobs.
Three phase transformers are preferred when more than one operator has to work simultaneously. An AC welding transformer (constant current power source) possesses drooping characteristics.
A typical specification of the same is as follows:
Current range up to 600 Amps.
Open circuit voltage 70 to 100 Volts.
Single or multi operator set.
Transformers are either air-cooled or oil-cooled depending upon their amperage rating (capacity). Big transformers are oil cooled.
Besides transformers, motor driven AC generators and oil/gas engine driven AC generators may also be employed for welding. A transformer being the simplest and the cheapest is most common as compared to the other AC welding machines.
A DC arc welding supply may be an AC transformer with a silicon or selenium rectifier, or a DC generator drive by an AC motor or a petrol diesel engine. A DC generator may possess all the three characteristics i.e. drooping, slightly drooping and flat.
One or maximum two operators can do welding simultaneously on a DC generator. A DC generator may have an open circuit voltage between 45 to 75 volts and current supply up to 600 amps. A DC generator is air cooled.
A transformer cum rectifier unit may be cooled by forced air circulation. It has characteristics similar to an AC transformer. Other types of welding machines supplying direct current may employ storage batteries and rectifier tubes.
2. Electrode holder : It can hold the electrode at various angles and energizes it at the same time. The jaws of the holder, which retain the electrode, remain under spring pressure. The jaws may or may not be insulated, but insulated jaws avoid danger of short circuiting.
Electrode holders are available to work from 100 to 500 amps and are provided with a heat shield to protect welder’s hand during welding. An electrode holder should be light, but sturdy at the same time. It should provide easy holding of the electrode. The handle of the electrode should be highly resistant to heat and electricity.
3. Welding leads
4. Ground connection
5. Hand-shields, etc.
- Flux Shielded Manual Metal Arc Welding is the simplest of all the
- arc welding processes.
- The equipment can be portable and the cost is fairly low.
- This process finds innumerable applications, because of the avail ability of a wide variety of electrodes.
- A big range of metals and their alloys can be welded.
- Welding can be carried out in any position with highest weld quality.
- The process can be very well employed for hard facing and metal deposition to reclaim parts or to develop other characteristics like wear resistance etc.
- Joints (e.g. between nozzles and shell in a pressure vessel) which because of their position are difficult to be welded by automatic welding machines are easily accomplished by flux shielded metal arc welding.
- Because of the limited length of each electrode and brittle flux coating on it mechanization is difficult.
- In welding long joints (e.g., in pressure vessels), as one electrode finishes, the weld is to be progressed with the next electrode. Unless properly cared, a defect (like slag inclusion or insufficient penetration) may occur at the place where welding is restarted with the new electrode.
- The process uses stick electrodes and thus it is slower as compared to MIG welding.
- Because of flux coated electrodes, the chances of slag entrapment and other related defects are more as compared to MIG or TIG welding.
- Because of fumes and particles pf slag, the arc and metal transfer is not very clear and thus welding control in this process is a bit difficult as compared to MIG welding.
- Today, almost all the commonly employed metals and their alloys can be welded by this process.
- Shielded metal arc welding is used both as a fabrication process and for maintenance and repair jobs.
- The process finds applications in
(a) Air receiver, tank, boiler and pressure vessel fabrications;
(b) Ship building;
(c) Pipes and Pen stock joining;
(d) Building and Bridge construction;
(e) Automotive and Aircraft industry, etc.
References : A Textbook of Welding Technology by O. P. Khanna