Shielded Metal Arc Welding (SMAW) also known as Manual Metal Arc Welding (MMAW), is a manual arc welding process that uses a consumable electrode (covered with a flux). Electrode is connected with one terminal of the power source and the work is connected with the other terminal of the power source. Welding cable (or lead) is used for this purpose. The power source can be used in AC (alternating current) or DC (Direct current). In AC the polarity of terminals keeps on changing (almost 100 times in second) but in DC one terminal always acts as Positive and the other acts as negative.
The basic components of SMAW (Figure – 1) are the following;
- Power Source (Figure – 2)
- Electrode Holder (Figure – 3)
- Electrodes (Figure – 4)
Please see the below schematic diagram (Figure – 1) of SMAW for a better understanding.
(Figure – 1)
In SMAW, electric arc is used to melt the base metal, this arc is generated by striking the electrode with the work piece. Soon after generation of arc the electrode is withdrawn from the work piece but it remains in close contact with the work piece. The gases present between the gap (between electrode and work piece) gets ionized and smooth flow of electrons takes place hence, in spite of the gap between the electrode and the work piece the circuit remains closed (or energized) and hence arc doesn’t get extinguished.
A power source is shown in figure – 2.
(Figure – 2)
If electrode is connected with the positive terminal and the work piece with negative terminal than it is known as DCEP (i.e. direct current electrode positive or Reverse Polarity). Whereas if the electrode is connected with the negative terminal and the work piece with positive, then it is known as direct current electrode negative (ie. DCEN or Straight Polarity).
In DCEP, the electricity flows into the tip of the electrode and concentrates about two-thirds of the heat, which gives good penetration. Hence it is usually used on thicker steels.
In DCEN, the electricity flows out of the rod, concentrating about one-third of the heat on the electrode. Hence the penetration is less, this a very good choice for thinner steels.
An electrode holder is shown in figure – 3.
(Figure – 3)
Shielded metal arc welding electrodes are covered with flux , Purpose of flux is as follows;
- During welding, the flux material decomposes and produces fumes. These fumes shield the molten weld pool from the atmosphere. In the absence of those fumes, molten weld pool will be exposed to atmosphere and may react with oxygen present in the environment and thus oxidation may take place, which is very dangerous and may lead to failure.(i.e prevents oxidation of molten weld pool).
- Density of flux material is less than the weld material hence the flux comes upwards and floats on the weld pool and thus protects the weld pool. During solidification of molten weld pool, flux also solidifies and forms a thin layer on the surface of weld called as slag. This thin layer of slag helps in slow cooling of weld material. In the absence of this thin layer of slag, rapid cooling of molten weld pool will take place which is very dangerous because rapid cooling results in microstructural changes of weld metal (martensite formation).
- Flux also helps in initiation of arc. Especially when welding is done in AC (Alternating Current), polarity keeps on changing at a constant rate, i.e. it attains a positive value then a negative value and it happens within a fraction of second. Polarity changes more than 100 times in a second. While going from positive to negative it attains a value ‘0′ and every time when it goes to zero, arc has to be re-initiated and the flux plays its role. It happens very fast (i.e. within a fraction of seconds) hence we can not see this with our eyes.
SMAW electrodes are shown in figure – 4.
(Figure – 4)
Advantages of SMAW:
- This process is suitable for most of the commercially available metals and alloys.
- The equipment is comparatively inexpensive and portable.
- Equipment is relatively simple.
- This process can be used in all welding positions.
- This welding process is flexible and can be applied to a variety of joint configurations and positions.
- No need of separate gas shielding.
- Less sensitive to wind and drafts if compared to gas shielded arc welding processes.
- It can be used at area where access is limited (electrodes an be bent and even mirror can be used at tight spaces).
Disadvantages of SMAW:
- Low deposition rate than GMAW and FCAW
- More skilled welding operator required than many other welding processors.
- Not suitable for reactive metals such as Titanium, Zirconium, Tantalum, and Niobium.
- Not suitable for metals with low melting temperature such as Lead, Tin and Zinc and their alloys.
- This process cannot be automated.
- More slag due to flux shielded electrode.
- Repeated changing of consumed electrodes with a new one makes this process quite slow if compared to GMAW.
Please watch the following videos for a better understanding of Shielded Metal Arc Welding. In the first video you will see the practical setup and the second video is about the theory behind SMAW.
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