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.
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.
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 Soon after generation of arc the electrode is withdrawn from the work piece but it remains in close contact with the work piece.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.
SMAW Electrodes are covered with flux. During welding these fluxes gets decomposed and produces fumes. These fumes covers (shields) the weld pool from external environment (or contamination). Hence we can say that the fumes shields weld pool from external contamination. In the absence of flux our weld pool will be exposed to the environment and chances of oxidation will always be there that’s why shielding is necessary. Flux material is lighter in density hence it easily floats over the weld pool and covers the weld pool. It helps in slow transfer of heat as well as protection from environment.
Please see the below schematic diagram of SMAW for a better understanding.
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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