Shielded Metal Arc Welding (SMAW) or Manual Metal Arc Welding (MMAW)

Shielded metal arc welding (SMAW) is a commonly used arc welding process manually carried by welder. It is an arc welding process in which heat for welding is produced through an electric arc set up between a flux coated electrode and the workpiece. The flux coating of electrode decomposes due to arc heat and serves many functions, like weld metal protection, arc stability etc. Inner core of the electrode supply the filler material for making a weld. The basic setup of MMAW is depicted in Fig.

Principle of arc welding

Arc welding process setup

 The configuration of weld zone is shown in Fig.

Arc welding operation

 If the parent metal is thick it may be necessary to make two or three passes for completing the weld. A typical multi pass bead in this case is shown in Fig.

A typical multi pass bead

Advantages of Shielded metal arc welding (SMAW)

1. Shielded Metal Arc Welding (SMAW) can be carried out in any position with highest weld quality.

2. MMAW is the simplest of all the arc welding processes.

3. This welding process finds innumerable applications, because of the availability of  a wide variety of electrodes.

4. Big range of metals and their alloys can be welded easily.

5. The process can be very well employed for hard facing and metal resistance etc.

6. 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 can be easily accomplished by flux shielded metal arc welding.

7. The MMAW welding equipment is portable and the cost is fairly low.

Limitations of Shielded metal arc welding (SMAW)

1. Due to flux coated electrodes, the chances of slag entrapment and other related defects are more as compared to MIG and TIG welding.

2. Duo to fumes and particles of 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.

3. Due to limited length of each electrode and brittle flux coating on it, mechanization is difficult.

4. 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

5. The process uses stick electrodes and thus it is slower as compared to MIG welding.

Applications of Shielded metal arc welding (SMAW)

1. Today, almost all the commonly employed metals and their alloys can be welded by this process.

1. Shielded metal arc welding is used both as a fabrication process and for maintenance and repair jobs.

2. The process finds applications in

(a)  Building and Bridge construction

(b)  Automotive and aircraft industry, etc.

(c)  Air receiver, tank, boiler and pressure vessel fabrication

(d)  Ship building

(e) Pipes and

(f) Penstock joining

Functions of Electrode Coating Ingredients

The covering coating on the core wire consists of many materials which perform a number of functions as listed below:

1. Welding electrodes are used to join various similar and dissimilar metals as plain carbon steels, cast iron, copper, aluminium, magnesium and their alloys, stainless steels and other alloy steels.

2. Slag forming ingredients, like silicates of magnesium, aluminium, sodium, potassium, iron oxide, china clay, mica etc., produce a slag which because of its light weight forms a layer on the molten metal and protects the same from atmospheric contamination.

3. Arc stabilizing constituents like calcium carbonate, potassium silicate, titanates, magnesium silicates, etc.; add to arc stability and ease of striking the same.

4. Gas shielding ingredients, like cellulose, wood, wood flour, starch, calcium carbonate etc. form a protective gas shield around the electrode end, arc and weld pool

5. Deoxidizing elements like ferro-manganese, and ferro-silicon, refine the molten metal.

6. It limits spatter, produces a quiet arc and easily removable slag.

7. Alloying elements like ferro alloys of manganese, molybdenum etc., may be added to impart suitable properties and strength to the weld metal and to make good the loss of some of the elements, which vaporize while welding.

8. Iron powder in the coating improves arc behavior, bead appearance helps increase metal deposition rate and arc travel speed.

9. The covering improves penetration and surface finish.

10. Core wire melts faster than the covering, thus forming a sleeve of the coating which constricts and produces an arc with high concentrated heat.

11. Coating saves the welder from the radiations otherwise emitted from a bare electrode while the current flows through it during welding.

12. Proper coating ingredients produce weld metals resistant to hot and cold cracking. Suitable coating will improve metal deposition rates.

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