Shielded Metal Arc Welding -SMAW

SHIELDED METAL ARC WELDING

Introduction :

The shielded Metal Arc Welding Process (SMAW) is also widely known by the common term Stick Welding. The term stick is a reference to the electrode used to provide the filler metal for welding with this process.

The Shielded metal Arc Welding process is one of the oldest and most widely used methods of joining metals by welding. The process is popular because of the simplicity of equipment involved, its portability, reliability and adaptability to outdoor use. The process may be used to weld a variety of metal types and shapes in any position.

Shielded metal arc welding was first invented in Russia in 1888. It consisted of a bare metal rod with no flux coating for providing a protective gas shield.The first coated electrode was developed and patented by the Swedish engineer Oskar Kjellsberg in 1905, but it took some years of refining coatings and testing the reliability of welded joints before the process was accepted in the fabrication of steel constructions. In 1938, however, the world's first wholly welded oceangoing ship was launched, thereby introducing SMAW welding as a production process for the maritime market.

The Process :

In the SMAW process, welding is done by setting up an electrical circuit using a welding machine to produce the electricity, a welding cable with an electrode holder to hold the electrode and a ground cable with a clamp to fasten to the work-piece to complete the circuit. The weld is made by touching the electrode to the work-piece closing the electrical circuit and causing the electrode to melt and form the weld. 

The arc is initiated and maintained between the end of a consumable electrode (the filler metal) and the work piece. The Intense heat produced from the arc causes the surface of the work piece to melt and form a weld pool. At the same time, the tip of the electrode melts, and small Globules of filler metal travel across the arc into the molten weld pool to form a weld.

 To initiate the arc, the welder touches the electrode tip momentarily  on the work piece, causing current to flow. The electrode is immediately withdrawn to give a gap of around 3mm between the electrode tip and work piece. Current continues to flow across the gap, initially in the form of a small spark. This spark rapidly ionises the air in the gap, forming an intense welding arc. The electrode has a pre-coated, dense layer of dry flux over most of its surface. A short length of the electrode is left uncoated where it fits into the electrode holder, and at the opposite end, the tip where it makes contact with the work piece to initiate the arc is also bare. As soon as the arc starts, the rapidly heated flux forms both a slag and gaseous shield to protect the weld from atmospheric contamination. Liquid slag, which appears brighter than the molten metal and is more free-running, forms on top of the solidifying weld metal, and the gaseous shield protects the weld pool, hot electrode tip and globules of filler metal from atmospheric contamination. As globules of filler metal transfer to the weld pool, the electrode becomes shorter. The welder continuously compensates for this and keeps the arc length constant by feeding the electrode towards the weld using a carefully controlled hand movement. Most SMAW electrodes are fairly short (around 350-450mm in length) which means that relatively short lengths of weld are made before having to install a new electrode, which is a quick and simple job. As the weld cools, the slag cools and solidifies and must be chipped off the weld bead once the weld run is complete or before the next weld pass is is carried out. Suitable eye protection needs to used while carring out the chipping off the slag from the weld. (e.g use of safety glasses is recommended). This cleaning process is important in multi-pass welding where slag may become entrapped, resulting in inclusions, which can weaken the weld.

Arc stability, depth of penetration, metal deposition rate and positional capability are greatly influenced by the chemical composition of the flux coating on the electrode.  


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