Principle and procedure of welding
In submerged arc welding, the bare metal or copper coated electrode acts as a filler and melts along with the welding process and granular flux is used which is a protection from atmospheric contamination. The heat is generated by the arc formation between the electrode and the metal work piece which remains masked beneath the flux. Flux turns conductive when the molten pool is formed and current path is stuck between electrode and the metal work piece. This flux also serves as shield to avoid spatter.
The process of submerged arc welding may be semi-automatic or automatic. Welding equipment mainly will have DC Power source, Feed Drive rolls to feed the electrode continuously, welding gun having flux hopper and nozzle.
In semi-automatic welding process, the trigger of the gun is pulled and the flux starts depositing on the work piece joint to be welded. Here, the gun type used may be of gravity flux feed or forcing the flux in pressure. The flux is non conductive and is as good as insulator when cold, so the arc is struck either by tapping the electrode on the metal work piece or before putting on the weld current, a steel wool is kept between the metal work piece and electrode. Alternatively, a high frequency current is also used for this welding process. In all the 3 methods mentioned above, flux is used to cover the arc and molten metal.
The supplies are put on and the weld current is allowed to flow between the electrode and the metal work piece. The flux becomes highly conductive and maintained through the molten flux. A backing plate made of either steel or copper is used to support the penetration and the molten metal which is in large amount. Due to the contact of the flux with atmosphere, the upper portion of the flux which is visible continue to be in solid granular form thereby making it for reuse and savings in cost. The welding head in such semi-automatic machines are moved manually along with the metal work piece joints whereas in automatic welding equipment, either the welding head moves on the metal work piece fixed by separate drives or the work piece rotates moves beneath the welding head. Principle of self-adjusting arc is applied here to keep the arc length constant where if the arc length gets decreased for some reasons, simultaneously, arc voltage and arc current will also decrease. Hence burn off rate gets increased thereby causing the arc to lengthen and the reverse happens if the arc length increases than the normal.
SAW is found suitable only for flat or horizontal fillet welding positions.
Advantages of submerged arc welding
The deposition rate is high
Penetration of weld is high
Weld joints are strong and of good quality
Due to automation option, the production speed is high
Welding operator skill required may not be high
Due to the shielding of arc by flux, fumes and arc light is low
Limitations of SAW
Restrictions to weld mild steel, stainless steel and nickel alloys only
Welding positions are limited
Good for only continuous and long length welding
Possibilities of irregular wire feeding
Flux handling is time consuming and added work
Post welding slag removal