Trigger is pulled and the flux starts depositing on the joint to be welded.
Since the flux when cold is non conductor of electricity, the arc may be struck either by touching the electrode with the job or by placing steel wool bet ween electrode and job before switching on the welding cur rent or by using a high frequency unit.
In all the cases the arc is struck under a cover of flux Flux otherwise is insulator but once it melts due to heat of the arc, it becomes highly conductive and hence the current flow is maintained between the electrode and the job through the molten flux.
The upper portion of the flux, in contact with atmosphere, which is visible remains solid granular i.e., unchanged and can be reused. The electrode at a predetermined speed is continuously fed to the joint to be welded.
In semi-automatic welding sets the welding head is moved manually along the joint whereas in automatic welding a separate drive move either the welding head over the stationary job or the job moves/rotates under the stationary welding head.
The arc length is kept constant by using the principle of a self-adjusting arc i.e., if due to certain reasons arc length decreases, arc voltage will decrease, arc current and therefore burn off rate will increase thereby causing the arc to lengthen. The reverse occurs if the arc length increases than the normal.
Backing plate of steel or copper may be used control penetration and to support large amounts of molten metal associated with the process
- Welding head. It feeds flux and filler metal to the welding joint.
Electrode (i.e., filler metal) gets energized here.
- Flux hopper. It stores the flux and controls the rate of flux deposition on the welding joint.
- Welding power source. Any AC transformer or a DC generator rated up to 1500 Amps may be used for submerged arc welding. The amperage of the set will vary with the job conditions.
- Flux : The granulated flux shields and thus protects molten weld metal from atmospheric contamination. The flux cleans weld metal and can modify its chemical composition also. The flux is granulated to a definite size. It may be of fused, bonded or mechanically mired type. The flux may consist of fluorides of calcium and oxides of calcium, magnesium, silicon, aluminium and manganese.
Alloying elements may be added as per requirements. Substances evolving a large amount of gases during welding are never mixed with the flux. Flux with fine and coarse particle sizes are recommended for welding heavier and smaller thickness respectively.
- Electrodes :The electrode composition depends upon the material being welded. Alloying elements may be added in the electrodes.
Electrodes are available to weld mild steel, high carbon steel, low and special alloy steels, stainless steel and some of the nonferrous alloys of copper and nickel. Electrodes are generally copper coated to prevent rusting and to increase their electrical conductivity. Electrodes are available in straight lengths and coils. Their diameters may be 1.6, 2, 2.4, 3.2, 4, 4.8 and 6.4 mm. The approximate value of current to weld with 1.6, 3.2 and 6.4 mm diameter electrodes are 150350250800 and 6501350 Amps respectively.
Joint Preparations :
- Butt, fillet and plug welds and surfacing are carried out using submerged arc process.
- Plates up to 19 mm thick can be butt welded in one-pass. Bigger thicknesses require multiple passes from one or both sides of the joint.
- Typical welding conditions for butt welds in steel plate are:
Square butt weld, plate thickness 12.7 mm, root opening 4.8 mm, current 1100 Amps, voltage 34 volts, electrode dia. 5.6 mm, steel backing strip 9.5 mm thick, 25.4 mm wide, welding speed 8 mm/second. Single electrode, one pass.
- Fillet welds up to 9.5 mm in throat size can be made in the horizontal position with one pass.
Weld Backing :
Submerged arc welding produces a large volume of highly fluid weld metal which needs to be supported (backed), until it solidifies when making butt welds in one pass where complete penetration is desired. The following methods are used to back up the welds:
1. Backing strips. (in figure below). The weld penetrates into and fuses with the backing strip, which temporarily or permanently becomes an integral part of the weldment. The backing strip metal should be such that it does not contaminate the weld metal.
2. Backing weld. (in figure below). One or two layers of weld metal are applied on the underside of the seam to support weld metal that will be deposited from the opposite side.
3. Copper backing (in figure below). Bar grooved or ungrooved, straight or circular is used to support molten weld pool. Such a bar does not fuse and become a permanent part of the weld. Copper chills molten metal rapidly.
4. Flux backing. (in figure below). Moderate flux pressure is developed on the back side of the weld.
5. Gas backing. (in figure below).
References : A Textbook of Welding Technology by O. P. Khanna