It is an automatic process developed primarily for the production of high quality butt welds in thicker steel plates. SAW differs from other arc-welding processes in a way, that a blanket of fusible, granular material (flux) is used for shielding the arc and the molten metal. The deposition rate in this process is high and penetration is deeper. In this welding process, machine units are supplied which control the feed of wire as it is consumed during the process.
In operation, a bare wire is fed through the welding heat into the granular material. This material is placed along the seam to be welded and the entire action of welding takes place beneath the granular material and hence this process is named as submerged arc welding.
The arc is started either by striking beneath the flux on the work or initially by placing some conductive medium like steel-wool. The intense heat of the arc produces a pool of molten metal in the joint and at the same time it also melts a portion of granular material.
Since the arc is fully covered by the flux, it is not visible and the weld is run without the flash spatter and sparks that characterise the open-arc process. The nature of the flux is such that very little smoke or visible fumes are developed.
The flux floats on the top of molten metal, forming a blanket which eliminates spatter losses and protects the welded joint from oxidation. Upon cooling, the fused slag solidifies and is easily removed. The granular material not fused can be used again. The flux is applied through a feed tube, just ahead of the electrode and takes its supply from a feed hopper.
Submerged-arc welding may be done with either DC or AC power. DC gives better control of head shape, penetration, and welding speed, and arc starting is easier with it. Bead shape is usually best with DC reverse polarity (electrode positive), which also provides maximum penetration; Higher deposition rates and minimum penetration are obtained with DC straight polarity. AC minimises arc blow and gives penetration between that of DCRP and DCSP.
It is a favoured process for rebuilding and hard surfacing. Any degree of mechanisation can be used—from the hand-held semiautomatic gun to boom or track-carried and fixture-held multiple welding heads.
The characteristics like high quality of welds obtained by submerged arc process, high deposition rates, deep penetration, adaptability of the process to full mechanisation, and no glare, no sparks, no spatter, no smoke and no extra heat radiation make it a preferred process in steel fabrication. The high deposition rates attained with this process are chiefly responsible for the economies achieved.
It is observed that welds obtained by this process are stronger than the base plate (the carefully controlled ingredients in submerged arc fluxes and electrodes, combined with the protecting and cleaning action in the weld puddle, produce refined weld metal with virtually no porosity or inclusions).
The welds are low in hydrogen and thus excellent crack resistance. Human error is minimised due to machine control of welding parameters and arc placement, and it is possible to produce welds of the same size and quality repeatedly and there is maximum operator comfort and minimum fatigue.
This process is limited to flat welding, although welds can be made on a slight slope or on circumferential joints also. It is also advisable to use backing strip of steel, copper or some refractory material on the joint to avoid the loss of molten metal.
Very high direct current of the order of 300 to 4000 amperes is used in this process as it permits rates of metal transfer and welding speeds. 12 mm thick steel plate can be welded at 10 mm/sec.
The submerged arc welding is mostly adopted for low carbon and alloy steels but sometimes non-ferrous metals are also welded. It is used for manufacture of large boilers, heavy marine components, ship and barge buildings railroad to building, pipe manufacture in fabricating structural beams, girders etc.
Arc can be initially generated by using a high voltage and high frequency current. Joints can be prepared with a shallow V groove resulting in lesser electrode consumption. Weld spatter is eliminated and nearly 100% deposition efficiency achieved. High welding speeds are possible. Wire electrodes are inexpensive. Entire welding takes place beneath a bath of molten flux, without sparks, smoke or flash.
It can’t be used for plates less than 5 mm thick. It can’t weld cast iron because of high heat input. Slag has to be removed continuously after it has melted in order to avoid entrapment between passes. Flux is subjected to contamination and adsorption of moisture.