13 May 2017

Submerged arc welding

Introduction :

Submerged are welding is a common arc welding process in which the formation of an arc between a continuously fed electrode and the workpiece. It is mainly used for doing faster welding joints. The arc is produced while the consumable electrode wire is continuously fed into the weld zone as in gas metal arc welding. The welding zone is completely covered by means of a large amount of flux. The arc occurring between the electrode and the workpiece is completely submerged under the flux and is not visible from outside. A part of the flux melts and forms the slag, which covers the weld metal. The unused flux is collected and reused.

How it works?

This welding process may be automatic or semi-automatic. The main principle of this process is the flux starts depositing on the joint to be welded. The power source used with submerged arc welding can either be AC or DC. Both constant-voltage and constant current type machines can e effectively used. The arc may be stuck either by touching the electrode with the workpiece or by placing steel wool between electrode and job before switching on the welding current. The arc is maintained between the end of the bare wire electrode and the weld. The electrode is constantly fed into the arc as it is melted. In all cases, the arc is stuck under the cover of flux. Flux otherwise is an insulator but once it melts due to the heat of arc, it becomes highly conductive and hence the current flow is maintained between the electrode and workpiece through the molten flux. The upper portion of flux is in contact with the atmosphere which is visible and remains unused. The lower portion of flux becomes slag, which is waste material and it is removed after the welding process is done.

The electrode at a constant predetermined speed continuously fed to the joint to be welded. The arc length is also kept constant by using the principle of self-adjusting arc because if the arc length decreases the voltage will increases. Therefore burn off rate will increases. 

In this process also some backing plate of the material like steel or copper may be used for control penetration and to support a large amount of molten metal associated with this process. 
Submerged arc welding

SAW process mainly depending upon the following factor :
  • Arc voltage or Arc length 
  • Electrode travel speed 
  • Electrode stick out or Contact tip to work 
  • Current type AC or DC 
  • Wire feed speed 
Advantages of submerged arc welding :
  • High deposition rate.
  • High operating factors in some applications.
  • Deep weld penetration so welded joints are strong.
  • It prevents hot materials from splattering and splashing onto workers because arc is always covered under the blanket of flux.
  • High speed welding for thin plates.
  • SAW is suitable for indoor and also outdoor works.
  • Single pass welds can be made in thick plates. 
  • Most of the flux are reused in this process. 
  • Very neat appearance and smooth weld shapes can be got.
  • Good ductility and corrosion resistance and good impact strength are formed in joints.
  • In this process we did not required to add pressure because it already generated by electrode.
Limitations of submerged arc welding :
  • This process is limited to steel, stainless steels and nickel.
  • Limited to the 1F, 1G and 2F positions. 
  • Limited to high thickness metal plates.
  • Flux is subjected to contamination that may caused porosity in welded joints.
  • The flux needs replacing on the joint which is not always possible.
  • Requires backing strips for proper penetration.
  • Flux and slag residue can present a health and safety concern.