12 May 2017

Electroslag welding

Introduction :

The electro slag welding process is developed mainly to weld very large metal plates without any edge preparation. This process is most of the cases single-pass process using a consumable electrode for filling the gap between the two heavy plates. The heat required for melting the plates and the electrode is obtained initially by means of an arc so that flux will form the molten slag. Once the molten slag is formed, the arc is extinguished and the heat of welding is obtained by the resistance heating of the slag itself. 

How it works?

First of all, in this process, the gap between the two workpieces is filled with a welding flux. It required to maintain a satisfactory amount of slag is fairly small in the order of 0.2 to 0.3 kg per meter of weld length, irrespective of plate thickness. The welding is initiated by an arc between the electrode and workpiece thus, heat utilized for melting the slag is much less. Most of the heat supplied in electroslag welding it melts the fluxing powder and forms the molten slag. The slag, having low electrical conductivity and is maintained in a liquid state due to the heat produced by the electric current.
Electroslag welding

By this process, a plate of 200 mm thickness can easily be welded in a single pass. The slag reaches a temperature of about the 1930 0C. This temperature is sufficient for melting the consumable electrode and workpiece edges. Metal droplets fall to the weld pool and join the workpiece.

For effective welding, it is necessary to maintain a continuous slag pool and therefore the best way to maintain it. The slag pool is contained in the groove with the help of two water-cooled copper dam plates which move along with the weld.

Electroslag welding is mainly used for welding the steels.

The quality of weld in this process mainly depends upon :
  • Slag depth.
  • The ratio of a width of the weld pool and its maximum depth is known as the form factor.
  • Weld current and voltage.
  • A number of electrodes used.

Advantages of electro slag welding :
  • Most of the cases welding are done easily by a single pass.
  • If any gas is present easily bubbles out through the slag and therefore, a better weld can be done.
  • The heating and cooling of the edge are more gradual.
  • Whatever be the thickness of the plate,  no edge preparation is required.
  • It is also useful for very thick plates. 
  • High deposition rate - up to 45 lbs/h (20 kg/h).
  • Low slag consumption and low distortion.
Disadvantages of electro slag welding :
  • The coarse grain structure of the weld.
  • An only vertical position of the weld is possible.
  • Low toughness of the weld. 
Application of electro slag welding :
  • Fabrication of high-pressure vessels.
  • Frames of heavy mechanical and hydraulic presses. 
  • In Rolling mill frames. 
  • Ship hulls and locomotive frames.