FUNCTIONS OF FLUX COATING
The coating of the electrode performs very important functions which are essential for obtaining a weld of required quality; in terms of chemical composition, mechanical properties and weld metal cleanliness.
a) The primary function of the coating is to initiate and maintain the arc column between the electrode and the work. This requires a continuous supply of ionized gas or continuous ionisation of the medium between the two terminals. Arc initiation requires a higher voltage than that to sustain it. The ionizing elements help the easy initiation and further maintenance of a column of ionized gas.
b) Having established a stabilized arc, the coating carries out its second function of protecting the molten metal that is transferred from the electrode to the weld below. The gas forming materials in the coating produces a gas shield by burning in the arc heat. The shielding mostly comprises active gases like carbon dioxide, hydrogen and oxygen. These isolate the weld area from the surrounding atmosphere.
c) The slags that forms a result of the reaction between the flux and the oxides, further protects the weld pool by covering it complete. This prevents any possible absorption of gas by the molten weld metal. In addition, the slag slows down the freezing rate of molten metal. Also it slows down the cooling rate of solidified weld metal.
d) To compensate the loss of alloying elements additional quantity of these elements are introduced into the weld metal through the electrode coating.
e) Deoxidisers which reduce the oxidation of the base metal are brought to the reaction zone through the medium of coating.
f) Because of the cup that is formed at the tip of the electrode during welding, the arc stream is concentrated and is directed towards the weld pool. The concentrated arc agitates the weld pool. This allows the dissolved gases to be released and also helps to achieve an uniform weld structure.
g) Flux acts as scavenger.
h) Finally the electrode coating, except in the case of iron powder electrodes, is a poor conductor of electricity. This property is very much useful when welding has to be carried out in a deep narrow groove.