The Welding Arc:

The arc is a sustained electrical discharge over a gaseous path among two poles. The arc current is carried by ionised gas known as plasma, which consists of nearly equal number of ions and electrons. The current is mostly conducted by electrons, which flow out of negative pole (cathode) and move towards a positive pole (anode). In a welding arc, the plasma might be mixed with other states of matter such like slags, molten metals, vapours, neutral and excited gaseous atoms and molecules. From the practical point of view, a welding arc might be considered to be a gaseous conductor, which converts electrical energy into heat.

Welding arc temperatures have been found to range among 5,000 and 30,000 K, based on the nature of the plasma and the current passing through the arc. In covered electrodes which carry easily ionised materials such like sodium and potassium in the coating, maximum arc temperatures are around 6,000 K. In an inert gas shielded arc, the axial temperature might be as high as 30,000 K. The highly concentrated heat of the arc and its small size make this possible to weld thick metallic sections at quite high speeds with outstanding bonding and minimum dissipation of heat, resulting in narrow heat-affected zones.

In a welding arc, heat generation occur at the cathode, at the anode and in the plasma. At the cathode , Heat is generated mainly by ionic bombardment. The plasma heat comes from the collisions between ions , electrons and atoms. At the anode, the incoming electrons strike the anode surface and liberate considerable heat, due to the energy, that they took upon emission from the cathode and the added energy, which they attained during acceleration across the plasma under the potential difference of the poles.

The heat liberated at the cathode and anode regions is greater than that from the plasma or arc column usually. Also the amount of heat produced at the anode and cathode differ appreciably, based on the metals that make up the electrode and base metal, and on the nature of the plasma. The heat distribution among anode and cathode frequently determines the melting rate of the electrode and penetration into the base metal. It ought to be understood that the above remarks apply to the DC arc and not to the AC arc, since in the latter case the polarity is not constant.

Throughout welding the entire heat generated by the arc is not utilized for heating and melting of joint edges, but a portion is utilized for heating and melting the electrode and its coating. Some portion is lost in convection and radiation.

The heat build by an arc or arc energy might be calculated by using the simple formula :

W = V × I × T

here,

 W        =          heat in joules,

V          =          arc voltage in volts,

I           =          arc current in amperes, and

T          =          time in seconds.

If the arc is travelling at a speed of S mm/sec, the heat-input per unit length of the weld is specified by the formula :

H = W /S =( V × I ) /S

 The welding arc may not be initiated merely by applying voltage needed by the arc to the cold poles the voltage needed by the arc. A conducting or ionising link ought to be provided between them. Usually this is done by either applying a adequately high voltage among the poles to cause a discharge or by bringing them together to make a contact and then drawing them apart.

Once an arc has been struck, re-initiating it after a momentary extinction is comparatively easy. Re-initiation is further enhanced if materials which are good thermal emitters are present in the electrode. A DC arc is simple to maintain once it is initiated, but it is not the case with the AC arc that is extinguished at zero current point on each reversal, twice in every cycle. For the arc to re-initiate, the needed voltage must be available at the point of zero current. It is ensured using a low operating power factor (usually 0.3) in an AC power source (welding transformer) so that the current wave lags the voltage wave, and so the full open-circuit voltage of the transformer is available at the zero current point to re- initiate the arc.

Arc length is a significant factor in arc welding, since it controls arc voltage. A shorter arc means less arc voltage and enhanced current, and it results in enhanced weld deposition rate and welding speed. A longer arc means lower current and higher arc voltage, which lead to lower welding productivity. While the arc is too long, heat is lost to the air, spatter enhanced and the weld metal picks up nitrogen, with the result that the welded joint indicates porosity and reduced toughness. In welding along DC, the shortest possible arc ought to be used to minimise arc blow and contamination by air.