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ARC Welding and Its Methods

 Arc Welding is the most extensively employed method of joining metal parts. Here the source of heat is an electric arc.

The arc column is generated between an anode, which is the positive pole of dc (direct current) power supply, and the cathode, the negative pole. 

When these two conductors of an electric circuit are brought together and separated for a small distance (2 to 4 mm) such that the current continues to flow through a path of ionized particles (gaseous medium), called plasma, conductor that enables more ions to flow from the anode to the cathode. Heat is generated as the ions strike the cathode. This ion theory does not, of course, completely explain the arc column., Perhaps the electron theory of the arc, which is beyond the scope of this book, explain what happens more fully. However, electrical energy is converted to heat energy. Approximately 1 kWh of electricity will create 250 calories (1000 J), the temperature at the center of  the arc being 6000*C to 7000*C. The temperature of an electric arc, of course, depends upon the type of  electrodes between which it is struck. 

The heat of the arc raises the temperature of  the parent metal which is melted forming a pool of molten metal. The electrode metal ( in metal arc welding) or welding rod ( in carbon-arc welding ) is also melted and is transferred into the metal in the form of globules of molten metal. The deposited metal serves to fill and bond the joint or to fuse and build up the parent metal surface. Two-thirds of the heat is developed near the positive pole while the remaining one-third is developed near the negative pole. As a result, an electrode that is connected to the positive pole will burn away approximately 50 per cent faster than when it is connected to the negative pole. This is helpful in obtaining the desired penetration of the base metal. 

The blast of the arc forces the molten metal out of the pool, thus forming a small depression in the parent metal, around which molten metal is piled up. This is known as the arc crater. The distance through the center of the arc from the tip of the electrode to the bottom of the arc crater is termed arc length. Arc length is a vital variable in a welding process and should be 3 to 4 mm. An important reason for this is that the globules of molten electrode metal in the process of deposition should have the smallest possible chance of coming in contact with the ambient air and should absorb as little oxygen as possible , because oxygen as an adverse effect on the mechanical properties of the weld metal. It is obvious that with a shorter arc the time of contact will be shorter than with longer arc. The arc is extinguished by widening the arc sufficiently.


Arc Welding

TYPES OF ARC WELDING :

 The main types of arc welding are :

1. Carbon-Arc Method.

2. Metal Arc Method.

3. Metal-inert-gas arc (MIG).

4. Gas-Tungsten-Arc (TIG).

5. Atomic Hydrogen Arc.

6. Plasma Arc.

7. Submerged Arc.

8. Flux-cored Arc.

9. Electro-slag welding.


Welding Methods or Welding Types








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