ARC WELDING (DIRECT CURRENT): Welding Terminology, Principle of Arc welding, Straight and Reverse Polarity, Heat Affected Zone (HAZ), Tensile Test


Aim: To conduct arc welding and study the effect of polarity on weld strength and heat effected zone.

Equipment and Material Required: D.C Welding machine, Bench vice, Tensile testing machine, M.S.

Plates of 100x50x5(2), Metallurgical microscope.

Tools Required: Hack saw, Chipping hammer, wire brush, safety goggles, Hand gloves, Face shield, Files.

Welding Terminology:

1) Backing: 

It is the material support provided at the root side of the weld to aid in the control of the penetration.

2) Base Metal: 

The metal to be joined or cut.

3) Bead or Weld bead: 

It is the metal added during a single pass of welding. The bead appears as strikers.

4) Crater: 

In arc welding, a crater is the depression in the weld metal pool at the where the arc strikers.

5) Deposition Rate: 

Rate at which weld metal is deposited per unit time and expressed in kg/hr.

6) Fillet Weld: 

The metal fused into the corner of a joint made of two pieces placed at approximately 90 degrees to each other.

7) Penetration: 

Depth up to which the weld metal combines with the base metal as measured from the top


8) Puddle: 

Portion that is melted by the heat of welding.

9) Root: 

The point at which the 2 pieces to joined are nearest.

10) Tach weld: 

A small weld used to temporarily hold the two pieces together during actual welding.

11) Weld face: 

Exposed surface of the weld.

12) Weld pass: 

A single movement of the welding torch or electrode along the length of the joint, which results in beats, is weld pass.


Principle of Arc welding: 

An arc is generated below 2 conductor cathode and anode. When they are touched to establish flow of current. An arc is sustained electric discharge through ionized gas column called plasma b/w 2 electrodes. Electrons liberated from cathode move towards anode at high speed large amount of heat is generated. To produce are potential diff b/w 2 electrodes should be sufficient.

Straight and Reverse Polarity: 

The positive terminal of DC supply is connected to work piece and the negative terminal to electrode and known as DCSP.

The positive terminal of DC supply is connected to electrode and negative to work piece and is known as DCSP.

Heat Affected Zone (HAZ): 

A HAZ of a weld is the part of welded joint, which has been heated to temperature up to solidify temperature resulting in varioud degree of microstructure as

Tensile Test: 

This test is carried out to determine the ultimate tensile strength under static loading of the base metal weld metal on welded joint.


1. Given 2 M.S. plates are filled at an angle of 450 at 2 surfaces to be joined (V groove is formed)

2. Electrode is fixed to electrode holder.

3. Connections to be given such that electrode- negative and work piece positive.

4. Welding is to be done carefully for the half-length of the plates.

5. The work piece is to be cut into two halves by power hacksaw.

6. The beads are polished, etched with two percent natal solution and studied under the microscope whose magnification factors 10X for the heat effected zone.

7. By gripping the beads b/w the jaws pf Tensile testing machine and load is applied until the work piece breaks and the readings is to be noted.

8. The same procedure is repeated for the remaining half which is welded by reverse polarity and th results are to be compared


1. Edge preparation should be done very carefully.

2. Before welding ensure that the surfaces are extremely clean.

3. While welding always use face shields or goggles.

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