Electric Arc Welding

Electric Arc Welding

The welding in which the electric arc is produced to give heat for the purpose of joining two surfaces is called Electric arc welding.

PRINCIPLE

Power supply is given to electrode and the work. A suitable gap is kept between the work and the electrode. A high current is passed through the circuit. An arc is produced around the area to be welded. The electric energy is converted into heat energy, producing a temperature of 3000 to 4000 C. This heat melts the edges to be welded and the molten pool is formed. On solidification the welding joint is obtained.

ELECTRIC POWER OF WELDING

AC current or DC current can be used for arc welding. For most purposes, DC current is preferred. In DC welding, a DC generator or a solid state rectifier is used. DC machines are made upto capacity range of 600A. The voltage in open circuit is kept around 17-25V. DC current can be given in two ways:

Straight polarity: 

In straight polarity welding work piece is made anode and the electrode is made cathode. Electron flow from cathode to anode thus heat is produced at the materials to be welded.

Reverse polarity: 

In reverse polarity system the work is made cathode and the electrode is made anode. This welding is done specially for thin sections.

AC welding has the advantage of being cheap. Equipment used is simpler than DC welding. A transformer is used to increase the current output at the electrode. The current vary from 150-1000A depend upon the type of work. 

EFFECT OF ARC LENGTH

Arc length is distance from the top of the electrode to the bottom of the arc. It should vary from 3-4mm. In short arc length the time of contact will be shorter. And will make a wide and shallow bead. The penetration is low as compare to long arc length.

EQUIPMENTS USED IN ARC WELDING

DC welding equipment

a. AC motor generator: 

In this generator is driven by suitable AC motor. The average voltage of the generator is 25V. the current ranges from 25-100A, the voltage in the generator is variable. The voltage can be set to the desired value with the help of rheostat.

b. Diesel engine generator set: 

In this set, the drive is given by a diesel engine. Rest of the system is same as in case of AC motor generator. Diesel engine generator sets are used in the area’s where electricity is not available.

c. Transformer rectifier set: 

It allows the current to flow through it only in one direction because it has a one way valve or solid rectifier installed on the electrode side of the secondary coil. The set can supply straight polarity and reverse polarity power supply. The rectifier are of two types:
1. Silicon diode
2. Selenium plate

 

AC equipments

Welding transformer set: 

It is used to step down the voltage supply. It consist of primary and secondary circuits. The input is given to primary windings. By electromagnetic induction the current flows through the secondary coil. The output can be controlled as per requirement.

Equipment accessories

a. Cables or leads: 

These leads are made up of copper or aluminium wire. The wires are insulated with rubber and cloth fibre. A heavy insulation is necessary for these cables.

b. Face shield: 

When arc is produced around job, infrared rays and UV rays are produced. To protect face and eyes from these dangerous rays, a shield is necessary.

c. Holder

d. Connectors

e. Ground clamps

Operator’s tools

a. Chipping hammer

b. Wire brush

c. Closed shoe

WELDING POSITIONS

In horizontal position it is very easy to weld. But many times it is impossible to weld the job in horizontal position. Other positions are classified as:

Flat position: 

In this position the work piece is kept in nearly horizontal position. The surface to be welded is kept on upper side.

Horizontal position: 

In this position the work piece is kept in vertical position. Two surfaces rest, one over the other with their flat faces in vertical plane. Welding is done from right side to left side. This axis of the weld is in a horizontal plane and its face in vertical plane.

Vertical position: 

In this position the axis of the weld remains in approximate vertical plane. The welding is started at the bottom and proceeds toward top.

Over head position: 

Here the work piece remains over the head of the welder. The work piece and the axis of the weld remains approximate in horizontal plane. It is most difficult position of welding.

ELECTRODES

Electrodes are of two types:

Coated electrodes: 

These electrodes are generally applied in arc welding processes. A metallic core is coated with some suitable material. The material used for core is mild steel, nickel steel, chromium molybdenum steel etc. One end of the coated core is kept bare for holding.

Bare electrodes: 

These electrodes produce the welding of poor quality. These are cheaper than coated electrodes. These are generally used in modern welding processes like MIG welding.

FUNCTIONS OF COATINGS

1. To prevent oxidation.
2. Forms slags with metal impurities.
3. It stabilizes the arc.
4. Increases deposition of molten metal.
5. Control depth of penetration.
6. Controls the cooling rate.
7. Adds alloy elements to the joint.

SPECIFICATIONS OF ELECTRODES

An electrode is specified by six digits with profile letter M. These six digits indicate the following matter:

M: 

It indicates that it is suitable for metal arc welding.

First digit: 

First digit may be from 1 to 8, which indicate the type of coating on the electrodes.

Second digit: 

It denotes the welding position for which electrode is manufactured. It varies from 1 to 6.

Third digit: 

It denotes the current to be used for an electrode. It is taken from 0 to 7.

Fourth digit: 

Fourth digit is form 1 to 8. Each digit represents the tensile strength of welded joint.

Fifth digit: 

It carries any number from 1 to 5. This digit denotes a specific elongation in percentage of the metal deposited.

Sixth digit: 

It carries any number from 1 to 5 and denotes impact strength of the point.