ARC WELDING

ARC WELDING:

In arc welding, the source of heat is an electric arc. The temperatures reached in an electric arc may be as high as 5500°C. A spark is produced in an electric circuit carrying current, if the circuit is broken accidentally. An electric arc is a sustained spark created intentionally by a gap between welding electrode and the work piece. Because of larger heat output and less oxidation, the quality of weld produced by electric arc is much better than gas-weld.

Either A.C. or D.C. power supply may be used for arc welding. For A.C., a transformer type machine is used to supply current. For A.C., an open circuit voltage of about 75–80 V is required. The current requirement is however very heavy and the welding machine should be capable of delivering 100–300 Amperes.

D.C. supply is charaterised by the +ve and –ve terminals. With D.C., a slightly lesser open

circuit voltage of 70–75 volts will be adequate to strike the arc. Normally the electrode is connected to –ve terminal and the work piece to +ve terminal. Such an arrangement is called D.C. straight polarity (DCSP). In this arrangement about 2/3rd of the heat is produced on the work piece end and 1/3rd at the electrode end. Under certain circumstances like overhead welding, D.C. reverse polarity (DCRP) arrangement is preferred. In this arrangement, the electrode is connected to +ve terminal and the workpiece to –ve terminal.

The process of electric arc welding with coated metal electrode is shown in Fig

Welding action of a cellulosic-coated stick electrode

STRIKING AN ARC

To strike an arc, the electrode should be shorted by touching the work. At the moment of contact, a very heavy current starts flowing through the circuit, while voltage drops. Now, the electrode is lifted slowly so that a gap of 2–3 mm between the tip of the electrode and the work piece is maintained. The voltage across the arc rises to about 15–20 volts and the amperage drops. Due to heat generated in the arc, the tip of the metal electrode starts melting and the gap increases. Unless the electrode is slowly moved towards the work at the same rate at which the tip of the electrode is melting maintaining the gap at 2–3 mm, the arc will extinguish. If the gap increases too much the machine voltage will not be able to maintain the arc.

A great amount of heat (and intense light) is generated by the arc. It not only melts the electrode tip, but also melts the work piece at the location of the arc maintaining a pool of molten metal as shown in Fig. Without some manner of shielding, this metal will oxidise. The metal electrodes are therefore, given a layer of coating throughout its length (except for about 35–40 mm at the stub end, where the metal core of electrode is exposed and held in the electrode holder). Under the action of heat, this coating at the tip of the electrode vaporises and creates a gaseous shield around the molten metal pool and saves it from oxidation. The electrode coating also contains flux (which reacts with impurities to form slag) and other ingredients which help stabilise the arc. Many types of coatings are in use.

As the electrode is slowly moved over the joint, the molten metal pool solidifies creating a joint. The joints produced by this process are often stronger than the parent metals being joined. 

Electrodes are available in many sizes. The size of electrode is specified by the diameter (in mm) of the core metal wire. Size of electrode depends upon the thickness of parts to be joined. Thicker electrodes are required to weld thick plates. The current depends upon the size of electrode used. Thus for a 3.15 mm dia electrodes, the recommended value of current is 100–120 Amp.

HEAT AFFECTED ZONE:

In the arc welding process, a great amount of heat output takes place resulting in formation of a molten pool in the arc area. The heat is also conducted into the vicinity of the joint on either side. The temperature of the material on both sides of the weld bead may not be as high as the melting point of the metal, but, is very close to it. As we move away from the joint or weld bead, the metal may be heated to lesser and lesser temperature. As the electrode travels over the joint and moves away, the heated metal cools as quickly as it was heated. Thus, we can conclude, that the metal adjacent to the weld bead has been subjected to a heat treatment. If steel is being welded, this heating and quick cooling may result in formation of martensitic and other structures which may be prone to cracking and hardness. The area so affected by welding is called “heat affected zone”.

ARC BLOW:

A difficulty associated with D.C. welding is arc blow. Arc blow means that arc is deflecting from its intended path making the job of welding difficult due to shifting of arc. We know that when a conductor carries D.C., a magnetic field is set up whose strength is proportional to the value of the current. In D.C. welding, heavy currents are passing through the electrode and the magnetic fields set up deflect the arc to one side or another. This phenomenon is called arc blow and it becomes particularly serious, when welding is being done at the start of the metal pieces or at their end.

The ways in which arc blow may be reduced are:

1. Switch over to A.C. welding, if possible. Changing polarity of A.C. does not cause arc blow.

2. Reduce current, as practicable,

3. Use as short an arc as possible, and

4. Wrap the ground cable around the work piece several times.

WELDING POSITIONS:

These are four welding positions from the point of view of the welder. These affect execution of sound welding.

These positions are:

1. Downhand welding position: This is the most comfortable position for welder to work in and he is able to produce welds of a good quality.

2. Horizontal welding position (on a vertical surface).

3. Vertical welding position (on a vertical surface).

4. Overhead welding position (say on the ceiling of a room): This is the most difficult welding position. Not only the operator has to crane his neck upwards and raise his arm to maintain arc, it is also difficult as molten metal tends to fall down due to gravity.

For important jobs, manipulators are used, which are capable of turning over the jobs and as

much welding is done in down hand welding position as possible.

ARC WELDING DEFECTS:

Improper welding procedure and lack of skill on the part of welder may result in many welding defects.

The major welding defects are described below:

(i) Incomplete fusion and lack of penetration: Incomplete fusion can be avoided by proper weld joint preparation, using adequate current and travel speed of electrode should not be too high.

(ii) Porosity: Molten metal has a tendency to absorb gases. The entrapped gases cause porosity or blow holes in the weld bead. Remedy lies in cleaning the work piece surface of all oil, grease and paint etc. before welding and ensuring that electrode coating is free from dampness. If necessary, electrodes can be dried in an oven before use.

(iii) Slag inclusion: It refers to slag or other non-metallic inclusions getting entrapped in the weld bead. The most common reason for slag inclusion is that between two electrode runs, the slag, has not been completely removed by chipping and wirebrushing.

(iv) Undercut: Undercutting is often caused due to high amperage used. It denotes the melting away of the base metal at the line where the final layer of weld bead merges into the surface of the base metal. The undercut portion must be rectified by depositing weld metal on it.

(v) Cracking: Cracks can take place either in the weld bead itself (called hot cracks) or in the heat affected zone (cold cracks). Hot cracks may take place due to narrow deep welds and are caused due to shrinkage of weld metal, particularly if impurities like sulphur are present in the weld metal. Excessive joint restraint can also cause such cracks. Cold cracks occur due to inadequate ductility or presence of hydrogen in hardenable steels. Preheating and post heating of base material will help in avoiding cold cracks.