This article metions about the way how to operate an electric arc welding machine. It includes :
1. Definition of Electric Arc Welding
2. Operation of Electric Arc Welding
3. Electric Current for Welding
4. Significance of Polarity
6. Edge Preparation of a Joint
Definition of Electric Arc Welding:
The arc welding is a fusion welding process in which the heat required to fuse the metal is obtained from an electric arc between the base metal and an electrode.
The electric arc is produced when two conductors are touches together and then separated by a small gap of 2 to 4 mm, such that the current continues to flow, through the air. The temperature produced by the electric arc is about 4000°C to 6000°C.
Arc Welding with a Coated Electrode
A metal electrode is used which supplies the filler metal. The electrode may be flux coated or bare. In case of bare electrode, extra flux material is supplied. Both direct current (D.C.) and alternating current (A.C.) are used for arc welding.
The alternating current for arc is obtained from a step down transformer. The transformer receives current from the main supply at 220 to 440 volts and step down to required voltage i.e., 80 to 100 volts. The direct current for arc is usually obtained from a generator driven by either an electric motor, or patrol or diesel engine.
An open circuit voltage (for striking of arc) in case of D.C. welding is 60 to 80 volts while a closed circuit voltage (for maintaining the arc) is 15 to 25 volts.
Procedure of Electric Arc Welding:
First of all, metal pieces to be weld are thoroughly cleaned to remove the dust, dirt, grease, oil, etc. Then the work piece should be firmly held in suitable fixtures. Insert a suitable electrode in the electrode holder at an angle of 60 to 80° with the work piece.
Select the proper current and polarity. The spot are marked by the arc at the places where welding is to be done. The welding is done by making contact of the electrode with the work and then separating the electrode to a proper distance to produce an arc.
When the arc is obtained, intense heat so produced, melts the work below the arc, and forming a molten metal pool. A small depression is formed in the work and the molten metal is deposited around the edge of this depression. It is called arc crator. The slag is brushed off easily after the joint has cooled. After welding is over, the electrode holder should be taken out quickly to break the arc and the supply of current is switched off.
The Arc Welding Setup
Electric Current for Welding:
Both D.C. (direct current) and A.C. (alternating current) are used to produce an arc in electric arc welding. Both have their own advantages and applications.
The D.C. welding machine obtains their power from an A.C. motor or diesel/petrol generator or from a solid state rectifier.
The capacities of D.C. machine are:
Up to 600 amperes.
Open Circuit Voltage:
50 to 90 volts, (to produce arc).
Closed Circuit Voltage:
18 to 25 volts, (to maintain arc).
The A.C. welding machine has a step down transformer which receives current from main A.C. supply. This transformer step down the voltage from 220 V-440V to normal open circuit voltage of 80 to 100 volts. The current range available up to 400 amperes in the steps of 50 ampere.
The capacities of A.C. welding machine are:
Up to 400 ampere in steps of 50 ampere.
Actual Required Voltage:
80 – 100 volts.
Significance of Polarity:
When D.C. current is used for welding, the following two types of polarity are available:
(i) Straight or positive polarity.
(ii) Reverse or negative polarity.
When the work is made positive and electrode as negative then polarity is called straight or positive polarity, as shown in Fig. 7.16 (a).
In straight polarity, about 67% of heat is distributed at the work (positive terminal) and 33% on the electrode (negative terminal). The straight polarity is used where more heat is required at the work. The ferrous metal such as mild steel, with faster speed and sound weld, uses this polarity.
(a) Straight polarity.
(b) Reverse polarity
Polarity for D.C. Arc Welding
On the other hand, when the work is made negative and electrode as positive then polarity is known as reverse or negative polarity, as shown in Fig. 7.16 (b).
In reverse polarity, about 67% of heat is liberated at the electrode (positive terminal) and 33% on the work (negative terminal).
The reverse polarity is used where less heat is required at the work as in case of thin sheet metal weld. The non-ferrous metals such as aluminum, brass, and bronze nickel are welded with reverse polarity.
Equipments Required for Electric Arc Welding:
The various equipments required for electric arc welding are:
1. Welding Machine:
The welding machine used can be A.C. or D.C. welding machine. The A.C. welding machine has a step-down transformer to reduce the input voltage of 220- 440V to 80-100V. The D.C. welding machine consists of an A.C. motor-generator set or diesel/petrol engine-generator set or a transformer-rectifier welding set.
A.C. machine usually works with 50 hertz or 60 hertz power supply. The efficiency of A.C. welding transformer varies from 80% to 85%. The energy consumed per Kg. of deposited metal is 3 to 4 kWh for A.C. welding while 6 to 10 kWh for D.C. welding. A.C. welding machine usually work with low power factor of 0.3 to 0.4, while motor in D.C. welding has a power factor of 0.6 to 0.7. The following table 7.9 shows the voltage and current used for welding machine.
Voltage and Current for Welding Machine
2. Electrode Holders:
The function of electrode holder is to hold the electrode at desired angle. These are available in different sizes, according to the ampere rating from 50 to 500 amperes.
3. Cables or Leads:
The function of cables or leads is to carry the current from machine to the work. These are flexible and made of copper or aluminum. The cables are made of 900 to 2000 very fine wires twisted together so as to provide flexibility and greater strength.
The wires are insulated by a rubber covering, a reinforced fibre covering and further with a heavy rubber coating.
4. Cable Connectors and Lugs:
The functions of cable connectors are to make a connection between machine switches and welding electrode holder. Mechanical type connectors are used; as they can he assembled and removed very easily. Connectors are designed according to the current capacity of the cables used.
5. Chipping Hammer:
The function of chipping hammer is to remove the slag after the weld metal has solidified. It has chisel shape and is pointed at one end.
6. Wire Brush, Power Wire Wheel:
The function of wire brush is to remove the slag particles after chipping by chipping hammer. Sometimes, if available a power wire wheel is used in place manual wire brush.
7. Protective Clothing:
The functions of protective clothings used are to protect the hands and clothes of the welder from the heat, spark, ultraviolet and infrared rays. Protective clothing used are leather apron, cap, leather hand gloves, leather sleeves, etc. The high ankle leather shoes must be wear by the welder.
9. Screen or Face Shield:
The function of screen and face shield is to protect the eyes and face of the welder from the harmful ultraviolet and infrared radiations produced during welding. The shielding may be achieved from head helmet or hand helmet.
Edge Preparation of a Joint:
The efficiency and quality of welded joint also depends upon the correct preparation of the edges of the plates to be welded. It is necessary to remove all scales, rust, grease, paint, etc. from the surface before welding.
The cleaning of the surface should be carried out mechanically by wire brush or power wire wheel, and then chemically by carbon tetrachloride. Proper shape to the edges of the plate should be given to produce a proper joint.
The shape of edges may be plain, V-shaped, U-shaped, reshaped, etc. The choice of various edge shapes depends upon the kind, thickness of metal to be welded. Some different types of grooves for edges of the work :
(i) Square Butt:
It is used when the thickness of the plate is from 3 to 5 mm. Both the edges to be weld should be spaced about 2 to 3mm apart as shown in Fig. 7.17 (a).
(ii) Single- V-Butt:
It is used when the thickness of the plates is from 8 to 16 mm. Both the edges are bevelled to form an angle of about 70° to 90°, as shown if Fig. 7.17 (b).
It is used when the thickness of the plates is more than 16mm and where welding can be performed on both sides of the plate. Both the edges are bevelled to form a double-V, as shown in Fig. 7.17 (c).
(iv) Single and Double-U Butt:
It is used when the thickness of the plate is more than 20mm. The edge preparation is difficult but the joints are more satisfactory. It requires less filler metal, as shown in Fig. 7.17 (d) and (e).