Introduction
Welding is the art of joining materials. It is a fabrication or sculptural process that joins the materials, usually metals or thermoplastics by causing coalescence. Normally in welding operations, the joining of metal pieces is done by raising their temperature to the fusion point so that, they form a pool of molten metal that cools to form a strong joint. Sometimes, the pool is supplemented with a filler metal (wire or rod) which has almost the same composition as that of the workpiece.
History of Welding
Until the end of the 19th century, the only welding process was forge welding, which blacksmiths had used for centuries to join iron and steel by heating and hammering. Later arc welding and gas welding techniques were developed. Many different energy sources can be used for the welding process such as gas flame, an electric arc, a laser, an electric beam, friction, and ultrasound. In an industrial process, welding may be performed in many different environments including open air, underwater, closed room,
etc. So, the welding process is an important method of joining two similar or dissimilar metal parts either by using a filler rod or not, with the application of heat and pressure or no pressure. In this chapter, we will study the types of welding processes and their various related aspects.
Classification of Welding
Based on the materials to be welded, welding can be classified as
(i) Homogeneous Welding
In this joining process, two similar metal parts are jointed either by using a filler rod or without a filler rod, and no pressure is applied during the welding process, e.g., welding of a mild steel part.
(ii) Heterogeneous WeldingÂ
In this joining process, two dissimilar metal parts are joined by using the filler metal. The filler metal is soluble in both the parent metal. The filler metal is less than that of metal parts to be joined such that its melting point is less than the parent metals.
Based on the pressure applied for welding, welding can be classified as.
(i) Pressure or Plastic Welding
In this welding process, the surfaces that are to be welded are first heated up to their molten stage and then they are joined by applying pressure. On solidification, a joint is formed between the surfaces. By using this process two types of welding are done, such as resistance welding and forge welding.
(ii) Non-pressure or Fusion Welding
In this process, the surfaces that are to be welded are kept together and heated by a heating source (such as flame or electric arc), till they reach the molten stage. In addition to this, a filler metal is used. This filler metal mixes with the molten base metals and when this molten metal solidifies, they form a joint…
Arc Welding
Arc welding is a welding process in which coalescence is produced by heating with an electric arc. Mostly, arc welding is done without applying pressure and with or without the use of filler metal, depending upon the thickness of the workpiece.
In arc welding, the electric arc is formed when an electric current passes between two electrodes separated by a short distance from each other. One electrode is a welding rod while the other one is a workpiece (metal) to be welded whose positive terminal is connected with the electron and negative terminal with the workpiece.
An arc is produced by momentarily touching the electrode on the workpiece and then withdrawing it to about 3 to 4 mm from the workpiece. When the electrode touches the workpiece, a current flame is produced and it is withdrawn from the plate, a current continues to flow in the form of a spark, across the gap. To form an arc, the electrode must be always touched with the workpiece and then drawn back, to form a gap for spark. Arc is generated by electrons flowing from the negative to the positive terminal and electric energy is changed into heat and light energy in the form of an arc. The temperature of the arc produced ranges from 2700 to 5500°C.
The basic limitation or drawback of arc welding is that edge preparation is required before welding. The different types of edge preparation can be used based on the following factors.
(i) Thickness of workpiece
(ii) Types of process
(iii) Access for arc and electrode
(iv) Dilatation
(v) Shrinkage and distortion
Arc Welding Equipments
Arc welding consists of the following set of equipment.
1. Welding Set
In arc welding, to generate an arc proper current and voltage are required and the machines used for this purpose are known as welding sets. In arc welding, both AC and DC are used. The welding set consists of equipment.
DC Generator Set
A DC generator set is used to generate electricity in electric arc welding. It is costlier than an AC transformer. It is used for submerged arc welding and TIG welding. In DC arc welding, heat generated at the positive pole is approximately two times, that generated at the negative pole.
The DC generator set is of two types as follows.
(i) Motor generator setÂ
The set starts with electricity and is mostly used for welding. The motor fitted in it can be started with AC or DC supply or current. It has a capacity of 2 A to 100 A.
(ii) Engine generator setÂ
The set in which generator diesel or petrol is used is called the engine generator set. It is very costly compared to a motor generator set. It produces more noise and is used in marriage functions for lighting.
(iii) AC/DC rectifier setÂ
As the name shows, these types of devices supply both AC and DC. In this set, firstly AC is generated and then it is converted to DC with the help of a rectifier.
(iv) AC transformerÂ
The AC transformer used in a welding set is a step-down transformer, which is used to supply electric current for arc generation in electric arc welding. AC transformer is not costlier and requires less maintenance. The chance of arc blow is minimal as the polarity changes continuously.
AC transformer is made for both single and double phase. The transformer may be air or oil-cooled. In oil oil-cooled transformer, a special type of transformer oil is used and the coils of the transformer are completely dipped in oil. These oils also act as an insulator. In an air-cooled transformer, a fan is used to cool the coils of the transformer.
2. Welding Cable or Lead
Welding cables or leads are special types of wires, that are used to carry the current in different parts of welding pieces of equipment. The wire used in the welding process must be of good quality and have a high current carrying capacity. The selection of cable wire depends upon the thickness of the electrode and the distance of the workpiece from the machine. Generally, a single-core cable is used.
Quality Features of Cable
(a) Cable must be flexible.
(b) Insulation of cable should be good.
(c) Proper working capacity cable should be used according to different works.
3. Cable Connector
A cable connector is used to connect the cable wire with the different equipment of a welding machine. It helps in preventing the spark produced at joints.
4. Electrode Holder
An electrode holder is a device, which is used to hold the electrode and connect it with a welding set through a cable. In an electrode holder, an insulator handle is provided for holding the device with a hand. In it, jaws are made in the front end, to hold an electrode at different angles.
Properties of Good Electrode Holder
(a) It can hold the electrode strongly.
(b) Its griping should be good.
(c) It should have less weight.
(d) Easy to change the electrode.
(e) Its balancing should be proper and welded in any direction.
5. Earth Clamp
The Earth clamp is used to clamp the workpiece or welding table with the earth cable. Earth clamping is done to flow the eddy current into the earth. The earth clamping is essential for obtaining the good quality of weld.
6. Welding Table
The table on which the workpiece is kept and the welding operation is performed is known as the welding table Generally, it is made up of cast iron and clamped with the earth cable. The height of the table was either fixed or adjustable and the table may be made of rotatory type.
7. Helmet and Hand Shield
The helmet and hand shield are used to protect the welder from the ultraviolet or infrared rays emitted during the welding process.
8. Goggles
It is used during the welding process and after the welding process for the removal of flux from the weld bead. The flux is removed by a chipping hammer. Goggles are made up of black glasses.
9. Gloves, Apron, and Leg Guard
During the welding process, the welder must wear gloves, an apron, and a leg guard to protect himself. These things are made up of chrome leather, which has a high degree of thermal resistance.
10. Welding Booth
A room that is developed for welding purposes is known as a welding booth. The walls of these booths are black, so the reflection of light does not take place.
11. Chipping Hammer and Wire Brush
A chipping hammer is used for removing the slag of flux from the welded surface and the wire brush is used to clean the welded parts.Â
12. Tool Kit
A tool kit is essential in the welding shop to keep the tools. It contains various types of small tools, which are used from time to time.
Some types of tools are given below.
(a) Spanner set
(b) Files
(c) Scriber
(d) Tongs
(f) Try square
(e) Plairs
(g) Bevel protector
(i) Chisels
(k) Centre punch
(h) Plum bob
(j) Hammer
(l) Screwdriver
These are some essential phenomenon, which has a great impact on obtaining good-quality weld. Therefore, one must have proper knowledge about them. Let us discuss those phenomena.
Arc Initiation
Arc initiation refers to the phenomenon of generating or starting an arc between the electrode and workpiece, during welding operation. An arc is initiated due to the ionization of air present in the small gap between the electrode and the workpiece. For arc initiation, the electrode is first made contact with the workpiece, then slightly pulled back to create a small air gap. An arc initiation can be done by following methods, which are as follows.
(i) Touch Start Arc Initiation Method
In the touch start method, the electrode is first touched with the workpiece and later, immediately drawn apart. The electrode is touched with the workpiece for a very short time, creating short-circuiting conditions (i.e., resistance becomes zero), and due to this heavy current flows between them. When they are apart by a small distance, the current ionizes the air present in the gap. This results in producing an arc.Â
Touch start arc initiation is of two types.
(a) Touch start or short-circuiting method.
(b) Scratching the job with an electrode.
In the scratching method, the electrode is scratched on the job and then fixed out definite height to produce an arc.Â
(ii) High Voltage Discharge Method
The touch start method may contaminate the electrode (as in the case of a non-consumable electrode). Therefore, another method of high-voltage discharge method was adopted. In this method, the arc is initiated by incorporating a high frequency. An oscillator is used to discharge the high voltage. When the arc is formed, then the arc can be obtained by supplying simple voltage. This method is generally used in TIG welding.
(iii) Steel Wool Method
In this method, electrode contact with the workpiece through a steel wool. The steel wool reduces the resistance of current flow (between electrode and workpiece) and an arc is generated. After the arc formed, this steel wool melted and mixed with welded metal. This method of arc initiation is generally used in submerged arc welding.