What is Welding Process? – Definition, Classification, Advantages and Disadvantages:
Welding Process is a materials joining that produces coalescence of materials by heating them to the welding temperature with or without the application of pressure or by the application of pressure alone, and with or without the use of filler metal. It is used to make welds. A weld is a localized coalescence of metals or non-metals produced either by heating materials to the welding temperature, with or without the application of pressure, or by the application of pressure alone and with or without the use of filler metal.
All metals can be joined by one welding process or another. There is a saying, “If it’s metal, weld it,” and it is certainly true. This should be qualified by stating that all metals commercially employed for structural or strength parts are weldable. Some metals are easy to weld, and others are difficult to weld. The metals that are easily weldable can be welded in thickness from the very thinnest, about the thickness of this paper page, to the thickest or heaviest produced. The difficult-to-weld metals require special procedures and techniques that must be developed for specific applications. Some metals may never be welded or joined. For example, mercury is a liquid at room temperature and cannot be welded, while sodium and potassium melt just below the temperature of boiling water and are of no use as a strength member and cannot be welded. In general, metals that have a low melting point or low strength would not be welded. Some metals are so scarce or expensive that they would not be used where welding would be required. The physical and mechanical properties (such as melting temperature, density, thermal conductivity, tensile strength, ductility etc.), availability, and price all help in determining if a metal will be used in applications where welding process is required.
Importance of Welding:
Welding Process is the most efficient way to join metals. It is the only way to join two or more pieces of metal to make them act as one piece. Welding is widely employed to manufacture or repair all products made of metal. Look around, almost every thing made of metal is welded; the world’s tallest building, moon rocket engines, nuclear reactors, home appliances, and automobiles barely start the list.
The use of welding is still increasing. If a joint is welded, it is permanent joint. Obviously, if the joint must be disassembled occasionally, it should not be welded. Thus we should change our statement to “welding is the most economical method to permanently join metal parts.” To join two members by bolting or riveting requires holes in the parts to accommodate the bolts or rivets. These holes reduce the x-sectional area of the members to be joined by up to 10 per cent. The joint may also require the use of one or two gusset plates, thus increasing the weight of material required and the cost. This expense can be eliminated by the use of a weld. The greatest economy of a welded design will be obtained if the x-sectional area of the entire structural member is reduced by the amount of the bolt holes. This can be done since the entire cross section of a member of a welded design is utilized to carry the load. The amount of material required is reduced as well as its cost. This same design concept applies to joining plates used to build a ship or a container. In view of this material savings, ships and storage tanks are no longer riveted.
Pipes joined by welding offer similar economies. The wall thickness of a pipe should be heavy enough to carry the required load. However, if the pipe is joined by screw threads a heavier wall thickness is used to allow for cutting away a portion of the thickness for the threads. A thinner pipe wall thickness is used for the entire welded pipe system. This reduces the amount of metal required and the cost. The inside surface of the welded joint is smoother. Larger diameter pipes are no longer connected together with screw threads and pipe fittings.
Converting castings to weldments allows the designer to reduce weight by reducing metal thickness. Welding Process is a design concept which allows freedom and flexibility not possible with cast construction. Heavy plates can be used where strength is required and thin ones can be used where possible. The uniform thickness rule and minimum thickness required for foundry practice are not necessary for weldments. Additionally, high-strength materials can be employed in specific areas, while normal-strength materials are used where required.
Welding Process is the best way to protect and conserve materials by protecting their surface with special metal overlays. Corrosion and wear of metals account for losses running into billions of dollars annually. Together they are responsible for an untold loss of lives. Waste from both of these destructive forces can be largely reduced by welding. Special alloys are weld deposited on base metals to provide corrosion resistance surfaces. Hard surfacing overlays can be made by welding to provide special alloys with wear resistant surfaces. A typical application is the resurfacing of a cinder crusher roll with hard weld metal. Weld surfacing is used to reduce the costly abrasive and corrosive wear of machinery.
Thus in short we can say, “today, the science and art of welding encompasses a wide range of processes and procedures applicable to materials of any thickness and shape—from tiny electronic components to mammoth machines and structures. Welding has opened up prospects for radical improvements in the manufacture of a large variety of machines, devices and structures. It has promoted mechanization and automation in their manufacture. The use of automatic, semi-automatic and mechanized welding equipment has relieved a large number of workers for employment elsewhere in the national economy. Welding is nowadays extensively used in automobile industry, aircraft machine frames, structural work, tanks, machine repair work, shipbuilding, pipeline fabrication in thermal power plants and refineries, fabrication of metal structures.”
Advantages and Disadvantages of Welding:
Some of the advantages of welding process are given below :
- Welding is the lowest-cost joining method.
- It affords lighter weight through better utilization of materials.
- It joins all commercial metals.
- It can be used anywhere.
- It provides design flexibility.
The limitations of welding are :
- Some welding depends on the human factor.
- It often needs internal inspection.
Most of these limitations can be overcome by means of good controls and supervision.
Welding is also an economical manufacturing method wherever there is a need to join parts permanently. In the high-volume production industries it is common to see welding operations intermixed with bending, machining, forming, assembly and so on. Welding is an important manufacturing process taking its place with other metal working operations to help bring us good-quality products at economical prices.
Personnel Protection and Safety Rules:
There are a number of safety and health problems associated with welding. When correct precautionary measures are followed, welding is a safe occupation. The hazards that are more or less peculiar to welding are electrical shock, are radiation, air contamination, fire and explosion, compressed gases, welding cleaning and other hazards related to specific processes or occupations.
Combustible materials must not he allowed to collect in or near the welding workplace. Good housekeeping practices should always be employed in the welding shop. Adequate safety devices should be provided, such as fire extinguishers, life saving and support equipment, first-aid kits, and so on, plus the training of personnel to utilize this equipment properly. Only approved equipment be used, and it must be properly installed and maintained in good working conditions.
Classification of Welding Processes:
There are many ways to make a weld and there are many different kinds of welds. The welder behind the hood making sparks is using one of the more popular welding processes, known as arc welding. Some welding processes do not cause sparks; in some cases electricity is not used, and in some cases there is not even extra heat. Welding has become complex and technical. It needs considerable knowledge to select the proper welding process for critical work.
Modern methods of welding may be classified under two broad headings—plastic welding and fusion welding. They are also called pressure welding, and non-pressure welding respectively. In the plastic welding or pressure welding, the pieces of metal to be joined are heated to a plastic state and then forced together by external pressure. This includes forge welding, resistance welding, thermit welding and gas welding, in which pressure is required. In the fusion welding or non-pressure welding, the material at the joint is heated to a molten state and allowed to solidify. This includes gas welding, arc welding and thermit welding without pressure.
The various welding processes used in general engineering are given below :
- Gas Welding — (i) Oxyacetylene (ii) Air-acetylene and (iii) Oxy-hydrogen.
- Resistance Welding — (i) Butt (ii) Spot (iii) Projection (iv) Seam and (v) Percussion.
- Arc Welding — (i) Carbon arc (ii) Metal arc (iii) Gas metal arc (iv) Gas tungsten arc (v) Atomic hydrogen arc (vi) Plasma arc (vii) Submerged arc (viii) Flux-cored arc and (ix) Electro-slag.
- Thermit Welding.
- Solid State Welding — (i) Friction (ii) Ultrasonic (iii) Diffusion and (iv) Explosive.
- Newer Welding — (i) Electron beam and (ii) Laser.