In industry most of the materials are fabricated into the desired shapes mainly by one of the four methods, casting, forming, machining and welding. The selection of a particular technique depends upon different factors which may include shape and the size of the component, precision required, cost, material and its availability. Sometimes it is possible to use only one specific process to achieve the desired object. However, more often it is possible to have a choice between the processes available for making the end product. In the latter case economy plays the decisive role in making the final choice.
Casting is perhaps the oldest known method of giving shapes to metals and alloys. When found suitable, it is the shortest route from the ore to the end product and usually the most economical. Through these days techniques have been developed to cast almost all metals and their alloys but still there are certain specific materials which have very superior casting properties, for example grey cast iron.
After casting followed the forming process in which the metals and their alloys are given desired shapes by the application of pressure, either by sudden impact as in the case of hammer blows or by slow kneading action as in hydraulic presses. Mechanical working of a metal below its recrystallisation temperature is called ‘Cold Working’ and that accomplished above this temperature is known as ‘Hot Working’. Both hot and cold working (and forming) is practised extensively in the industry.
It is the process of giving the desired shape to a given material by removing the extra or unwanted material by cutting in the form of chips. The cutting tool material is by necessity harder and stronger than the material to be cut. The machining processes commonly employed are turning, milling, drilling, shaping, planning, reaming, boring etc. Through lathes and milling machines were used in connection with watch making even in the fifteenth and sixteenth centuries but most of theses processes were introduced into the high volume industries in their present forms for making stream engine parts in the late nineteenth century but have come of age in the present century.
Welding as it is normally understood today is comparatively a new corner amongst the fabrication process through smith forging to join metal pieces was practised even before Christ. Through there are a number of well established welding process but arc welding with coated electrodes is still most popular welding process the world over.
These days different welding machine developed for different welding process like arc welding machine, MIG welding machine, TIG welding machine, welding rectifier, spot welding machine, plasma cutting machine and also portable welding machine like inverter welding machine (IGBT welding).
Arc welding in its present form appeared on industrial scene in 1880’s. Through there are conflicting claims about the inventor of this process but very often it is attributed to a Russian named Slavianoff who is claimed to have patented it in 1881. Arc welding machine, however, was not accepted for fabrication of critical components till about 1920 by which time coating for electrodes had been well developed. However, the demand for large scale production of heavy items like ships, pressure vessels, construction of bridges and the like provided the necessary impetus for welding to come of age and the Second World War firmly established it as the major fabrication process.
Welding which is a process of joining two or more parts of material (s) through provides a permanent join but does normally affect the metallurgy of the components. It is therefore usually accompanied by post weld heat treatment (PWHT) for most of the critical components.
Most materials can be welded by one process or the other. However, some are easier to weld than others. To compare this ease in Welding a term “Weldability” is often used. Weldability of a material depends upon various factors like the metallurgical changes that occur due to welding, changes in hardness in and around the weld, gas evolution and absorption, extent of oxidation, and the effect on cracking tendency of the joint. Depending upon these factors plain low carbon steels have the best weldability amongst metals. Quite often materials with high castability usually have low weldability.
Welding process widely used in the industry include oxy-acetylene, manual metal arc or shield metal arc (SMAW), submerged arc welding (SAW), metal inert gas (MIG), tungsten inert gas (TIG), thermit welding and cold pressure welding. Most of these processes have special fields of influence like resistance welding is popular with the automobile industry, thermit welding for joining rails in situ, MIG welding is particularly suited for welding of low carbon steel structures as also welding of stainless steels and aluminium, TIG welding is more popular with aeronautical and nuclear industries, SAW welding for ship building, cold pressure welding by food processing industry, and the like. However, SMAW or stick electrode welding and oxy-acetylene welding processes are the general purpose processes with a wide range of applications.
Some of the typical applications of welding include the fabrication of ships, pressure vessels, automobile bodies, off-shore platforms, bridges, welded pipes, sealing of nuclear fuel and explosives etc.
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