EFFECTS OF WELDING HEAT

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EFFECTS OF WELDING HEAT

EFFECTS OF WELDING HEAT

EFFECTS OF WELDING HEAT

During the process of welding, the metal is heated over a range of temperature up to fusionĀ and then allowed to cool. Heating and cooling causes metallurgical and mechanical effects onĀ the metal pieces being welded. The region of the base metal which is affected by metallurgicalĀ change due to the welding heat is called the heat affected zone. In this zone, the metal hasĀ been heated and cooled through a range of temperature great enough to cause changes in theĀ structure of the metal. The amount of change may be controlled by using the suitable weldingĀ processes. Sometimes, the original properties may be restored by heat treatment after welding.Ā Although the mechanical strength of weld metals is at least equal to that of the base metalĀ but the metallurgical changes can reduce greatly the associated properties of the jointĀ particularly for shock and fatigue.

Mechanical effects of welding heat are the distortion and the residual stress resultingĀ from welding. The magnitude of these effects depends on the factors of design and weldingĀ procedure. But the effects will always be present to a certain extent in any structure fabricatedĀ or repaired by non-pressure welding. Because of expansion and contraction of the heatedĀ portion the distortion and residual stresses set up. The base metal heated locally will expandĀ and up set owing to the restraint imposed by tile surrounding cold metal. This upset portionĀ of the base metal will contract on cooling beyond its original dimensions and thereby setupĀ internal stresses causing distortion. If the pieces being welded are not tree to move, highĀ residual stresses will be formed which may cause cracking during welding.

The residual stresses also reduce load carrying capacity of the structure. The residualĀ stresses may be relieved by heat treatment. Preheating the whole structure is helpful toĀ reduce residual stresses. Certain procedures and proper welding sequences are also used inĀ removing the distortion and internal stress. It is to be noted that the flow of heat in the weldĀ zone is highly directional towards the adjacent cold metal, which produces columnar grainsĀ at right angle to the fusion line. The columnar structure is a characteristic of the metal ofĀ single pass welds. Thus the original structure consisting of ferrite and pearlite in slabs isĀ changed to another microstructure. The composition of the first crystal which form a moltenĀ alloy may quite different from the composition of the liquid, but as the freezing proceeds, theĀ crystals readjust their composition to that of the initial liquid alloy in order to satisfy theĀ condition of equilibrium. The weld metal when it is in the molten state can dissolve in oreĀ gases, which come into contact with it, like oxygen, nitrogen and hydrogen. But as the metalĀ cools it looses its dissolving capacity and the dissolved gases become free from the metalĀ creating gas pockets and porosity in the final weld.

Welding processes widely used in the industry include oxy-acetylene, manual metal arcĀ or shielded metal arc, submerged arc, gas metal arc, gas tungsten arc welding, resistanceĀ welding, thermit welding and cold pressure welding. Most of these processes have specialĀ fields of influence like resistance welding is popular with the automobile industry, thermitĀ welding for joining rails. Gas metal arc welding is particularly suited for welding of low carbonĀ steel structures as also welding of stainless steels and aluminium. It is more popular inĀ aeronautical and nuclear industries. Submerged arc welding is used for ship building. ColdĀ pressure welding is preferred by food processing industry. However, Arc welding and oxyacetyleneĀ welding, processes are the general purpose processes with a wide range of applications.Ā Some of the typical applications of welding include the fabrication of ships, pressure vessels,Ā automobile bodies, off-shore platform, bridges, welded pipes, sealing of nuclear fuel and explosives,Ā etc. The knowledge of welding is much essential to make welded fabrications a success.

Source A Textbook of Basic Manufacturing Processes and Workshop Technology by Rajender Singh.

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I have recently passed my Bachelor of Engineering in Mechanical Branch. I have expertise in CAD system. Also, I am interested to make new things.
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