Introduction to Basic Manufacturing Processes and ... - always yours

338 Introduction to Basic Manufacturing Processes and Workshop Technology
The residual stresses also reduce load carrying capacity of the structure. The residual
stresses may be relieved by heat treatment. Preheating the whole structure is helpful to
reduce residual stresses. Certain procedures and proper welding sequences are also used in
removing the distortion and internal stress. It is to be noted that the flow of heat in the weld
zone is highly directional towards the adjacent cold metal, which produces columnar grains
at right angle to the fusion line. The columnar structure is a characteristic of the metal of
single pass welds. Thus the original structure consisting of ferrite and pearlite in slabs is
changed to another microstructure. The composition of the first crystal which form a molten
alloy may quite different from the composition of the liquid, but as the freezing proceeds, the
crystals readjust their composition to that of the initial liquid alloy in order to satisfy the
condition of equilibrium. The weld metal when it is in the molten state can dissolve in ore
gases, which come into contact with it, like oxygen, nitrogen and hydrogen. But as the metal
cools it looses its dissolving capacity and the dissolved gases become free from the metal
creating gas pockets and porosity in the final weld.
Welding processes widely used in the industry include oxy-acetylene, manual metal arc
or shielded metal arc, submerged arc, gas metal arc, gas tungsten arc welding, resistance
welding, 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. Gas metal arc welding is particularly suited for welding of low carbon
steel structures as also welding of stainless steels and aluminium. It is more popular in
aeronautical and nuclear industries. Submerged arc welding is used for ship building. Cold
pressure welding is preferred by food processing industry. However, Arc welding and oxyacetylene
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 platform, bridges, welded pipes, sealing of nuclear fuel and explosives,
etc. The knowledge of welding is much essential to make welded fabrications a success.
17.12 WELDING DEFECTS
Defects in welding joints are given in 17.31 (i-viii)
Lack of penetration
Lack of penetration
Fig. 17.31(i)
Incomplete fusion
Lack of Fusion
Lack of fusion
Lack of fusion
Fig. 17.31(ii)
Incomplete Fusion