There are two principal forms of welding of steel, viz., electric arc welding and oxyacetylene gas welding.
The electrical welding is further divided into resistance arc, shielded arc and atomic hydrogen welding. In both the types of welding, the members to be welded are placed in contact and edges are melted so that the metal flows from both the members together. When allowed to cool, the members get welded with the aid of the weld. In order to get a perfect joint, additional metal is supplied with the aid of a metallic rod which can be used as an electrode.
In general, the defects noticed in welding are: undercut, porosity, incomplete penetration, lack of fushion at edges, slag inclusions, hot crack, gas pores and surface imperfections.
Undercut occurs when a groove is melted into the member adjacent to the arc formed by the arc action which has not subsequently included in the weld metal.
Porosity is the presence of cavities in the weld metal caused by the gas entrapment. This happens because of the presence of moisture, scales, oil and other contaminations. This is overcome by striking the arc and travelling for 15 mm and then going back over the initial arc to melt out the start porosity.
Incomplete penetration invariably occurs when larger diameter electrodes are used in fillet welds. In butt welding, incorrect, ‘back gauging’ can result in lack of penetration.
Lack of fushion at edges happens in fillet welds and is due to the failure to fuse together adjacent surfaces of the weld and the material of the member.
Slag is derived from the flux used with welding process. If it is entrapped in the weld it is harmful. This can be prevented by correct plate preparation.
Typical hot crack in a fillet weld is a longitudinal crack characterised by a blue appearance along the crack due to surface oxidation at high temperatures.
Gases are formed inside the weld. Main surface imperfections can happen due to (i) edge of the plate melt off and (ii) overlap.
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