(a) Welding requires much less time (which gives rapid production) than riveting, and is also more economical.
(b) Entire cross - section of tension members is utilized as there are no rivet holes to be deducted and can thus take more loads.
(c) There is saving of materials for end connections as no gussets, etc., are required.
(d) Reduction in weight of the structure.
(e) In certain works welded fabrication is the most practical solution.
There two principal forms of welding metals – Electric Arc Welding, and Gas Welding. The electric arc welding process is the most important and is most extensively used for mild steel structures with plate thickness over 1.5mm. Gas welding is done using oxy-acetylene flame and is not adopted to structural steel work but is used generally for small jobs, and has a very wide application. It requires heating of the members to be welded along with the welding rod and is likely to create temperature stresses in the welded members.
Electric arc welding is a process whereby the metal of the two members to be welded is fused together through heat generated by an electric arc. The electrode used is a specially prepared metal rod or wire which supplies the metal to be deposited in the weld. The heat produced by the electric arc, raises the work piece at point of welding and the electrode to a very high temperature, the end of the electrode is melted and fuses with contiguous metal surface to be joined. Fusion should be complete over the area of the joint surface. This type of welding can be done on most of the mild steels ranging from light articles with a wall or section thickness of 16 gauges to heavy fabrications.
Gas welding is done with a flame produced by the burning of oxygen acetylene (Acetylene gas burning in pure oxygen produces a flame) fed through a blow pipe which is ignited at its tip. The flame is played on the two pieces to be welded until the metal becomes hot enough to fuse together adding additional metal to the join as necessary by melting into it a suitable electrode.
The proportion of oxygen and acetylene forming the oxy-acetylene frame is an important factor and the gases must be mixed in correct proportions otherwise defective welds will be produced. Three flame conditions are developed, (a) Neutral flame, in which equal quantities of both the gases are supplied and which required for most of the iron works, steel, stainless steel, cast iron, copper, aluminum etc. in a neutral flame inner cone is sharply defined with a very slight haze or flicker at its end, disappearance of this flicker indicate excess of oxygen. This condition must be avoided as when excess of oxygen is supplied to the torch it becomes an effective cutting or burning tool – the heated steel burns in the presence of excess of pure oxygen. (b) Oxidizing flame, in which excess of oxygen is supplied by the blow pipe. This flame is used for welding brass and some bronzes. (c) A Carburising flame in which an excess of acetylene is delivered through the blow-pipe. This flame is useful for hard-surfacing applications.
Agnition of the acetylene is first achieved at the tip without any flow of oxygen ; when the flow of acetylene is such that the flame is about to leave the tip, the oxygen is turned on until the correct flame or welding or cutting is obtained.
Maximum Diameter of Electrodes for Welding
Average thickness of plate or section
Max. gauge or dia. Of electrode to be used
Less than 5mm 3.2mm – 10 SWG
5mm to less than 8mm 4mm - 8 SWG
8mm to less than 10mm 5mm - 6 SWG
10mm to less than 16mm 6mm - 4 SWG
16mm to less than 25mm 9mm
25mm and over 9mm
(See also under welding of Reinforcement Rods)
Cracks may occur in welding alloy steels owing to the rapidity with which these harden. This may largely be avoided by preheating the parent metal to 300 deg. C. or above in advance of welding to lower the normal cooling rate.