Types and Methods of Welding

WELDING

Uses and advantages of welding:

(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.

    Different process of Welding:

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:

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:

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.

           Welding Methods. (a) Weld should be made in the flat position as far as practicable. (b) Freedom of movement of one member should be allowed as far as possible. (c) The work should be securely held in position by means spot welds, service bolts, clamps or jigs before commencing welding so at to prevent any relative movement due to distortion, wind or other causes. (d)  The parts to be welded must be thoroughly cleaned and proper flux used. Any paint or rust and loose mill scales, etc. should be remove from the surfaces to be welded and surrounding materials for a distance of at least 12mm from the weld. A coating of boiled linseed oil may be permitted. Steel to be welded should not be painted or oiled until after erection, unless all ends to be welded are left bare. (e) The sequence of welding should be such that when possible the members which offer the greatest resistance to compression are welded first.