For pipes or tubes size 4 inch (10.2mm) OD and below, strip is fed into a set of forming rolls which consists of horizontal and vertical rollers so placed as to gradually from the flat strip in to pipes or tubes which is then allowed to pass the welding electrodes. The electrodes are copper disks connected to the secondary of a revolving transformer assembly.
The copper disk electrodes make contact on each side of the seam and temperature is raised to the welding point. Outside flash is removed by a cutting tool as the pipe or tubes leaves the electrodes, inside flash is removed either by an air hammer or by passing a mandrel through the welded tube after the tube has been cooled. This is termed as Electric Resistance welded or ERW pipe or tubes. If this ERW is being drawn further to get desired size of tubes or pipes, in cold condition is called as Cold Drawn welded or CDW.
This process is used for pipes or tubes from 24″ to 36″ i.e 610mm to 914mm OD. Flat plate is first pressed into U and later O shape. The O shape is placed in an automatic welder and backed up on the inside by a water cooled copper shoe. Two electrodes in close proximity and used. The electrodes are not in actual contact with the pipes or tubes. The current passes from on electrode through a granular flux and across the gap in the pipes or tubes to the second electrode. The high temperature of the arc heats the edges of the plate, a welding rod placed just over the seam is thereby melted and metal is deposited in the groove. After the outside weld has been made, the pipe is conveyed to an inside welder where a similar operation is carried on, except that no backup shoe is need.
Edit by Ms Enya Wang of KRM/BSW
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