ERW - 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 a tube 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 tube leaves the electrodes, inside flash is removed either by an air hammer or by passing a mandnel through the welded tube after the tube has been cooled. This is termed as Electric Resistance welded or ERW tube/pipe. 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.

EFW - Flat Plate known as Skelp, is prepared in proper width and thickness for the desired pipe inside and outside diameters. It is then charged into an electric furnace and when the proper welding temperature has been reached is drawn through a funnel-shaped die so shaped that the plate is gradually formed into the shape of a tube, with the edges of the plate being forced squarely together and fused. The formed pipe then passes through a series of rolls in which it is sized or drawn to final dimensions. The pipes obtained through this process is called ELECTRIC FUSION WELDED TUBE (EFW). 

SAW - This process is used for pipes 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 pipe. The current passes from on electrode through a granular flux and across the gap in the pipe 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. 

CARBON STEEL - Most frequently used as manufactured in area with ASTM specifications A-106 & A-S3. The chemical compositions of these two materials is identical, both are subjected to physical test, but those for A-106 are more rigorous. A-106 are made in grade A & B. Grade B has higher strength properties but is less ductile and for this reason Grade A is permitted only for cold or close coiling. Wehn carbon steel is intended for use in welded construction at temperatures in excess of 427oC and its use in welded construction above this value should be with caution.

CARBON MOLLY STEEL - This is one type of alloy steel as per A-204 (EFW), A-335(Seamless, A-309) etc. was developed in past years when steam temp. were approaching but not reaching 538oC under which conditions carbon steel was both unsatifactory and economical. It has been found that there is a tendency for carbon molybdenum to show graphitization at temperatures in excess of 427oC. It is used in welded construction.

Page II