Electric resistance welding (ERW) refers to a group of welding process in which parts to be welded are first heated to the fusion temperature by the resistance of work piece to the flow of electric current and then squeeze by mechanical pressure to accomplish weld. No filler metal or flux is added in this process.
Principal of Resistance Welding.
The principal involved in resistance welding is that when an electric current is passed through the junction, a high temperature is obtained by the resistance of the metal to the passage of the current. The contact surfaces are not in perfect contact and offer the highest resistance and are thus heated up first.
Low-frequency electric resistance welding, LF-ERW, is an obsolete method of welding seams in oil and gas pipelines. It was phased out in the 1970s but as of 2015 some pipelines built with this method remained in service.
Electric resistance welded (ERW) pipe is manufactured by cold-forming a sheet of steel into a cylindrical shape. Current is then passed between the two edges of the steel to heat the steel to a point at which the edges are forced together to form a bond without the use of welding filler material. Initially this manufacturing process used low frequency A.C. current to heat the edges. This low frequency process was used from the 1920s until 1970. In 1970, the low frequency process was superseded by a high frequency ERW process which produced a higher quality weld.
Over time, the welds of low frequency ERW pipe was found to be susceptible to selective seam corrosion, hook cracks, and inadequate bonding of the seams, so low frequency ERW is no longer used to manufacture pipe. The high frequency process is still being used to manufacture pipe for use in new pipeline construction.