Low-frequency electric resistance welding (LF - ERW), is an obsolete method that was used to weld 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 pipe (ERW steel 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. 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. 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 In 1970, the low frequency process was superseded by a high frequency ERW process which produced a higher quality weld. So the high frequency process is still being used to manufacture pipe for use in new pipeline construction. In the production of ERW steel pipe manufactured by Prime Steel Pipe, high-frequency ERW process is widely used, so that we can guarantee the quality of welding seam and produce high quality ERW steel pipe.