High-Frequency Resistance Welding (HFRW) is a category of high-speed, solid-state forge welding processes by which thousands of miles of longitudinal seams in steel tubular products are produced for pressure piping and structural applications. Attesting to its high reliability as a joining process are the 2.5 million miles of in-service transmission pipelines constructed from what is known in the oil and gas industry as electric resistance welded (ERW) line pipe. Although its thermo-mechanically processed weld bondline historically has received relatively little attention in the welding research literature, HF welds have at times been the object of misdirected criticism by those outside the field.
From a welding metallurgy perspective, the aim of this work was to clarify the nature of the bondline as a metallurgical heterogeneity. Physical simulations on reheated solid bars were used to isolate the properties of sound HF bondlines from the geometric discontinuities that occur in welds made with suboptimal process parameters, which in most cases are associated with nonmetallic inclusions or incomplete fusion. Low-carbon microalloyed, high-carbon, and austenitic and duplex stainless steels were included in the study.
In the steels examined, it was found that the microstructure of the bond line retained the original single-phase microstructure . Depending on cooling rate and steel composition, ferritic, austenitic and martensitic bondlines can be produced in the Gleeble, similarly to those that form in real HF welds. The width of the thermo-mechanically processed zone was found to control the mechanical and corrosion behavior of the weld. Local grain coarsening being limited by dynamic recrystallization was deemed less important, together with crystallographic orientation effects.
It was concluded that the reduced toughness and somewhat lower corrosion resistance in the weld area can be rectified by in-line post-weld heat treatments (seam anneal). Because weld defects in adjacent regions of sound bondlines can initiate HF weld failures, more effort should be directed toward eliminating weld discontinuities and improving NDE techniques.
The presentation should be useful to welding engineers not familiar with High Frequency Resistance Welding, and to designers who need to know that sound longitudinal weld seams in tubular products can be expected to be fit for service.