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  Title Longitudinal weld characteristics of HFW linepipe and its fracture performance
  Author(s) Dr Satoshi Igi, Takatoshi Okabe, Teruki Sadasue, and Satoru Yabumoto  
  Abstract A WELDING TECHNOLOGY FOR ERW pipe was developed in the 1970s by using high- frequency current welding (HFW) instead of low-frequency current welding. However, some pipeline engineers still set a limit on the application of HFW linepipe for pipelines which were to be used in harsh environments based on doubts concerning weld seam integrity. Thus, the development of an HFW weld seam with quality equal to that of the pipe body was a problem for many years.

A non-heat treated HFW linepipe with high quality seam weld toughness was developed by applying quench and temper PWHT. Using a combination of chemical composition, microstructure and oxide content optimization, the seam welds of X60-X80 grade linepipes showed extra low temperature toughness with Charpy transition temperatures much lower than -45? [1-4].

A series of large- and small-scale tests were conducted to verify the performance of the developed HFW linepipe with a high- quality weld seam [15-16]. A full gas burst test at a low temperature below -40ºC was performed using the developed HFW linepipe to investigate crack arrestability in HFW seam welds. The full gas burst test is conducted using a girth welded pipe specimen with the seam weld rotated 90º in order to introduce an initial notch in the base metal and the seam weld centre by an explosive cutter simultaneously. The purpose of the seam notch test is to verify the crack arrest property of the developed HFW linepipe with excellent low-temperature toughness as a result of control of the morphology and distribution of oxides generated in the welding process by temperature and deformation distribution control.

A ductile crack was initiated from the initial explosive notch tip and arrested after ductile crack propagation of about 1 m in the base material. On the other hand, a brittle crack initiated and propagated along the seam weld due to unexpected local overcooling to -130ºC at the initial notch tip. However, this brittle crack transitioned to a ductile crack after branching into the pipe body at approximately the -35ºC region.

 
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