Introduction

Independent reporting by those working among the high frequency (HF) electric-resistance welded (ERW) Equipment Producers [e.g., 1] makes it clear that there is little conceptual difference between the modern HF processes and the vintage low frequency (LF) processes. Reporting by the research community [e.g., 2] makes it equally clear that failures like those seen historically in LF seams continue to occur in modern HF seamed pipes. Reality in this context is that the fundamental differences between the LF and HF processes involve the method of heating, and the supporting quality practices, and its management.

The process details illustrated and discussed in the QR Segment titled Compare and Contrast LF and HF Processes from a Patent Perspective indicate that the positioning and alignment of the abutting edges as well the method of heating are process specific, and can be producer specific. While these LF and HF processes differ in various aspects, the early patents [e.g.3,4] as well as the modern processes [e.g.,5-10] both indicate the need to ‘tune’ their setup and process to the thickness and diameter of the pipe being produced. While these mechanical aspects have been known and well understood for decades, they can in some cases be as responsible for failures as are the skelp and bondline quality.

High Frequency versus Low Frequency ERW Seams, and Implications

As discussed above, a major change in the ERW process came with the shift to HF ERW and the use of sliding contact (HFC) in lieu of rolling contact. While many consider this transition to be completed at least for US pipe production by the early 1970s, it is likely that older LF mills producing pipe around the world might make this transition later in time. As is normal for most major process changes, this process transition brought with it some growing pains – much as can be anticipated for new and/or modernizing steel and pipe producers. With this transition some HF ERW production experienced multiple pre-service hydrotest failures, with similar issues also apparent with the use of the HF induction (HFI) process[1].

While changes affected via the HF ERW / HFI processes can limit the frequency and extent of bondline defects, they do not ensure a quality seam unless clean quality skelp is used, to avoid the hook-crack and other related concerns that occurred in the LF ERW / FW seams. It follows that HF ERW / HFI seams have been prone to many of the same issues that occurred for LFERW, particularly where dirty steel opens to hook cracks and selective-seam-weld corrosion (SSWC). Several papers have identified and discussed the defect types that can occur in seams made using HFI / HF ERW processes [e.g.,1,11], including susceptibility to selective attack in the seam, which in such cases tends to be termed grooving corrosion [12,13]. Regarding weld-process defects, one paper by an welding equipment producer [1] discusses the “most common defects” and lists nine in total for just the bondline, whereas as noted above process defects also can occur in the upset / HAZ, as well as via grooving corrosion in the bondline depending on the steel used. To be fair, many of the nine seam defects reflect the same concerns as noted for LF ERW in regard to cold welds, stitched welds, and penetrators. In fairness it is noted that much has been done in the context of detailed research into the HFI / HF ERW seam process to understand the causes of such defects [e.g.,14,15], and to modify them to limit the formation of defects in production.

Regardless of the improvements affected by the use of high (lower range AM radio / KHz) frequencies and the manner it is introduced into the pipe, there is always a chance for process upsets to cause defects. Thus, avoiding issues in-service is dependent on quality control (QC) and quality assurance (QA), and the use of appropriate pre-service testing in the mill, and then again post-construction. But even with such controls, failures have continued, albeit at reduced rates. In addition to the occurrence of cold welds, hook cracks, and SSWC (or grooving corrosion), there are some defect types that are appear unique to the high frequency process [1].

Summary

Good steel and a good seam are the essence of good pipe – so it takes QC and QA in the steel mill in order to ensure a good seam results from the same QC/QA in pipe-making to produce PSL2 line pipe. Keys to success in using ERW seamed pipe in new construction include a pre-service hydrotest designed to expose M&C threats that might become an issue in service, whereas for existing construction a study of prior failures on the subject pipeline or some sister construction can be useful in informing operations and maintenance decisions. Reality regarding the HF and LF seams is that they are conceptually similar in many ways, whereas they differ to the extent that the HF process has capitalized on modern QA/QC technologies. Differences are also evident in the combined effects of the heat input and the mechanical upset, and the thermal cycle of its post-weld heat treatment (PWHT). Differences in seam quality when the pipe leaves the mill develop due to process control issues in heat input and upset force, subject to other preparatory factors like alignment and edge trim and/or post-weld issues during expansion, mill testing, and other mill functions. Among the features that are or might continue to become defects in HF PSL2 pipe are: cold welds, paste welds, hook cracks, weld-area cracks, improper PWHT, inclusions, penetrators/pin-holes, offset edges, and excessive/inadequate trim. The LF vs HF Processes from a Patent Perspective QR segment presents a broader view of the historic versus modern ERW processes, whereas References 16 to 20 inform concerning the process and References 2, 21 and 22 include detailed discussion of the defects.

References

  1. Nichols, R. K., “Common HF Welding Defects,” Thermatool Corp., (www.thermatool.com/…/common-hf-welding-defects.pdf), undated
  2. Leis, B. N., “Time-Trending and Like-Similar Analysis for ERW-Seam Failures,” Battelle Interim Report, Subtask 4.2, US DoT Contract No. DTPH56-11-T-000003, June, 2014.
  3. Johnston, G. V., “Method And Apparatus For Butt Welding Thin Gage Tubing,” US Patent 1,388,434, 23 August, 1921.
  4. Johnston, G. V., (assigned to Elyria Iron & Steel Co) “Butt Welded Thin Walled Tubing,” US Patent 1,435,306, 14 November, 1922.
  5. Scott, P., “High Frequency Welding of Low Carbon Steel Tube,” Thermatool Corp Paper, undated, www.thermatool.com/…/welding/ High Frequency Welding o….
  6. Haga, H., Aoki, K., and Sato, T., “Welding Phenomena and Welding Mechanisms in High Frequency Electric Resistance Welding – 1st Report,” presented at the AWS 60th Annual Meeting Detroit, 2-6 April 1979, also in Welding Research Supplement, July 1979, pp 208s – 212s.
  7. Kim, D., Kim, T., Park, Y. W., Sung, K., Kang, M., Kim , C., Lee, C. and Rhee, S., “Estimation of Weld Quality in High-Frequency Electric Resistance Welding, with Image Processing,” Welding Journal, Research Section, March 2007, pp 71-S to 79-S.
  8. Koch, F. O. and Peters, P. A., “Distinguishing Characteristics of High-Frequency Induction-Welded Pipe,” presented at API Standardization Conference, Pipe Symposium, New Orleans, 23 June 1986.
  9. Baralla, E. and Tommasi, C., “Integrated System for Process Control of High Frequency Electric Resistance Welded Steel Pipe,” Proceedings PAN NDT Meeting, Rio de Janeiro, 2003
  10. Anon., “Complete Inspection Solutions for ERW Tubes (including Large Diameter Pipe),” Olympus Document, see http://www.olympus-ims.com/en/erw/, accessed July 2011.
  11. Wright, J., “Optimizing Efficiency in HF Tube Welding Processes,” Tube and Pipe Technology, November/December, 1999 (www.eheimpeders.com/uploads/TB1011.pdf)
  12. Lukezich, S. J., “Susceptibility of Modern ERW Pipe to Selective Weld Seam Corrosion in Wet Environments,” Project PR-15-9306, Cat #L51775, 1998
  13. Joosten, M. W., Kolts, J., Kiefer, J., Humble, P. G., Marlow, J. A., “Aspects of Selective Weld and HAZ Attack in CO2 Containing Production Environments,” Corrosion 96, Paper 79, 1996: see also Duran, C., Treiss, E., and Herbsleb, G., “The Resistance of High Frequency Inductive Welded Pipe to Grooving Corrosion in Salt Water,” Materials Performance, p. 41, September 1986.
  14. Kim, D., Kim, T., Park, Y. W., Sung, K., Kang, M., Kim , C., Lee, C. and Rhee, S., “Estimation of Weld Quality in High-Frequency Electric Resistance Welding, with Image Processing,” Welding Journal, Research Section, March 2007, pp 71-S to 79-S.
  15. Choi, J.-H., Chang, Y. S., Kim, C.-M., Oh, J.-S., and Kim, Y.-S., “Penetrator Formation Mechanisms during High-Frequency Electric Resistance Welding,” Welding Journal, Research Section, January 2004, pp 27-S to 31-S.
  16. Weimer, G., and Cagganello, R., “Electric Resistance Welding at a Glance: Process, Power Supply, and Weld-Roll Basics,” 13 June 2002 see www.thefabricator.com for this and others on resistance welding, on-line access in July 2011.
  17. Scott, P., “Choosing the Right HF Welding Process for API Large Pipe Mills,” Thermatool Corp Paper, 29 November 2005.
  18. Scott, P., “The Effects of Frequency in High Frequency Welding,” Thermatool Corp Paper, 1996.
  19. Pierson, J. and DiDonato, M., “High-Frequency Electric Resistance Welding: An Overview” June 1, 2010, see www.thefabricator.com for this and others on resistance welding, on-line access in July 2011.
  20. Warren, L., “Skelp Edge Preparation for Manufacturing ERW Pipe,” in thefabricator.com, dated May 30, 2001.
  21. Leis, B. N. and Nestleroth, J. B., “Battelle’s Experience with ERW and Flash Weld Seam Failures: Causes and Implications,” Battelle Interim Report, Subtask 1.4, US DoT Contract No. DTPH56-11-T-000003, September, 2013.
  22. Kiefner, J. F. and Kolovitch, K. M., “ERW and Flash Weld Seam Failures,” KAI Interim Report on Task 1.4, , US DoT Contract No. DTPH56-11-T-000003, September, 2012

  1. This trend is evident on metallurgical studies that were done in follow-up to pre-service hydrotesting, which would not be apparent in incident reporting. Such work pointed to microstructural issues due to source steel, as well as to process upsets. Such trends are more specific to the supplier than they are to the production era.