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Effect of Welding Parameters on PE Pipe Joint Integrity

TWI Core Research Project  1141/2020

Overview

There are a number of different BF welding procedures for PE100 pipes used around the world and the welding parameters used can be significantly different, especially the welding pressures.  This work was carried out to understand the factors that influence the integrity of BF joints and therefore determine welding procedures that produce joints with better integrity.

Objectives

  • Compare the short- and long-term mechanical properties and weld microstructure of BF joints in PE100 pipes made according to different national and international welding procedures
  • Determine the effect of the size, geometry and structure of the weld and weld beads on the mechanical properties of the joint
  • Ascertain the most appropriate method of qualifying BF welding procedures
  • Determine which of the current, standard BF welding procedures produces welds with the highest mechanical integrity

Results

Even though two of the welding procedures used had welding parameters significantly outside those recommended in any standards, of the short-term

tests used, neither the guided side bend test (ASTM F3183) nor the tensile impact test (ASTM F2634) generated failures at the weld.  The waisted tensile test (EN 12814-7), however, could discriminate between the six different welding procedures investigated.

FEA modelling using macrographs of sections through the weld beads suggested that the maximum stress concentration was always at the notch between the 

outer weld bead and the outer pipe surface.  This was confirmed by the WPTCR tests, which showed that the failures always initiated from the notch between the weld bead and the pipe surface.  These tests also suggested that welds made using a low welding pressure (0.15MPa) had longer times to failure than those made using higher pressures (>0.5MPa).

The results of the NI measurements revealed not only the boundary of the melt zone, which correlated well with the melt zone thickness measurements from transmission light microscopy (TLM) and DSC, but also the heat affected zone (HAZ) boundary, which was not visible using TLM (Figure 1).

The amount of melt generated during the heating stage of the welding cycle for each welding procedure, as predicted using an FEA thermal model, correlated reasonably well with the size of the weld beads measured from macrographs of the weld, added to the size of the melt zone measured from TLM.

 

Conclusions

The results showed that, for the welding procedures studied, there is no correlation between the short- and long-term performance of BF joints in PE100 pipes.  However, taking into account both the long- and short-term performance, joints made to WIS 4-32-08 appear to have the highest mechanical integrity.

Of the short-term tests investigated, the waisted tensile test was the most discriminating and should, therefore, be used for qualifying BF welding procedures.

WPTCR results suggested that the long-term performance of BF joints in PE100 pipes is dependent on the geometry of weld and weld bead or the molecular orientation in the region of the notch between the weld bead and pipe surface, rather than the intrinsic resistance to slow crack growth of the weld interface.

FEA modelling of the shape and dimensions of the weld beads identified the weld bead root length as having the greatest effect on the Von Mises stress at the notch between the weld bead and the pipe surface.

A new weld zone geometry is proposed, which consists of the melt zone bounded by the HAZ, identifiable using NI and DSC techniques (Figure 2).

Figure 1. Nanoindentation graphs of hardness and elastic modulus overlaid with TLM image of a section through the BF joint
Figure 1. Nanoindentation graphs of hardness and elastic modulus overlaid with TLM image of a section through the BF joint
Figure 2. Proposed melt zone and HAZ in a PE100 pipe BF weld
Figure 2. Proposed melt zone and HAZ in a PE100 pipe BF weld
Avatar Mike Troughton Technology Fellow

Mike has been carrying out research for the plastics industry for over 30 years, and at TWI, he is responsible for co-ordinating all R&D, consultancy and training activities in the area of plastics.  Mike’s main areas of expertise include the welding, inspection and mechanical testing of polyethylene (PE) pipes, on which he has written over 30 technical papers, and he is also the editor of the Handbook of Plastics Joining – A Practical Guide.  Mike has managed over 150 research and consultancy projects for clients around the world, he is Chairman of the British Standards Committee on plastics welding, and is also a member of various ISO, CEN, ASTM, IIW, AWS and ASME committees on the welding of plastics and plastics pipes.

 

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