A study of the micro-mechanical properties of butt fusion-welded joints in HDPE pipes using the nanoindentation technique

High-density polyethylene (HDPE) pipes are being used increasingly in many traditional industries as an alternative to metallic pipes. The weakest part of every pipe system is the welded joint. A number of standards are available that define the welding procedure for butt fusion welding of HDPE pipes. However, they define very different values for welding pressure and temperature, for the exact same materials. HDPE PE100 180 mm SDR11 pipes were welded using various standard and non-standard welding procedures. The ranges of welding parameters chosen were expected to produce detectable changes in the size of the weld regions and their micro-mechanical properties. The micro-mechanical properties across butt fusion welds were characterised using the nanoindentation technique. The indentations were spaced at 100 μm at the mid-wall position of the welded joint in a grid of 60 columns by 3 rows. The melt zone (MZ) showed a distinct drop in material properties consistently across all welding procedures. Nanoindentation also revealed the presence of a heat-affected zone (HAZ) surrounding the MZ, characterised by a gradual increase in material properties from that of the parent material to the boundary with the MZ. The size of the MZ measured using nanoindentation was correlated with transmission light microscopy images of microtomed slices.