The purpose of this Masters thesis was to characterize the morphology and its impact on short- and long-term behaviour of high density Polyethylene (PE-HD) butt-welds. Butt-welds of pipes with diameters of 160 mm and 2000 mm were produced with varying welding parameters, such as pressure mode and heat soak time. In order to determine the morphology of welds and pipes, orientation measurements via small angle X-ray scattering (SAXS) and infrared spectroscopy (IR) were performed. Furthermore, the cristallinty of the different welds was evaluated with the method of differential scanning calorimetry (DSC). Based on the examination of morphology mechanical and fracture-mechanical testing was performed in order to verify the aforementioned impact of the morphology on material behaviour. To determine short term behaviour and material parameters such as Young’s Modulus (E-Modul) and elongation at break etc., tensile testing was performed. Further on impact testing was performed to identify material resistance against rapid fracture. In contrast, long term material behaviour (slow crack growth) was determined with methods of linear elastic fracture mechanics (LEBM). To provide a ranking of used welding parameters cyclic tests with cracked round bar specimens (CRB) were performed. With these tests specimens were tested at the same stress-levels and ranked according to the cycles until fracture occurred. The results confirm a change in morphology due to welding parameters. Both mode of pressure and heat soaking time during welding influence the cristallinity and mechanical material properties, although the profile of pressure during welding shows a more significant impact. Results also suggest that welding with “Dual-pressure mode”, where the welding pressure is reduced to 1/6 of its initial level after ten seconds, improves long term behaviour of welds in comparison to “Single-pressure mode”, where the welding pressure remains the same for the whole process. The findings were also most informative, seeing that material behaviour inside the welding zone behaves vastly different from expectations due to experience with fractures of pipe welds in the field, where fracture usually occurs in the bulk-material after crack initiation at welding beads. This abnormality is explained by the fact that fractures of pipe welds usually initiate at cracks, induced by welding-beads, but actually occur in the bulk material, not inside the welded zone as it was observed during testing. The results of the tests on large diameter pipes suggest that the morphology of large diameter pipes still needs investigation and research in terms of long term behaviour with respect to the non-uniform morphology due to larger wall-thickness.