Cold-Welding and Over-Welding in Polyethylene Pipe Butt Fusion Joints: Formation Mechanisms, Structure–Property Degradation, and Emerging Microwave-Based Detection Strategies

Polyethylene (PE) pipes are extensively used in gas- and water-distribution systems because of their corrosion resistance, toughness, and installation flexibility. However, the field reliability of PE pipeline systems often depends on the quality of butt fusion joints, whose integrity is highly sensitive to welding parameters and thermal history. Among process-induced defects, cold welding and over-welding are particularly critical because they are usually concealed, difficult to identify by visual inspection or short-term pressure testing, and strongly associated with long-term brittle failure. Existing reviews have discussed butt fusion joining or polymer non-destructive testing in a broad sense, yet a defect-oriented synthesis linking process deviation, interfacial evolution, material degradation, and microwave response remains limited. This review therefore focuses specifically on cold welding and over-welding as the two most safety-critical hidden defects in PE butt fusion joints. First, the paper summarises how insufficient heat input, poor wetting, and limited molecular-chain interdiffusion lead to weak interfacial healing and low effective entanglement density, whereas excessive heat input and unfavourable thermal history promote chain scission, thermo-oxidative degradation, abnormal recrystallisation, and microvoid formation. It then compares how these mechanisms influence tensile behaviour, slow crack growth resistance, and long-term service performance. On this basis, conventional radiography and ultrasonics, phased-array ultrasonics, nonlinear acoustics, and microwave non-destructive testing (MNDT) are critically compared. Particular emphasis is placed on the structure–property–signal pathway that links local perturbations in complex permittivity to changes in microwave scattering parameters, including |S11|, |S21|, phase drift, resonance shift, and quality-factor reduction. Finally, a hierarchical evaluation framework is proposed to connect microwave features with defect severity and engineering decisions, and the major barriers to wider engineering adoption—benchmark datasets, quantitative inversion, lift-off robustness, and standardisation—are discussed. By organising the field around a mechanism-based structure–property–signal framework, this review aims to provide a more rigorous basis for defect-oriented quality control and future intelligent inspection of PE butt fusion joints.