What structural hazards will be caused by excessive welding temperature during PE pipe hot-melt butt welding?
Release Time : 2025-10-23
During the hot-melt butt welding of PE pipe, exceeding the specified welding temperature can lead to multiple structural hazards. These hazards not only affect the short-term quality of the pipe connection but also pose a long-term threat to the safety and stability of the pipe network. The core issue caused by excessive temperatures is the irreversible destruction of the PE material's molecular structure, which is the root cause of all subsequent hazards.
When the welding temperature exceeds the tolerance of the PE pipe material, the material's molecular chains undergo excessive breakage. Under normal circumstances, hot-melt butt welding uses localized heating to melt the pipe ends, creating molecular entanglement and bonding under pressure. However, excessively high temperatures can cause the molecular chains to break faster than they recombine, resulting in a large number of short-chain molecules. These short-chain molecules have significantly weaker bonding than long-chain molecules, significantly reducing mechanical properties such as tensile strength and impact toughness in the weld area. In actual engineering, this material degradation often manifests as crack propagation in the weld, especially when the pipe is subjected to internal pressure or temperature fluctuations, when cracks can rapidly propagate along the direction of molecular chain breakage.
Excessive temperatures can also cause thermal oxidative degradation of the PE pipe material. Under high temperatures, unstable groups in the PE molecular chain react with oxygen to form oxygen-containing functional groups such as carbonyl and hydroxyl groups. The presence of these functional groups disrupts the regularity of the molecular chain and creates localized stress concentration points. Furthermore, low-molecular-weight products produced by oxidative degradation can form bubbles in the weld, creating microscopic pores. These pores not only weaken the weld's tightness but also serve as penetration pathways for corrosive media, accelerating chemical corrosion of the pipe's inner wall.
From a rheological perspective, excessive temperatures can cause a sharp drop in the viscosity of PE pipe melts. Ideal hot-melt butt welding requires a suitable melt viscosity to ensure sufficient melting of the end faces while maintaining a certain degree of dimensional stability. However, excessively high temperatures lead to excessive melt fluidity, which can easily cause excessive extrusion under pressure, resulting in a "mushroom-shaped" weld. This abnormal weld shape leads to uneven stress distribution. During pipeline operation, stress concentration at the weld root can lead to fatigue damage.
Excessive temperatures can also negatively impact PE pipe welding quality by altering crystallization behavior. PE material forms a spherulite structure during the cooling process. Appropriate welding temperatures ensure a uniform distribution of spherulite sizes. However, excessively high temperatures can lead to the formation of overly large spherulites during cooling. This coarse spherulite structure reduces the material's toughness and crack resistance. Actual testing has found that the elongation at break of PE pipe welds welded at temperatures exceeding the specified temperature is lower than that of standard welds, indicating a significant decrease in material toughness.
Improper temperature control can also lead to interfacial contamination during PE pipe welding. Under high temperatures, low-molecular-weight substances on the surface of PE material precipitate, forming an adhesion layer. When the two pipe ends are butt-jointed, this adhesion layer hinders direct molecular contact, resulting in a weak bonding layer at the weld interface. This interfacial contamination manifests macroscopically as uneven weld flanges and microscopically as reduced molecular chain entanglement density, seriously impacting the weld's long-term sealing performance.
Excessive temperatures can pose hidden risks to PE pipe welding quality. During initial inspection, the weld may appear to meet standard requirements, but over time, material degradation and oxidative degradation will gradually become apparent. This lag causes many pipeline systems to leak only after years of operation, which can be costly to repair and potentially lead to secondary disasters. Therefore, strictly controlling the PE pipe hot-melt butt welding temperature is crucial not only to ensuring the quality of the weld but also to maintaining the long-term safe operation of the entire pipeline system.