How to control residual stress and prevent cracking during the welding process of empty pipe joints in iron products?
Release Time : 2026-01-13
During the welding of empty pipe joints in iron products, the rapid localized heating and cooling constrains the material's thermal expansion and contraction characteristics, inevitably generating residual stress. If this stress is not effectively controlled, it can lead to joint cracks, reduced structural strength and sealing performance, and even failure during use. Therefore, residual stress must be comprehensively controlled from multiple dimensions, including welding process design, material selection, operating procedures, and post-treatment, to prevent cracking.
Preheating and slow cooling are fundamental measures for controlling residual stress. Preheating the empty pipe joint, either overall or locally, before welding reduces the temperature difference between the weld and the base material, slows down the cooling rate, and thus reduces thermal stress. The preheating temperature needs to be adjusted according to material properties (such as carbon equivalent and thermal conductivity) to ensure minimal expansion differences between high-temperature and low-temperature zones. After welding, wrapping the joint with insulation or slow cooling in a furnace ensures uniform cooling and avoids stress concentration due to inconsistent shrinkage rates. For example, slow cooling can significantly reduce the risk of cracking when welding thick-walled empty pipe joints at low temperatures.
Optimizing the welding sequence and direction is key to adjusting the distribution of residual stress. Symmetrical welding and segmented back-welding sequences ensure uniform heating of the joint and reduce stress accumulation on one side. For multi-pass welds, welds with larger shrinkage should be welded first, or stress should be balanced through cross-welding. For example, when connecting an empty pipe joint to a flange, welds away from the constrained end first, then gradually advance towards the constrained end, reducing the impact of restraint on residual stress. Furthermore, avoiding stress superposition at weld intersections and reducing overlapping areas by adjusting the welding direction can effectively control cracking tendency.
The appropriate selection of welding parameters directly affects heat input and stress levels. Excessive current and welding speed lead to concentrated heat input, excessively high local temperatures, reduced material yield strength, and increased residual stress after cooling; while insufficient current may cause incomplete fusion or slag inclusions, forming stress concentration sources. Therefore, appropriate current, voltage, and welding speed must be selected based on material thickness and joint type to ensure a moderate weld pool size and controllable heat-affected zone. For example, thin-walled empty pipe joints are best welded with low current and short arc to reduce heat input; thick-walled joints require layered welding, controlling interpass temperature to avoid overheating. The choice of welding method is crucial for residual stress control. Welding methods with concentrated energy (such as laser welding and electron beam welding) can reduce the heat-affected zone and lower residual stress; while traditional arc welding, due to its high heat input, results in higher residual stress. For empty pipe joints, if the material thickness allows, argon arc welding or plasma welding is preferred, as their concentrated heat source and high welding speed can significantly reduce stress. Furthermore, automated welding equipment (such as welding robots) can stably control welding parameters, reducing human error and further improving joint quality.
Post-weld heat treatment is an effective means of eliminating residual stress. Annealing, tempering, or vibration aging can redistribute the internal stress of the material to achieve equilibrium. Annealing requires selecting the temperature and holding time based on material characteristics to ensure full stress release; vibration aging uses mechanical vibration to adjust the material's microstructure, reducing residual stress. For empty pipe joints, if the structure allows, overall post-weld annealing can completely eliminate stress; if size is a constraint, localized heating or vibration aging can be used for targeted treatment.
Material selection and compatibility are equally important for residual stress control. The chemical composition and mechanical properties of the base metal and welding materials must be similar to avoid stress concentration due to differences in thermal expansion coefficients. For example, when welding dissimilar steel empty pipe joints, welding materials that can form a good metallurgical bond with both sides of the material must be selected to reduce stress in the fusion zone. Furthermore, strictly controlling the impurity content (such as sulfur and phosphorus) of the base metal and welding materials can reduce crack susceptibility and improve the joint's crack resistance.
Controlling residual stress during the welding of iron empty pipe joints needs to be maintained throughout the entire welding process. From preheating and slow cooling, optimizing the sequence and parameters, to selecting appropriate methods and materials, and finally to post-weld heat treatment, each step requires precise control. Through systematic process design, residual stress levels can be significantly reduced, crack formation can be avoided, and the reliability and durability of the joint under complex working conditions can be ensured.