Stress concentration is a key factor affecting the lifespan and safety of stainless steel hot water pipes during installation. Its occurrence is largely related to installation techniques, material properties, and environmental conditions. If not effectively controlled, it can lead to pipe deformation, cracking, or even leakage. This is especially true in hot water systems, where thermal expansion and contraction further exacerbate the risk of stress concentration. Therefore, a comprehensive approach is needed, encompassing design, installation, welding, support, and maintenance, to reduce the probability of stress concentration.
Before installation, the pipe design should be optimized to avoid sharp bends or complex routes. Stainless steel hot water pipes expand thermally due to temperature changes. Excessive right-angle bends or tight U-shaped structures in the pipe layout can hinder local deformation and easily lead to stress concentration. The design should utilize natural bends or large-radius bends as much as possible to reduce sharp turns. Sufficient expansion space should be provided, such as by installing expansion joints or flexible compensators in long straight pipe sections, to absorb thermal deformation and prevent stress accumulation at fixed points.
Welding is a high-risk process for stress concentration, requiring strict control of welding parameters and operating procedures. During welding, localized high temperatures can alter the metal's grain structure, generating residual tensile stress upon cooling. Excessive welding current, speed, or poor weld formation can easily lead to defects such as undercut, incomplete penetration, and weld beads, which can become the starting point for stress concentration. Therefore, fine machining of the bevel is necessary before welding to ensure uniform gaps; low current and multi-pass welding should be used during welding to control interpass temperature; post-weld heat treatment or hammering should be performed to relieve stress, ensuring the weld area's mechanical properties are consistent with the base material.
The support and fixing methods directly affect the stress distribution of the pipeline. Fixed supports should be installed at rigid points on the pipeline, such as both sides of elbows or equipment interfaces, avoiding fixed points in the middle of the pipeline, as this can lead to excessive local stress due to impeded thermal expansion. Simultaneously, the spacing of guide supports should be rationally set according to the pipeline diameter and temperature changes to ensure the pipeline can expand and contract freely without lateral displacement; for long straight pipe sections, sliding supports or spring supports can be used to reduce the constraint of fixed points on the pipeline.
Material selection and pretreatment are also crucial for reducing stress concentration. Stainless steel hot water pipes should be made of materials with strong corrosion resistance and good ductility, such as 316L stainless steel. The molybdenum content enhances resistance to chloride ion corrosion, and solution treatment eliminates the tendency for intergranular corrosion generated during processing. Before installation, the pipe surface must be pickled and passivated to remove oil, scale, and other impurities, forming a dense passivation film to prevent stress concentration caused by localized corrosion. Cut or bent pipe ends must be ground to remove burrs and cracks.
Mechanical damage during installation must be strictly avoided. If pipes collide with or are scratched by hard objects during handling, hoisting, or assembly, the surface passivation film will be damaged, forming corrosion micro-cells and leading to stress corrosion cracking. Therefore, a soft padding layer should be laid at the installation site, and nylon slings or special clamps should be used to secure the pipes. During welding, non-welding areas must be covered and protected to prevent weld spatter from damaging the pipes.
Heat treatment and stress relief are necessary steps after installation. For welded joints or cold-formed fittings, residual stress must be released through heat treatment. For example, for stainless steel pipe fittings that do not contain titanium or niobium, tempering treatment can be used. Heating to 300-350℃ and holding for a period of time allows the grains to rearrange, reducing internal stress. For thick-walled pipe fittings, overall stress-relief annealing is required after welding. Heating to 850-870℃ and then slowly cooling eliminates the tensile stress generated during welding.
Regular maintenance and inspection are essential for preventing stress concentration in the long term. During the operation of hot water systems, it is necessary to regularly check for loose pipe supports, cracks in welds, and malfunctions of compensators. For pipes exposed outdoors, insulation measures should be taken to prevent stress concentration caused by sudden temperature changes. Simultaneously, the corrosiveness of the transported medium should be controlled, such as reducing the chloride ion content in the water and adding corrosion inhibitors, to reduce the risk of stress corrosion cracking.
