The issue of the superposition of residual stress and thermal stress in heat exchange tubes

Discussion: 1 Is it necessary to introduce the concept of stress region location? For example, the residual stresses that arise during the manufacturing process are mainly concentrated in the bent pipe areas, expanded pipe areas, and welding areas ； As for the stress areas resulting from temperature differences during operation, whether they completely overlap with these areas is important; if they do overlap and the stress vectors are in opposite directions, the stresses can partially cancel each other out, while if they are in the same direction, the stresses add up. From a microscopic analysis perspective, in a expansion+welding structure, the expansion section bears the axial forces exerted on the heat exchange tubes. Therefore, even if axial loads arise due to operational temperature differences, they only affect the expansion section, having little impact on the welds where residual stresses are higher. In other words, residual stress and temperature difference stress do not have a simple uniform additive relationship

Thank you very much for your professional response. I narrowed down the scope; by residual tensile stress in the heat exchange tubes, I mean the stress that remains after straightening the tubes following solution treatment, and it has nothing to do with the welds. Due to differences in thermal expansion, the heat exchanger causes the shell to be under tension, while the tubes are under compression. Can the compressive stress and residual tensile stress in this case be superimposed on each other?

After the heat exchange tubes are dissolved in solution, the residual stresses remaining from straightening are extremely small; and if any such stresses do exist, they will be reduced to some extent during subsequent periods of rest or due to vibrations during transportation! After solution treatment, the stress in normal heat exchange tubes is completely eliminated. Straightening generally involves two steps: pre-straightening and final straightening; unless a particular manufacturer carries out hard straightening in one go, which can result in excessive deformation and the retention of stress! Here are 3 common methods for stress relief: 1. Use thermal aging by placing the workpiece in a thermal aging furnace for heat treatment to gradually eliminate stress. 2. Vibratory stress relief is used to eliminate stresses. This technique is known abroad as “Vibrating Stress Relief” (abbreviated as VSR). It involves using specialized vibratory stress relief equipment to cause resonance in the workpiece being treated; through this resonance, a certain amount of vibrational energy is transmitted to all parts of the workpiece, resulting in microscopic plastic deformation within it – the distorted crystal lattice gradually returning to its balanced state. 3. Natural aging is a method for eliminating residual stresses; it involves exposing the parts outdoors over periods of several months to several years, thereby achieving stability in their dimensional accuracy. Numerous experimental studies and production practices have shown that natural aging has a favorable effect on stabilizing the dimensional accuracy of workpieces. The above are my personal opinions for reference only!

Have you measured the residual stress level in tubes that have been straightened after solution treatment?