Analysis of Stresses and Strains in Stainless Steel 316 L Tubes Subjected to Die Expansion

SS316 L finned tubes are becoming very popular in high-pressure heat exchangers and particularly in CO₂ cooler applications. Due to the high pressures present during operation, these tubes require an accurate residual stress evaluation generated by the die expansion process. Die expansion of air cooler fin tubes creates not only high stresses that can surpass the ultimate tensile strength when combined with operation stresses but microcracks during expansion when the process is not well controlled. This research work aims to study the elastic-plastic behavior and estimate the residual stress state of fin tubes subjected to the die expansion process. The stresses and deformations of the expanded SS316 L tube are analyzed numerically using the finite element method. The expansion and contraction process are modeled considering the elastic–plastic material behavior for different die sizes. The maximum longitudinal, tangential, and contact stresses are evaluated to verify the critical stress state of the joint during the expansion process. The importance of the material behavior in evaluating the residual stresses using kinematic and isotropic hardening is addressed. Finally, an experiment was conducted to assess the tangential and longitudinal strains of a 3/8 in. stainless steel tube subjected to expansion with an oval-shaped die.