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

Abstract

SS316L finned tubes are becoming very popular in high-pressure gas exchangers and particularly in CO2 cooler applications. Due to the high-pressure requirement during operation, these tubes require an accurate residual stress evaluation during the die expansion process. . Die expansion of gas coolers finned tubes creates not only high stresses that can surpass the UTS when combined with operation stresses but also micro-cracks during expansion when the process is not very well controlled. This research work aims to study the elastics-plastic behavior and estimate the residual stress state of tubes subjected to the die expansion process. The stresses and deformations of the expanded SS316L tube are analyzed numerically using the finite element method. The expansion and contraction process is modeled considering 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 stainless steel subjected to expansion with an oval shape die.