Investigating the impact of roller position on pressure tube-end fitting rolled joint performance using 3D explicit finite element analysis

Abstract

Pressurized Heavy Water Reactors (PHWRs) utilize Zr-2.5 Nb pressure tubes that are connected to SS-403 end fittings through rolled joints. These rolled joints are made using a rolling tool equipped with five rollers and a tapered mandrel. The action of the rollers induces plastic deformation on the inner surface of the tube. The tapered mandrel promotes rotation and axial advancement of the rollers into the pressure tube through friction. However, prolonged usage of this rolling tool can lead to the wearing down of both the mandrel and the rollers. Consequently, this wear and tear can cause the rollers to deviate from their intended position within the rolled joint. Such misalignment can lead to excessive rolling of the pressure tube into the entry taper section of the end fitting. This, in turn, generates higher residual stresses than initially anticipated. This issue may also arise from accumulated tolerances within the rolling tool or positioning errors made by operators during the rolling operation. The overextension of the rollers can significantly increase tensile residual stresses within the pressure tube, and management of these stresses is critical to prevent failure of the pressure tube due to Delayed Hydrogen Cracking.The primary objective of the present investigation is to explore the effects of the overextension of rollers on the residual tensile stresses in the transition region of the rolled joint. This objective is achieved by employing a three-dimensional explicit finite element analysis methodology. In order to ensure the reliability and authenticity of the finite element model, the computational results are cross-validated with the experimental findings documented in the existing literature. The current investigation involves the examination of two distinct configurations of pressure tube to end fitting rolled joints, which are typically found in PHWRs. The results of the present inquiry reveal an optimal roller position that minimizes residual tensile stresses within the transition region of a rolled joint. Furthermore, it is observed that any deviations in the roller position within a range of 5 mm, beyond the optimal position, result in a negligible increase in residual tensile stress.