Residual Stresses in Cylinder Girth Butt Welds

Abstract

ABSTRACT The build-up of stress in the multipass girth butt welding of cylinders has been analysed theoretically. The computer model has been validated experimentally by measurement of residual stress distributions through the cylinder wall thickness. Trends have been identified as weld pass heat input is varied. Empirical formulae are proposed for axial residual stress at the weld root section. INTRODUCTION The proper assessment of defects in a welded joint requires knowledge of the stress fields due both to fabrication and to live loading. In default of more precise data, it is usually assumed that residual stresses of tensile yield magnitude exist throughout the weld region. It may be argued qualitatively, however, that in multipass welds the shrinkage of later passes tends to superimpose compression on the tensile stress field induced by the earlier passes. This project sought to establish how far the assumption of general tensile yield could be relaxed in the specific case of cylinder girth butt welds. The investigation was both theoretical and experimental. A finite difference computer model was formulated to analyze the build-up of stress in multitask girth butt welds. The model was validated by comparing computed residual stress distributions with experimental results from three girth butt welded, structural steel cylinders (1 m diam. × 10 to 20 mm thickness). Through-thickness stress distributions were determined experimentally by sectioning; surface stresses were measured independently by the centre hole method. The experimental results were mutually consistent and there was good agreement between theory and experiment. The theoretical model was therefore used to explore levels of residual stress in similar cylinders outside the range of the experimental programme. Underlying trends were identified and empirical formulae are here presented for estimating local values of axial residual stress at the weld root. THEORETICAL MODEL - AN OUTLINE Determining welding residual stress is an inelastic thermal stress problem for which a rigorous treatment is computationally severe. The following simplifying assumptions have therefore been made in the present study of cylinder girth butt welds:that there is rotational symmetry and symmetry about the weld centre-line;that the material is homogeneous, isotropic, elastic/perfectly plastic and initially stress free;that the material obeys the von Mises yield criterion and the Prandtl-Reuss flow rule;that the material yield stress is temperature dependent but that other 'properties are temperature independent.that the Bauschinger effect, the influence of strain rate on yield, and creep effects are all negligible. The theoretical model consists of two parts: a thermal conduction model to predict transient temperatures, and a non-linear structural model to predict the stress-strain effects of the thermal loading. The governing differential equations at discrete points over the cylinder cross-section are solved numerically using the finite difference method. The parameters considered are shown in Fig. I.