Welding residual stresses in tubular steel joints and their behavior under multiaxial loading

The lack of insight into the initial welding residual stress field and its behavior during the different phases of the fatigue damage namely crack initiation and propagation has led to conservative assumptions in fatigue design codes of welds. Since a significant amount of fatigue failures in welded joints are caused by torsion or combined tension-torsion in machinery components, estimating the potential threat of the inevitable residual stresses to structural integrity seems to be mandatory for the design of the future tubular lightweight welded components and structures. In this paper the development of residual stresses in different TIG-welded tubular specimens out of S355J2H and S690QL steel is studied and compared. The mechanisms of the development of residual stresses based on heat input and cooling rate are discussed. The welding parameters and thus heat inputs are varied and the mechanisms leading to different residual stress states are investigated. X-ray and neutron diffraction methods have been used for residual stress state characterization. Welding residual stresses as high as almost the yield strength of the base metal were observed in the weld bead. At the weld toes which are potentially the fatigue crack initiation sites, however lower tensile residual stresses or even compressive residual stresses were present. It will be also discussed here how the welding residual stresses behave under pure torsion and tension-torsion loading.