Welding Residual Stress Distributions in the Thickness Direction under Constraints Using Neutron Diffraction and Contour Methods

Abstract

Using high-strength steel for offshore structures with a yield stress of 500 MPa, this study evaluated the distribution characteristics of welding residual stress in the thickness direction under the influence of constrained conditions during welding using cutting, neutron diffraction, and contour methods. Welding residual stress inevitably occurs during welding and impacts fracture stability in structures. As high-strength steel better reflects the effects of phase transformation, the behavior of welding residual stress is known to differ from that of general steel. This study fabricated fully constrained and unconstrained specimens and evaluated them under identical conditions to evaluate welding residual stress according to the influence of constraints on high-strength steel welds. The results indicated that the maximum tensile residual stress of the fully constrained specimen occurred in the first layer of the weld joint, while the maximum tensile residual stress of the fully unconstrained specimen occurred in the last layer of the weld joint. Additionally, the welding residual stress of the fully constrained specimen was larger. Although some errors occurred in the residual stress values in the thickness direction depending on the measurement method, both methods applied in this study exhibited nearly identical distributions. Meanwhile, a maximum angular deformation of about 6° occurred in the fully unconstrained specimen, and we considered that the residual stress decreased owing to the occurrence of angular deformation. The maximum welding residual stress is generally the degree of yield stress. When residual stress greater than the yield stress occurs, it changes to the plastic range and appears in the form of angular, longitudinal, and lateral deformation. Under the fully unconstrained condition, reduced residual stress is considered to appear in the form of angular deformation. The welding tensile residual stress decreased to around 42% in the unconstrained specimen compared to the constrained specimen, and at that time, angular deformation of approximately 6° occurred. Therefore, it is estimated that an angular distortion of about 2.4° occurs as the stress of 100 MPa decreases.