Assessment of Residual Stresses in MAG-Welded Steel Joints by X-Ray Diffraction and Incremental Hole-Drilling

This study provides a detailed comparison between X-ray diffraction (XRD, sin2ψ method) and incremental hole-drilling (IHD, regularised integral method) for assessing residual stresses in steel plates MAG-welded with three commonly used joint configurations (II-, X- and V-grooves), with and without mechanical constraints. Surface and in-depth residual stresses were evaluated on both faces of each sample, supported by hardness and metallographic analysis. After removal of the mechanical constraints and sectioning, XRD revealed similar tensile stresses at the weld toe for all joints, with longitudinal values of approximately +200 MPa, but marked differences at the weld centre, where compressive stresses of approximately −390 MPa were measured in the II-joint, associated with the combined effects of groove geometry, welding sequence and pass-specific heat input during welding under common restraint conditions. IHD provided smooth depth profiles but consistently overestimated stress magnitudes relative to XRD, with differences reaching approximately 150–200 MPa at depths ≥ 0.2 mm in the X-joint. Plasticity effects were identified as an important contributor to these discrepancies. The plasticity factor was used as a complementary indicator of when local yielding may influence IHD results, exceeding the adopted reference value of f=0.2 near the surface in all joint configurations and supporting the interpretation that yielding occurred during the first drilling increments in all joints. The integral method, used for non-uniform in-depth stress evaluation by IHD, propagates near-surface inaccuracies into deeper layers. A preliminary application of a plasticity-correction procedure, developed for uniform stress fields, reduced IHD stresses but was not fully effective. Overall, the results highlight the strengths and limitations of XRD and IHD for residual-stress evaluation in MAG-welded joints and emphasise the need for caution—and ideally correction—when applying IHD to determine welding residual stresses.