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Post-weld treatments can enhance the fatigue strength of weldments by means of improving their geometry or residual stresses, or both. In this study, the fatigue strength of TIG-dressed non-load-carrying cruciform joints made of ultra-high-strength steel were analyzed using stress-based approaches, i.e. nominal, structural, and effective notch stress (ENS) methods, and a multi-parametric approach, entitled the 4R method. Comparisons were made between the methods, along with the applied parameters, and the results showed the 4R and ENS approaches to have the best correspondence with the experimental test data, whereas the nominal and structural stress methods resulted in conservative fatigue strength estimations. Post-print / Final draft

TIG-dressing is a technique used to improve the fatigue strength of welded joints by a remelting of the weld toe region that promotes both a smoother transition between the plate and the weld crown and a residual stress redistribution. These effects are very hard to be quantified by numerical simulation since a highly coupled thermo-fluid-mechanical analysis is required. However, if the final weld toe geometry is supposed to be known or a-posteriori measured, a simplified numerical method can be used to simulate the residual stress redistribution that uses the activation-deactivation function of elements. This technique is applied to a real steel weldment and the results, in terms of phases proportions and residual stress redistribution, were found in good agreement both with data coming from metallurgical analysis and the improved fatigue strength observed on welded joints after the TIG-dressing operation

Fatigue strength of welded joints is controlled above all by notch defects such as weld toe and weld root that act as stress concentrators. This is the reason why over the last years different post-weld fatigue improvement techniques have been developed with the main propose of reducing geometrical discontinuities. Among these, TIG-dressing is the most used because of its simplicity and effectiveness in lowering the residual stress concentration. By this process the weld toe is re-melted to provide a smoother transition between the plate and the weld crown and to beneficially modify the residual stress redistribution. However, because of the intrinsic difficulty to numerically simulate the TIG-dressing process due to the high coupling between fluid and mechanical analysis, the effects of a weld toe remelting in terms of residual stress redistribution is hardly quantified in literature. The present paper is aimed to analyse the influence of TIG-dressing process on metallurgical and mechanical properties of a steel T-joint. Finally, a numerical model, recently published in literature, was used to quantified the residual stress redistribution. © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).

Weld improvement techniques are aimed at reducing the notch effects of welds and generally focus on two aspects: a change of geometry of the weld toe and a change of the weld residual stresses. In this paper, fatigue tests are discussed, performed on butt welded specimens in steel grades ranging from S460 to S1100 in the TIG-dressed condition. The test results are compared with modelled fatigue strength according to the notch stress theory and similar specimens in the as-welded condition. The changed notch geometry of the joints is based on the results of a parallel program of weld toe geometry measurements.The individual data points and their failure mode give good qualitative insight in the effect of TIG-dressing on butt welded joints in high strength steel. Overall, a positive effect of TIG-dressing on the weld toe is visible.On the basis of a large dataset of measured weld toe geometries a model has been developed based on the notch stress theory. The theoretical results of the model are compared with the results of the physical tests, showing significant differences between the model and the fatigue tests.

This paper presents the results of extensive measurements on weld toe geometry of as-welded and TIG-dressed butt welded connections in high strength steels S460, S690 and very high strength steels S890 and S1100. Descriptions of the measurement techniques and data analysis are presented. Four weld toe parameters have been formulated to characterize the weld toe: weld toe radius, weld toe angle, weld height and undercut. From the data can be concluded that TIG-dressing, on average, has a beneficial effect on the geometry of the weld toe. However, TIG-dressed weld toes have different appearances and as a result, weld toe parameters show more variation after TIG-dressing. An improvement of the weakest link is therefore not guaranteed.The reheating of weld material and surrounding parent material results in a new fusion zone (FZ) and heat affected zone (HAZ). Hardness measurements have been performed in all different zones and show an increase of hardness as a result of TIG-dressing, with an occasional exception in the very high strength steels.

Fatigue of metals is yet an area of material science that is not completely well understood, and there are many topics that are still being investigated. However, fatigue of welded components is an even more complex phenomenon. Apart from thermal cycles during welding, which strongly affect the base material and induce residual stresses and distortion, the fusion process and the existence of a filler (in some processes) lead to very heterogeneous microstructures, varying in mechanical properties and chemical composition throughout the area of the joint. Additionally, weld defects like porosity, slag inclusions, undercuts and overlap among others, can significantly reduce the fatigue strength. All these features create suitable conditions for weld toe cracking that is apparently the most problematic region. Based on this, several improvement techniques have been developed and they have proven to enhance fatigue resistance by either inducing compressive residual stresses in the weld toe or modifying its shape into smoother geometries.In the present work, fatigue strength of transverse non-carrying load fillet joint was analysed. High frequency axial fatigue tests were performed in as-welded and improved joints. TIG-dressing and shot peening techniques were employed to modify the weld toe into more convenient configurations. Microstructures obtained in both cases are discussed and related to crack initiation phase. Crack path and fracture surfaces were also investigated.It was found that as-welded (AW) samples welded according specifications presents enhanced fatigue strength above FAT 90. TIG dressing fulfilled to create smoother toes and increased fatigue life by at least 50%. Testing of shot peened samples is still ongoing. An average increase of surface hardness around 10% was measured in the base metal (BM) with this technique. Compressive stresses can delay nucleation and growing of microcracks from the toe, which is translated into longer fatigue lives. Validerat; 20140619 (global_studentproject_submitter)