PSI - Issue 42

Mihai A. Popescu et al. / Procedia Structural Integrity 42 (2022) 1626–1633 M.A. Popescu et al. / Structural Integrity Procedia 00 (2022) 000–000

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Moreover, when working with steel structures requested to behave in a reliable manner in regards to fatigue endurance as shown by Nelson (1982), Lawrence, Burk, and Yung (1982), and Baumgartner (2016), extra care needs to be taken during all steps of the production process. Wind turbines are subjected to a large number of loads, some of them steady, such as a steady wind stream, or unsteady loads, such as crosswinds and wind turbulence as described by Hau (2006). Due to the cyclical nature of the loading conditions, the tower itself needs to behave safely in regard to fatigue (Albuquerque et al. (2012)). As shown and detailed by Radaj (1992), Klas Weman (2012), and Michaleris (2014), one of the main by-products of the welding process is welding residual stress, and while compressive residual stress is beneficial, tensile residual stress is very detrimental to steel structures and can lead to a shorter lifespan, especially when constantly subjected to cyclical loading. To reduce production costs and time, it is necessary to assess the welding residual stress (Jacob et al. (2018)) present in the parts after welding and its e ff ects, in order to prevent distortions and achieve a product that will have a long service life.

Nomenclature

BM Base metal CGSC Coarse-grained supercritical zone EDM Electrical discharge machining FGSC Fine-grained supercritical zone HAZ Heat a ff ected zone

IC Intercritical zone LN2 Liquid Nitrogen OM Optical microscopy SAW Submerged-arc welding SC Subcritical zone

VWT V: Charpy V-notch – W: notch in weld metal – T: notch through the thickness VHT V: Charpy V-notch – H: notch in heat a ff ected zone – T: notch through the thickness WM Weld metal

2. Specimens and Test procedures

The specimens were extracted from door cut-outs of tubular steel towers (Fig. 1), which are used as a support struc ture for wind turbines, obtained from two di ff erent manufacturers. The specimens were obtained from two di ff erent manufacturers, having been extracted from the bottom sections of the towers containing circumferential welds. The materials used for the tower fabrication are EN 10055-2:2019 S355 J0 + N, and EN 10055-2:2019 S355 J2 + N, both from plates with 59 mm thickness. The chemical composition of the base metals (BM) is given in Table 1. The specimen obtained from the S355 J0 + N tower will be referred to as Specimen 1 (S1) and the one obtained from the S355 J2 + N tower will be referred to as Specimen 2 (S2). The solid wire electrodes and flux used were as follows, ESAB OK Autrod 12.22 (4 mm) / ESAB OK flux 10.72 for S1, and OERLIKON OESD3 (4 mm) / OERLIKON OP121TT flux for S2. Both tower sections were welded using multi-pass submerged-arc welding (SAW) technique, adjusting the parame ters in accordance with the internal welding specifications of each individual manufacturer. The chemical composition of the flux and wire electrodes is shown in Table 2 and Table 3, obtained from the manufacturers. The edge preparation of the bevels was prepared manually for S1 and machined for S2. No information was provided regarding the welding parameters, heat input, type of weld, number of passes, or bevel geometry.

2.1. Macro and Microstructural Analysis

Microstructural analysis of the weld and base material was performed by standard metallography and the mi crostructure images having been recorded by optical microscopy (OM). In order to analyze the microstructure of the