PSI - Issue 5

Rui F. Martins et al. / Procedia Structural Integrity 5 (2017) 633–639 Diogo F. Almeida et al. / StructuralIntegrity Procedia 00 (2017) 000 – 000

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1. Introduction

Welding processes play a key role in several industries, such as aerospace, automotive, aeronautics, naval or civil engineering. Due to the high temperature gradients that occur in the heating and cooling phases, the welded components frequently present distortion and geometrical imperfections. Furthermore, the resulting residual stresses may be responsible for increasing corrosion, fatigue and brittle fracture or produce changes in the component’s geometry during service. Consequently, it is essential to carefully control the welding procedures during the production phase and/or during the post-welded stage and numerical simulations can help considerable to fine tune the welding parameters, contributing to substantially reduce the amount of experimental tests needed.

2. Materials and methods

2.1. Base and weld material

In this work it was used an austenitic stainless steel (SS), namely AISI 316L. The AISI 316L SS is often used because of its good mechanical behaviour to thermal and/or mechanical stresses and its good corrosion resistance. In fact, this material has a much lower percentage of carbon in its chemical composition when compared with AISI 316 SS, which makes it an adequate choice for structures which are subject to operating temperatures in excess of 400°C and/or having intensive welding processes during manufacturing stage due to its lower propensity for the precipitation of chromium carbides according Depradeux (2004), Cruz et al. (2010), Martins et al. (2009) or Martins et al. (2008). In addition, it is a material that, according to the Delong diagram, has a stable austenitic microstructure with nickel equivalent and chromium equivalent values equal to 15.65 and 20.08, respectively. The filler metal used during the welding process was Thermanit GE-316L Si (material no. 1.4430) in the form of wire and with a diameter of 1.6 mm. Prior to the start of the welding process, one of the plates was welded along the outer edges (spots) to a fixed structure, so as to minimize its deformation (Fig. 1). Thus, due to the boundary conditions defined, the welded joint will tend to exhibit a high level of residual stresses and small distortion/deformation; instead, if the plates were welded without any constraint, high distortion would occur, while the values of residual stresses would be relatively low. Additionally, offsetting the plates to be welded was done prior to welding, in order to allow some thermal expansion of the plates and minimise distortion. 2.1. Welding parameters

Measurement of residual stresses (location of HBM rosette)

Fig. 1. Butt-welded joint under study during the welding procedure and after being carried out using the TIG welding process. Overall view of the thin welded plate ’ s main dimensions, together with the location where the residual stress es’ measurement had taken place.

The welding of the plates was carried out by the two sides, with one weld pass on each side, at a welding speed of 45 mm/min, in order to allow an adequate deposit of weld metal, as well as full penetration and no lack of fusion of the welds being carried out. At the end of the welding procedures, two weld beads were obtained with an average width of approximately 5 mm and a length of 315 mm. The value for the electrical current used was 70 A, corresponding to a voltage, U, of 13 V. The protective gas used was ALCAL 1 (Ar + CO2), of Air Liquide , with a