Issue 51

M. Cauwels et alii, Frattura ed Integrità Strutturale, 51 (2020) 449-458; DOI: 10.3221/IGF-ESIS.51.33 450 Several possible theories for hydrogen embrittlement have been proposed, with the operational mechanism depending on the material, the hydrogen charging condition and the load type [6, 7]. No single mechanism can comprehensively describe the HE phenomena and different combinations of mechanisms have also been described [8]. For non-hydride forming materials, hydrogen enhanced decohesion (HEDE), hydrogen enhanced localised plasticity (HELP) and hydrogen enhanced strain-induced vacancies (HESIV) are most commonly cited. HEDE is the oldest proposed mechanism and proposes hydrogen induced weakening of interatomic bonds, as such promoting decohesion in the lattice [9–11]. The HELP model is based on atomic hydrogen enhancing dislocation mobility through an elastic shielding effect. This locally reduces the shear strength, leading to premature fracture [12–14]. Finally, according to the HESIV theory, hydrogen stabilises vacancies and reduces their mobility, increasing the strain-induced creation of vacancies and reducing the material ductility [15, 16]. Hydrogen induced ductility loss of DSS under cathodic protection in artificial seawater or NaCl-containing solutions has been reported in several studies [4,17–21]. The hydrogen susceptibility was found to be more pronounced for welded DSS because of microstructural changes and a higher ferrite content in the heat affected zone as compared to the base metal [22]. The hydrogen properties of the two phases in DSS are very different. In ferrite, the hydrogen diffusion is relatively high but the hydrogen solubility is low, while for austenite the reverse is true [23]. Usually, α and γ are present in equal fractions in commercial DSS to obtain an optimal combination of mechanical properties. An austenite phase fraction that differs from this ideal 50% is relevant for example in the heat affected zone of welded DSS structures. This work aims to study the influence of the austenite phase fraction on the hydrogen embrittlement sensitivity of DSS. First, two heat treatments were devised in order to create DSS samples with a different austenite phase fraction. Thereafter, tensile tests were performed both with and without the presence of hydrogen. The tests without hydrogen were performed in air and tests with hydrogen were performed in-situ, i.e. with continuous electrochemical hydrogen charging during the tensile test, using pre-charged specimens. M ATERIALS AND EXPERIMENTAL PROCEDURE he material used was UNS S32205 duplex stainless steel, hot rolled to plates of 0.8 mm thickness. The chemical composition is given in Tab. 1. The as-received material contained 53% ferrite as determined via magnetic measurements (Feritscope® FMP30). The steel was subjected to two different heat treatments in order to create materials with a differing austenite fraction: one with equal fractions of ferrite and austenite, and another with approximately 40% austenite. The samples were heated to a temperature were the desired phase balance was present, followed by quenching in a brine solution (7 wt% NaCl). The required annealing temperatures were chosen based on Thermocalc ® calculations as 1190 °C and 1110 °C for the 40% γ and 50% γ samples, respectively. Annealing times were selected to ensure similar grain sizes in both heat treated materials, since grain size influences both the hydrogen behaviour of a material, as well as its mechanical properties. 10 min was selected for the samples annealed at 1190 °C and 30 min was chosen for the samples annealed at 1110 °C. The resulting two heat treated materials will be referred to as HT 1190 and HT 1110 in the following sections. wt% C Cr Ni Mo Mn Si Other UNS S32205 0.022 22.850 5.500 3.070 1.810 0.320 Cu 0.200, P 0.027, Co 0.162, N 0.173, Al 0.022 Table 1 : Chemical composition of UNS S32205 The heat treated materials were etched with Carpenter stainless steel etchant (8.5 g Ferric Chloride, 2.4 g Cupric Chloride, 122 ml ethanol, 122 ml hydrochloric acid and 6 ml nitric acid) and investigated using light optical microscopy (Keyence VHX-2000), scanning electron microscopy (Quanta FEG 450, accelerating voltage 20 kV, spot size 5 nm) in combination with energy-dispersive X-ray analysis (EDX), and electron backscatter diffraction (EBSD). Phase fractions were determined using a Feritscope ® FMP30. Tensile tests were performed on non-notched samples. The sample geometry is given in Fig. 1. Samples were heat treated in a resistance furnace (Nabertherm ® ) and subsequently ground and polished to remove oxides. Both materials were tested in uncharged condition in air, as well as in hydrogen charged condition, to evaluate the impact of hydrogen on the mechanical properties. The hydrogen charged condition consisted of pre-charging for one day followed by in-situ charging during the tensile test. Hydrogen charging was done electrochemically in a 0.5 M H 2 SO 4 solution containing 1g/l thiourea with an applied constant current density of 0.8 mA/cm 2 . Continuous in-situ hydrogen charging ensured that hydrogen did not T