Issue 51

M. Cauwels et alii, Frattura ed Integrità Strutturale, 51 (2020) 449-458; DOI: 10.3221/IGF-ESIS.51.33 454 In the presence of hydrogen (Fig. 4b), the appearance of the fracture surface is clearly different. Near the edges of the sample, brittle fracture features were observed. In the centre, still ductile fracture features such as dimples were observed. In between, a transition shear zone was observed. On Fig. 4c, the dashed line indicates the size of the brittle zone. This distinct fracture surface is a consequence of hydrogen not being present throughout the entire thickness of the sample after one day of pre-charging, considering the relatively slow diffusion of hydrogen in DSS. The side (ND) surfaces of the hydrogen charged samples showed secondary cracks which were not present when specimens were tested in uncharged condition. Fig. 5 shows the embrittled zone in detail for both heat treatments. These images show a combination of (quasi-)cleavage and intergranular fracture features. Quasi-cleavage is used to refer to features on a fracture surface that exhibits characteristics of both plastic deformation and cleavage. For quasi-cleavage, the fracture surface has the appearance of a cleavage fracture, but is not along a known cleavage plane [36, 37]. This is an often observed feature for fracture of ferrous alloys which are susceptible to HE [38]. Quasi cleavage is often characterised by fine lines that follow the crack propagation direction called ‘river markings’ (Fig. 5a). A smaller part of the brittle features for both heat treated samples was intergranular. There was some indication that intergranular fracture was more common for the HT 1110 sample than the HT 1190 sample. After one day of hydrogen pre-charging, the samples are not saturated with hydrogen. The depth of the area affected by hydrogen was measured on several SEM images and averaged out resulting in a depth of embrittlement of 66.78 ± 1.8 μm for HT 1190 and 56.43 ± 0.8 μm for HT 1110. The hydrogen concentration C H throughout the sample can be modelled by solving Fick’s second law for uniaxial diffusion in a thin plate [23]. A qualitative representation of the hydrogen profiles in HT 1190 and HT 1110 tensile samples with a thickness of 0.650 mm after one day of hydrogen charging is given in Fig. 6. As HT 1190 has a higher ferrite fraction, hydrogen can diffuse further into the material due to a higher average diffusion coefficient [39]. The average diffusion coefficient of hydrogen in DSS depends on austenite fraction, as well as the shape, size and spacing of the austenite and its orientation in relation to the direction of hydrogen entry into the material. The lower austenite fraction and larger grain size of the HT 1190 sample contribute to a less tortuous path for hydrogen diffusion and less opportunity for hydrogen to get trapped at the α/γ interface. The diffusion coefficients used for this calculation were 1.14·10 -14 m 2 ·s -1 for HT 1190 and 3.37·10 -15 m 2 ·s -1 for HT 1110 These coefficients were determined by fitting Fick’s diffusion law to a saturation curve for each respective material following the procedure established by Claeys et al. in [40]. Also based on Fick’s diffusion law, the distance x hydrogen can theoretically diffuse into the material in a given time t is given by the formula: x = Dt (2) Fick’s laws for diffusion, however, assume a homogeneous microstructure, which is certainly not the case for DSS. Application of Eqn. (2) using the experimentally determined diffusion coefficients mentioned above, results in 31.4 μm and 17.07 μm for HT 1190 and HT 1110, respectively. This is a considerable underestimation of the diffusion depth compared to the values measured on SEM images. In DSS, hydrogen will not be distributed uniformly over austenite and ferrite. Since the diffusion coefficient of hydrogen in ferrite is much larger than the one in austenite, hydrogen will diffuse deeper into the material following ferrite ‘paths’, where diffusion is faster than the overall, effective diffusion speed, while diffusion in austenite contributes little to the overall hydrogen transport [39] Figure 6 : Simulated normalised hydrogen concentration profile through the thickness of a tensile sample after one day of hydrogen charging for HT 1190 and HT 1110