Issue 51

M. Cauwels et alii, Frattura ed Integrità Strutturale, 51 (2020) 449-458; DOI: 10.3221/IGF-ESIS.51.33 451 diffuse out of the sample during testing. The tests were performed at a constant cross-head displacement speed of 0.6 mm/min, which corresponded to a strain rate of 1·10 -3 s -1 . Fracture surfaces were investigated using SEM. Tensile tests were also interrupted before fracture (stopped at an elongation where the ultimate tensile strength (UTS) was just reached) to study hydrogen-assisted cracking on the sample surfaces. Figure 1 : Geometry of tensile samples R ESULTS AND DISCUSSION Material characterization ptical microscopy (Fig. 2) revealed a strongly oriented microstructure as a result of hot rolling. On the longitudinal section (TD plane) elongated austenite ‘rods’ (light coloured phase on Fig. 2) were observed parallel with the rolling direction, embedded in a ferrite ‘matrix’ (darker coloured phase on Fig. 2). The samples were free of intermetallic sigma phase, which may precipitate during heat treatment of DSS [24]. The average grain diameter on the TD plane was determined using the linear intercept method until a relative accuracy (%RA) lower than 10 % at a confidence level of 95% was reached. This resulted in 5.18 ± 0.51 μm for HT 1190 and 4.42 ± 0.34 μm for HT 1110. The phase balance in both heat treated samples, determined by EBSD and Feritscope ® measurements, is given in Tab. 2. Magnetic measurements (Feritscope ® ) were considered to be more reliable since they analyze a relatively large volume of material compared to EBSD, which is a local 2D technique and might therefore not be representative for the entire sample. The HT 1110 sample had an austenite phase fraction close to the intended 50%, while the HT 1190 sample had an austenite content close to 44%. γ% HT 1190 HT 1110 EBSD 39.20 ± 2.40 46.10 ± 1.09 Feritscope® 43.91 ± 1.15 49.83 ± 0.95 Table 2: Average austenite fraction γ% for the different heat treatments, as determined by EBSD and Feritscope ® . Figure 2: Optical microscopy images of the TD plane of (a) HT 1190 and (b) HT 1110. O