Issue 51

M. Cauwels et alii, Frattura ed Integrità Strutturale, 51 (2020) 449-458; DOI: 10.3221/IGF-ESIS.51.33 453 to a combination of effects. First, the pinning of dislocations by hydrogen, impeding cross-slip and dislocation movement, leading to solid solution strengthening [34]. Secondly, Barnoush et al. [35] found that when the hydrogen concentration is high enough, the shear modulus of the austenite is reduced locally. Dislocation sources such as Frank-Read sources, that were previously inactive will then generate new dislocations under the high internal stresses inherently present in the DSS due to the differing thermal expansion coefficients of ferrite and austenite. The generation of new dislocations increases the dislocation density, which in turn increases the stress required to move dislocations, and thus increases the yield strength. Both effects may be increased in the HT 1190 samples compared to the HT 1110 samples due to the higher hydrogen content in those samples. Evaluation of the fractography and hydrogen-assisted cracks by SEM The fracture surface of both materials tested in air showed dimples. A lot of necking occurred for both samples, resulting in a very thin fracture surface due to the large reduction in area. Fig. 4a shows the fracture surface delineated by white lines. The fractured specimens also had a cup-and-cone-like appearance. This, as well as the presence of dimples on the fracture surface indicates a ductile fracture behaviour in air. Figure 4 : Fracture surface for a HT 1190 sample tested (a) in air, (b) and (c) in hydrogen charged condition. (c) shows the depth of the brittle zone (white dotted line) and the direction of hydrogen entry (white arrows). Figure 5 : SEM images of the embrittled zone in sample (a) HT 1190 and (b) HT 1110. Arrows indicate examples of river markings.