PSI - Issue 59

Hryhoriy Nykyforchyn et al. / Procedia Structural Integrity 59 (2024) 82–89 H. Nykyforchyn et al. / Structural Integrity Procedia 00 (2019) 000 – 000

87

6

a

b

Fig. 3. SEM image of the fracture surface of the 17H1S steel specimen in the as-delivered state (a) and after the PEH + LTT250 treatment (b) tested for determination of fracture toughness.

a

b

Fig. 4. SEM images of fracture surfaces of the 17H1S steel specimens after the LTT250 treatment (a) and the PEH + LTT250 treatment (b) tested by tension in NS4 solution.

In the case of SCC testing of the 17H1S steel specimens after the PEH + LTT250 treatment (Figure 4b), in addition to areas of ductile fracture, intergranular fragments were observed separating adjacent grains of ferrite and pearlite. Notably, the surface of intergranular facets of ferritic grains exhibited traces of structural components (cementite particles interspersed with ferrite layers) from adjacent pearlitic grains. These features visualized dominant hydrogen influence pathways. It is worth mentioning that these features were found not only in the vicinity of the specimen lateral surface, which had direct contact with the testing environment but also in the central part where hydrogen could diffuse due to its high mobility. In some cases, fragments of transgranular failure were also present, with clear morphological characteristics of pearlite (its lamellar structure). However, even in these cases, fragments of transgranular failure were decorated with secondary intergranular cracks. Thus, the revealed susceptibility of 17H1S steel after its strain aging to SCC is due to the preferential interaction of hydrogen with these microstructure components. The embrittlement is caused by the influence of internal hydrogen, for which the boundaries of heterogeneous (ferritic and pearlite) grains and interphase boundaries (between pearlite components) are the dominant paths of diffusion and, therefore, the accumulation of hydrogen in the metal. The conducted analysis of the implementation of the mechanism of strain aging in steels with the participation of hydrogen absorbed by the metal assumes its role in various states. On the one hand, this is atomic hydrogen, for the