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

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reduced its fracture toughness (by approximately 10%) and noticeably decreased its SCC resistance (the parameter RA dropped from 69% to 53%). All the analysed properties of the steel can be roughly divided into groups, with some reflecting resistance to fracture at the macro-scale (strength, plasticity, impact toughness), while others relate to the meso-scale (fracture toughness) and the micro-scale (SCC resistance). As a result, it was concluded that the treatment PEH + LTT250 did not affect the steel's properties at the macro-scale, slightly reduced them at the meso-scale, and had the most significant impact at the micro-scale. The negative influence of the treatment PEH + LTT250 on the steel's properties was accompanied by its embrittlement, as confirmed by fractographic analysis of the tested specimens. Figure 1 illustrates the multiple cracking patterns observed in specimens after the treatment PEH + LTT250 and tests for SCC susceptibility. In contrast, no such cracking patterns were observed for the specimens in the initial state and those subjected to the LTT250 treatment at SCC testing. Instead, they exhibited a ductile fracture mechanism with cup-and-cone fracture surface at the macro-scale. These observed regularities were explained by the diffusion of absorbed hydrogen within the metal through favourable microstructural pathways. Therefore, it is precisely in these areas, favourable for the diffusion, the maximum concentration of accumulated hydrogen and, accordingly, a high level of internal stresses caused by hydrogen charging should be expected. Thus, in these localized areas of the metal, conditions favourable for its local plastic deformation will arise, which is a necessary prerequisite for the implementation of the mechanism of strain aging of the steel. However, it cannot be excluded that hydrogen can enhance strain aging due to its ability to intensify diffusion processes (in particular, of carbon, as was shown by Student (1998)). In our case, the self-diffusion of carbon and nitrogen atoms to dislocation cores is important; however, this factor was considered additional, since it does not eliminate the need for plastic deformation as a source of dislocation generation.

Figure 1. An example of multiple cracking in the vicinity of the fracture surface of the 17H1S steel specimen subjected to the treatment PEH+LTT250 and SCC tested in the NS4 solution.

It was assumed that only those properties of the steel could be sensitive to strain aging induced by using the PEH + LTT250 treatment, which is influenced by microstructural features responsible for fracture formation predetermined structure induced hydrogen diffusion pathways. For example, as shown by Luu and Wu (1996), Stopher et al (2016), Galindo- Navaa et al. (2017), Hüter et al. (2018), Liu et al (2019), Díaz et al. (2020), Chen et al. (2020), Mogilny et al. (2020) and others, grain boundary hydrogen transport in steels is considered dominant. Moreover, it was demonstrated by Mogilny et al. (2020) that the electrolytic hydrogen charging of steels induced plastic deformation, which led to an increasing density of dislocations and caused crystallographic texture. Accordingly, fracture resistance parameters with a grain boundary fracture character should be sensitive to the processes of local strain aging caused by metal embrittlement. It was suggested that the fracture topography for the tested steel under SCC conditions would follow such a pattern. Microfractography studies of specimens tested by tension in air showed that regardless of the steel's condition, its fracture occurred by dimpled ductile mechanism, and