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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no significant differences were observed for the fracture surfaces of the steel specimens in the different states at the micro-scale (Figure 2).

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b

Fig. 2. SEM images of fracture surfaces of the 17H1S steel specimens in the as-delivered state (a) and after the PEH + LTT250 treatment (b) tested by tension in air.

In the case of fracture toughness tests for non-hydrogen charged steel (in the as-delivered state and after LTT250 treatment), the fracture also occurred due to significant plastic deformation of the bridges between voids at non metallic inclusions located in ferrite, or due to decohesion of narrow pearlite grains from adjacent ferrite grains. As a result, on the dimple bottoms, non-metallic inclusions or carbides, or their traces, or even entire pearlite grains were observed, surrounded by relatively high tear ridges, which is an indication of a significant reserve of plasticity of the ferrite of the steel. On the fracture surfaces of specimens subjected to PEH + LTT250 treatment and tested for fracture toughness, despite the presence of fractographic features characteristic of non-hydrogen charged steel (after LTT250 treatment) in the area of static crack growth, areas exhibited noticeably flatter dimples with less deformation of the bridges between adjacent voids before their fracture were additionally identified (see Figure 3). It appeared that the fracture in the areas with ferritic grains occurred through the conventional mechanism of stretching up to the rupture of the bridges between adjacent voids, indicating the preservation of the plasticity reserve of ferrite. Meanwhile, within the areas with flat dimples, at the bottom of which pearlite grains were most often found, signs of local plastic deformation by shear were observed. These rounded areas of shear with traces of pearlite at their bottom were considered fractographic evidence of the localization of plastic deformation in hydrogenated ferrite, which is a necessary precondition for the implementation of strain aging. The rounded ridges outlining these dimples with characteristic parallel traces of stretching (by analogy with the stretch zones at the tip of a fatigue crack in ductile steels tested for fracture toughness) were considered evidence of the existence of these flat dimples even before the fracture toughness tests. Microfractographic analysis of the 17H1S steel specimens in the as-delivered state and after the LTT250 treatment tested by slow strain rate tension in the NS4 solution revealed fracture surfaces identical to those obtained for testing these steels in air, exhibiting typical ductile failure. With high resolution, it was evident that the specimens fractured through the mechanism of nucleation, growth, and coalescence of microvoids, forming the classical dimple relief on the fracture surface due to the rupture of bridges between adjacent voids (see Figure 4a). Therefore, it is clear that testing the steel after the LTT250 treatment with slow strain rate tension in the NS4 solution did not affect its fracture mechanism, correlating with its insensitivity to SCC.