PSI - Issue 72

Halyna Krechkovska et al. / Procedia Structural Integrity 72 (2025) 149–156

153

of steam pipeline steel. Formula 3 makes it possible to assess the combined effect of restoration and hydrogenation on the properties of restored steel relative to its corresponding characteristics after operation. = 2 – ∙ 100% (2) = 2 – ∙ 100% (3) The obtained data (Fig. 2a) illustrate that under the influence of hydrogen absorbed by the operated steel, both the strength characteristics and, especially, the plasticity characteristics decreased. In particular, due to hydrogenation, the strength characteristics of the operated steel tended to decrease (by up to 3%). As for the plasticity characteristics, the elongation of this steel decreased by 18% (moreover, on the samples cut near both surfaces of the pipe). Hydrogenation had a more significant effect on RA, the value of which near the outer and inner surfaces decreased to 40% and 36%, respectively. This indicates a tendency of the operated steel to hydrogen embrittlement caused by the hydrogen absorbed by it. In the presence of numerous damages in the form of pores (especially along the grain boundaries), the hydrogen accumulating in them promotes their merging, as a result of which microcracks and delaminations are formed in the steel structure.

Figure 2. The effect of hydrogen on the mechanical properties of 12Kh1MFsteel both after (a) its operation and (b) RHT, established on samples cut near the outer (1) and inner (2) surfaces of the pipe and tested for tension in air after preliminary electrolytic hydrogenation.

The proposed RHT mode was accompanied by recrystallization of the microstructure, a decrease in the proportion of large grains (greater than 30 μm), an increase in the proportion of medium grains (10…30 μm), and a decrease in the number of microdefects along the grain boundaries. As a result, there was a significant improvement in the strength and plasticity characteristics of the restored steel compared to the long-term exploited one. Therefore, it was expected that the susceptibility of the restored steel to hydrogen embrittlement would also be less than that of the operated steel. Under the influence of hydrogenation, the positive effect of restoration of exploited steel under the proposed RHT mode, its characteristics still exceeded those of the exploited steel (Fig. 2b). The negative effect of hydrogen absorbed by the samples during electrolytic hydrogenation was also evident in the restored steel. However, the positive effect of steel restoration (in terms of strength characteristics), although decreased, nevertheless remained positive at the level of 7 – 15% (compared to 18 – 11% observed during testing of non-hydrogenated samples, Fig. 1). As for the plastic properties, under the influence of hydrogen, the positive effect of steel restoration also decreased (to 38% and 41%, respectively, for elongation and RA), however, as in the tests of non-hydrogenated samples, it remained positive (Fig. 1). Moreover, near the outer surface of the pipe, the positive effect of restoration of hydrogenated samples remained more noticeable than near its inner surface. This is due to the more noticeable effect of restoration of the most damaged steel located near the outer surface of the steam pipeline bend. Both plasticity characteristics of the restored steel still exceeded the corresponding standard values even after its hydrogenation. Moreover, although the elongation of the restored steel decreased more noticeably after its hydrogenation, it nevertheless exceeded the permissible value (Table 2). Thus, the elongation values of steel near both pipe surfaces