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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1. Introduction Strain aging of structural steels is considered an important factor in operational embrittlement. The physical basis of the strain aging effect consists in the fixation of carbon and nitrogen atoms on dislocations, known as the formation of Cottrell clouds. This complicates the plastic deformation of the metal, which causes its embrittlement. Strain aging phenomenon is widely investigated (De et al. (2001), Vodopivec (2004), Belotteau et al. (2006), Veiga et al. (2011), Tsyrul’nyk et al. (2018), Yang et al. (2022) and others). Strain aging requires a few conditions to occur. The standard DSTU 9166:2021 (2021) regulates the procedure of strain aging of steels in laboratory, according to which samples are initially plastically deformed by 5-10%, and then held for one hour at a temperature of 250ºC. So, strain aging involves two main steps: first, the plastic deformation of the metal, which activates sources of dislocation generation, and second, moderate and relatively short-term heating of the deformed metal to facilitate the diffusion of interstitial atoms such as carbon and nitrogen to dislocations to form atmospheres around them. In our research, we took into account the well-known effect of internal stresses occurring in metal due to electrolytic hydrogen charging, as demonstrated by Mohtadi-Bonab et al. (2015), Kharchenko et al. (2016), Tiegel et al. (2016), Mogilny et al. (2020) and others. We assumed that the internal stresses induced by hydrogenation could reach such a high level as to cause microplastic deformation at certain local microstructural sites, accompanied by the generation of dislocations. These localized areas within the metal serve as traps for diffusible hydrogen, which contributes to the accumulation of molecular hydrogen in them under increased pressure. Therefore, the hydrogenation of steels can, to some extent, serve as a source of dislocation generation even without the need for previous plastic deformation at the macroscale. This is one of the necessary conditions for implementing the mechanism of steel strain aging. Consequently, metal hydrogenation can substitute for its plastic macro-deformation according to the regulated aging treatment for steels. However, the requirement for subsequent heating of the metal after hydrogen charging remains the same. It can be assumed that the necessary temperature for strain aging of steel will decrease with longer exposure beyond 1 hour. This expands the possibilities for strain strengthening structural steels in real operational conditions that involve both hydrogenation and periodic heating. In the presented paper, this assumption was confirmed experimentally under experimental tests of the low-alloyed pipeline steel 17H1S (API 5L X52 strength grade). 2. Methodological Aspects of Experiments To substantiate the proposed assumption, a series of experiments were conducted using the low-alloyed pipeline steel 17H1S (approximately equivalent to API 5L X52 strength grade) in the as-delivered state (from a reserve pipe), the ferrite-pearlite structure of which was formed as a result of cooling the pipes in the air from the temperature of their rolling, corresponding to the normalization mode. A series of specimens were tested: 1) in as-delivered state; 2) after low- temperature tempering for 1 hour at 250 ºС (LTT250); 3) after in-laboratory preliminary electrochemical hydrogen (PEH) charging followed by low-temperature tempering for 1 hour at 250 ºС (PEH + LTT250). PEH charging of specimens was carried out with the use of the following parameters: charging at a current density 50 mА/сm 2 for 100 hours in an aqueous sulphuric acid solution (pH2.0) with adding 2 g/l thiourea. This moderate PEH regime was chosen to avoid obvious hydrogen-induced damage of the steel as a result of the treatment. It was confirmed by comparing the mechanical properties of the specimens subjected to tension testing before and after PEH; the properties were found to be identical, leading to the conclusion that damage from hydrogen charging could be disregarded. In the last case, hydrogen desorption from the steel that can be relea sed at 250°C occurred simultaneously with aging. To ensure complete hydrogen desorption, the samples were additionally held at room temperature for one month. The assessment of hydrogen concentration in the steel after treatment PEH + LTT250 and additional holding confirmed the almost complete removal of hydrogen from the metal (its content practically did not exceed 0.6 ppm, which is typical for the steel in its as-delivered state).