Rostyslav Leshchak

It is established that the applying of mechanical tensile stresses to specimens of pipe L450M ferritic-pearlite steel leads to the following structural changes and the formation of defects in its near-surface layer. In particular, the local weakening of ferrite grain boundaries as¬sociated with pearlite colonies is observed; single crack-like defects of the order of ~ 10 mkm appear; the chains of small microcracks are formed along the boundaries of ferrite grains weakened by pearlite; defects in the form of voids with a size of 10 mkm, and more appear; surface cavities with a size of 10–70 mkm are formed. An empirical dependence for assessing the minimum thickness of the defective near-surface layer in steel is proposed, which can be used to predict the level of applied tensile stress in order to obtain a defective near-surface layer of steel of an assigned thickness.

It is established that plastic expansion of a pipe billet made by cold deformation of hot-rolled 12 mm thick L450M steel sheets causes the uniform mechanical characteristics along the pipe contour. This effect has a positive impact on the quality of straight-seam welded pipes, since due to the uniformity of the mechanical characteristics of the metal along the pipe contour, they will have increased operational characteristics. This, in turn, increases the level of safety and durability of the main gas and oil pipelines and heating networks during their operation.

Precipitate‐strengthened Fe‐based alloys, often referred to as body‐centered cubic superalloys, are considered as promising candidates for elevated‐temperature applications up to 700°C due to their low density, cost efficiency, and favorable mechanical performance in this temperature range. While the fundamental properties of the Fe–Ni–Al system are well studied, their behavior under hydrogen‐containing environments remains insufficiently understood, despite the high relevance of hydrogen‐rich and H ₂ O‐bearing atmospheres in next‐generation energy systems. Given the intrinsic room‐temperature brittleness of Fe‐rich Fe–Ni–Al alloys and the ability of hydrogen to act as an embrittling agent, systematic investigation of hydrogen–alloy interactions is essential. This study addresses this gap by examining the influence of hydrogen on Fe‐rich compositions within the Fe–Ni–Al system, providing insights into their mechanical response after hydrogen charging. Single‐phased and two‐phased precipitate‐strengthened Fe–Ni–Al alloys are compared in terms of mechanical properties after hydrogen charging, with the two‐phase alloys showing over 50% better strength in both the reference and charged states, despite their brittle behavior. The variety of investigated charging modes allows achieving a range of hydrogen concentrations that are related to the failure stress of the alloys.

The technological operation of expansion in the range of e = 0–1.2% has a negligible effect on the standard mechanical characteristics of the base metal of straight-seam welded pipes made of L450M steel. The tensile strength practically does not change, and the yield strength decreases by approximately 6% in comparison to the base metal of the unexpanded pipe. At the same time, the relative elongation and relative area reduction decrease by 6–8% with an increase in the expansion level. It is found that the impact toughness KCV of the base metal in straight-seam welded pipes is more sensitive to changes in the expansion level, and at e = 1.2% it increases by approximately 11% compared to the unexpanded pipe.

To increase the fragmentation of medium-carbon 60С2А steel in the conditions of a static explosion, a method of its preliminary treatment with electrolytic hydrogen is proposed. The Pf parameter is proposed for evaluating the steel fragmentation, which determines the number of steel fragments weighing 4–5 g or more per weight unit of the material. It was found that when the hydrogen concentration in steel changes in the range of 0.07–4.36 ppm, the value of the Pf parameter increases by 25%. Comparison of the proposed method with the method of preliminary quenching of 60C2A steel showed an increase in the value of the Pf parameter by 56%, which indicates its perspective use.

The change in the values of the parameters of different physical nondestructive testing methods (surface acoustic waves, magnetic structuroscopy, and electrochemical potential) was studied depending on the concentration of hydrogen in carbon steel in the range of 0.4–8.5 ppm. It was established that the most sensitive are the values of the coercive force, which changed by more than 25%. The values of the residual magnetic induction and the electrochemical potential of the metal surface changed by 23% and 20%, respec­tively, making them also applicable for assessing the hydrogen content in this steel. Other investigated parameters showed lower efficiency and their change was: for the area of the magnetic hysteresis loop of about 10%, and for the maximum magnetic permeability and the relative change in the velocity of propagation of the surface acoustic wave – approximately 2%.