Increasing the resistance of steel products to sulfide stress cracking (SSC) is one of the topical issues of the oil and gas industry. Among various factors determining the SSC resistance of a material is the structure-phase state of the material itself and the crystallographic texture associated with it. This paper analyzes these features using the scanning electron microscopy (SEM), transmission electron microscopy (TEM), and microroentgen electron backscattered diffraction (EBSD) techniques. As the research material, a production string (PS) coupling made of medium-carbon steel was selected, which collapsed by the mechanism of hydrogen embrittlement and subsequent SSC. For the first time, by the SEM method, using the location and mutual orientation of cementite (Fe3C) particles, at high magnifications, the authors demonstrated the possibilities of identifying the components of upper bainite, lower bainite, and tempered martensite in steels. The presence of the detected structural components of steel was confirmed by transmission electron microscopy (TEM). Using the EBSD method, the detailed studies of microtexture were conducted to identify the type and nature of the microcrack propagation. It is established that the processes of hydrogen embrittlement and subsequent SSC lead to the formation of {101} <0¯10>, {100} <001>, {122} <2¯10>, {013} <211>, {111} <¯100>, {133} <1 ̅2 ̅1>, {32 ̅6 ̅} <201> grain orientations. It is shown that the strengthening of orientations of {001} <110>, {100} <001>, {112} <111>, and {133} <1 ̅2 ̅1> types worsens the SSC resistance of the material. Using the EBSD analysis method, the influence of coincident site lattice (CSL) grain boundaries on the nature of microcrack propagation is estimated. It is found that the Σ 3 CSL grain boundaries between the {122} <2¯10> and {111} <¯100>, {012} <1 ̅1 ̅0>, {100} <001> plates of the upper bainite inhibit the microcrack development, and the Σ 13b, Σ 29a, and Σ 39a CSL grain boundaries, contribute to the accelerated propagation of microcracks. For comparative analysis, similar studies were carried out in an unbroken (original) coupling before operation.

In this paper, the authors consider the mechanisms of formation of high-strength states in the Zn–1%Li–2%Mg alloy as a result of its processing by the high pressure torsion (HPT) method. For the first time, the study showed that using HPT treatment, as a result of varying the degree of deformation at room temperature, it is possible to increase the ultimate strength of a zinc alloy from 155 to 383 MPa (with an increase in the yield stress from 149 to 306 MPa) without losing its ductility. To explain the reasons for the increase in the zinc alloy mechanical properties, its microstructure was analyzed by scanning electron microscopy (SEM), X-ray phase analysis (XPA), X-ray diffraction analysis (XRD), and small-angle X-ray scattering (SAXS). Using XPA, the authors established for the first time that Zn(eutectic)+β-LiZn4(eutectic)→~LiZn3+Zn(phase)+Zn(precipitation) and MgZn2→Mg2Zn11 phase transformations occur in the zinc alloy during HPT treatment. SEM analysis showed that at the initial stages of HPT treatment, cylindrical Zn particles with a diameter of 330 nm and a length of up to 950 nm precipitate in β-LiZn3 phase. At the same time, the SAXS method showed that needle-like LiZn4 particles with a diameter of 9 nm and a length of 28 nm precipitate in the Zn phase. The study established that, only spherical Zn and LiZn4 particles precipitate at high degrees of HPT treatment. Precision analysis of the zinc alloy microstructure showed that HPT treatment leads to grain refinement, an increase in the magnitude of crystal lattice microdistortion, a growth of the density of dislocations, which are predominantly of the edge type. As a result of the analysis of hardening mechanisms, the authors concluded that the increase in the zinc alloy strength characteristics mainly occurs due to grain-boundary, dislocation, and dispersion hardening.

In this paper, using the X-ray scattering method, the authors found the similaritues and differences in the structural-phase transformations in a Zn–Li–Mg alloy under the artificial and dynamic aging. The artificial aging (AA) of the alloy was implemented at a temperature of 300 ºС for 24 h, while the dynamic aging (DA) was performed through high-pressure torsion at room temperature for a few minutes. For the first time, using X-ray phase analysis, the authors identified the type and parameters of the LiZn2 phase crystal lattice (Pmmm, a=0.48635 nm, b=1.11021 nm, c=0.43719 nm, α=β=γ=90º) and the β-LiZn4 phase (P63/mmc, a=b=0.279868 nm, c=0.438598 nm, α=β=90º, γ=120º) to the eutectics in specified conditions. The study found that SPD leads to intensive precipitation of Zn particles in the primary β-LiZn4 phase, and β-LiZn4 particles precipitation in the Zn eutectics phase. While analyzing the diffraction patterns, the authors estimated the lattice parameter, the size distribution of coherent scattering regions, the averaged dislocation density, and the fraction of edge and screw dislocations after AA and DA. For the first time, by small-angle X-ray scattering, the authors identified the quantitative characteristics of the size, shape, and nature of the bimodal precipitate distribution in the above-mentioned conditions. In particular, it was found that fine Zn precipitates in the form of needles of 8 nm in diameter and up to 27 nm in length and coarse Zn precipitates in the form of rods of 460 nm in diameter and up to 1000 nm in length are produced in the alloy after AA. In the case of DA, fine Zn precipitates of a primarily spherical shape with an average diameter of 20 nm and coarse Zn precipitates, which formed in the primary β-LiZn4 phase a network with a cell diameter of 200–300 nm and wall thickness of 62 nm are produced in the Zn–Li–Mg alloy.