X-Ray Structural Analysis of Steels for Solving Applied Problems of the Oil Industry

The present work describes the development and adaptation of X-ray diffraction (XRD) methods for low-carbon steels to explain the reasons for the improvement in mechanical properties and changes in corrosion resistance of low-carbon ultrafine-grained (UFG) steels through the analysis of their fine-structure parameters. As the objects of study, steels grade 10 and 15Cr1Mo1V, widely used for components in the oil and gas industry, were selected. To expand the application range of these steels by enhancing their mechanical properties, a UFG structure was produced via equal-channel angular pressing (ECAP). As a result, steel billets were obtained that demonstrated high mechanical properties (steel 10: yield strength σYS = 1021 MPa, ultimate tensile strength σUTS = 1072 MPa, elongation δ = 11 %; steel 15Cr1Mo1V: σYS = 894 MPa, σUTS = 1136 MPa, δ = 10 %) compared to their coarse-grained counterparts. Corrosion tests showed that the corrosion rate of steel 10 increased from 0.299 to 0.345 mm/year, and in steel 15Cr1Mo1V from 0.121 mm/year to 0.163 mm/year after UFG formation. XRD analysis established that the reasons for the increased strength characteristics of these steels are the refinement of their grain structure, the increased density of introduced defects (dislocations), and the precipitation of MexCy particles of nanometer size. The increase in corrosion rate is also explained by XRD analysis: local depletion of the iron matrix in alloying elements due to the precipitation of MexCy-type nanoparticles, an increase in the fraction of grain boundaries, an increase in the energy of dislocation dipoles, and a rise in dislocation density, which leads to the formation of a cell dislocation structure as a result of ECAP. The new XRD findings on the microstructure of these steels obtained in this work make it possible to purposefully optimize thermomechanical processing parameters in the future to achieve the required physical and mechanical properties, and will be useful to researchers working in the field of materials science.