OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 24 No. 4 2022 martensitic structures. Such structures have a positive effect on the mechanical and corrosion properties of the material [5]. However, in addition to heat treatment, the properties of the material are affected by the defectiveness of its structure [6]. The study [6] indicates that there are also foreign impurities that get into steels at various stages of metallurgical processes. Such impurities, in addition to alloying elements, are introduced into the composition of steels to obtain a certain level of properties. At the same time, many impurities (most often these are: sulfur, oxygen, manganese, silicon, calcium, etc.) can not only dissolve in the matrix of the base material, but also participate in the formation of particles of non-metallic inclusions [7]. The presence of impurities in steel leads to the formation of areas where local internal stresses act. The authors in the study [8] believe that internal stresses arising near structural defects stimulate the migration of point defects to this area. It leads to the clusters of point defects around the impurities, its subsequent expansion and the disc-shaped clusters of vacancies. This process is typical for rapid material cooling. For example, during the quenching process, point and linear defects of the structure do not get around to migrate to the drains, which are the body surfaces and grain boundaries. As a result, the matrix is oversaturated with defects. In view of this, non-metallic impurities significantly reduce the mechanical properties of the material. In addition, the studies [9–12] indicate that the presence of non-metallic impurities of various compositions in steel directly affects the rate of corrosion in local areas. However, the authors in the study [9] note that there is no correlation between the percentage of impurities and corrosion in the local area when assessing the content of non-metallic impurities by the standard method [13]. The studies [11, 12] show that the main cause of abnormally high corrosion rates of oilfield pipelines is the steel contamination with non-metallic corrosive impurities [14], which are inclusions based on manganese sulfide (MnS). The most common grades of oilfield pipelines’ steels are 09G2S and 15ChSNC. There are situations when local corrosion sources are observed on the surface of these steels, which often have a spherical shape associated with the inclusions [14]. The influence of heat treatment on the shape and size of non-metallic inclusions determining the physical and mechanical properties of low-alloy low-carbon steel 09G2S is considered in this paper. This heat treatment leads to the formation of a ferrite-martensitic structure. Research methodology In this work, the samples made from sheet metal, steel grade 09G2S (S – 0.11%, Si – 0.15%, P – 0.05%; S < 0.028; Cr – 0.07%; Mn – 1.91%; Ni – 0.11%; Cu – 0.22%) are studied. The fabricated samples had the following linear dimensions: 4.0 x 70.0 x 25.0 mm. The process of heat treatment of the samples under study is shown in Table 1 The hardness measurement of the samples is carried out on a Vickers Indentec 6030LKV hardness tester with a preload of 20 kg. Each sample was indented five times. The hardness measurement error does not exceed 1% according to the passport data. The grain structure is analyzed with the software package “SIAMS 700” and “SIAMS 800”. Some of the results are reflected in the studies [16, 17]. Microphotographs of the local area are obtained and its chemical composition is determined using a JEOL 6008A scanning electron microscope. The samples are treated with a 3% solution of nitric acid to reveal the microstructure. Ta b l e 1 Heat treatment of the 09Mn2Si steel samples Heat treatment Heating up to 930±20 oС; water quenching Tempering at 200, 350, 500, 650 oС for 1 hour; air cooling
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