OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 27 No. 3 2025 Discussion of the results From the analysis of the literature, it is evident [1–12, 17–21] that the theoretical discussion of the depth of decarburization during heating remains a debatable issue. The works [1, 2] provide information that in 1946 Pennington conducted a comprehensive study of the decarburization of carbon steel in the temperature range of 691–927 °C in an atmosphere containing 20 % H₂O – H₂, in which oxidation of steel could not occur. In this study, it was found that a ferrite “band” formed on the steel surface at 732–893 °C, but not at 691 °C and 927 °C, while the maximum ferrite thickness was observed at 790–815 °C. Pennington explained the decrease in ferrite thickness above 905 °C by a decrease in the solubility of carbon in ferrite to zero at 905 °C. One of the limitations of Pennington’s analysis was the assumption that the carbon concentration in the formed ferrite was constant throughout the ferrite layer and, therefore, that carbon diffusion within the ferrite did not contribute to its growth. Simultaneous oxidation and decarburization were studied by Birks and colleagues in 1970. Based on these studies, it was stated that “the mechanism by which decarburization of steels occurs has been well studied, particularly in the case of plain carbon and low alloy steels” [1, 2]. However, the scope of the studies by Birks et al. was limited. First, the studies focused on decarburization occurring only in the austenite. Second, although it was recognized that in an oxidizing atmosphere decarburization occurs through a reaction between the scale and dissolved carbon in the steel, Birks and colleagues continued to assume that the carbon concentration at the scale-steel interface was zero because of its low value. It is known that decarburization and surface oxidation occur simultaneously on the steel surface when heated in an oxidizing environment. In essence, decarburization is a reaction between carbon in the matrix and oxygen, while oxidation is a reaction between iron and oxygen. Therefore, decarburization is closely related to oxidation, and the relationship between them is competitive. The formation of an oxide layer on the steel surface consumes part of the decarburized surface layer and, thus, reduces the final observed decarburization thickness. Therefore, to obtain more accurate results, it is necessary to take into account the oxidation process. The absolute thickness of the total decarburized layer can be considered as the sum of the thicknesses of the observed total decarburization and the oxide scale. In practice, the thickness of the oxide scale cannot be measured accurately, since the oxide layer tends to flake off from the sample surface during the cooling period and subsequent manipulations. When examining the Fe–O phase diagram [1, 2], we assumed that the oxide layer on the surface of iron and unalloyed steel consists of only one oxide, FeO, which is formed during heating in air at temperatures (T > 570 °C). In reality, according to [1, 2], the high partial pressure of oxygen and different iron chemical valence create an oxide layer consisting of three different oxides at these temperatures. The sequence from the oxide with the least amount of oxygen, closest to the metal, to the oxide with the highest amount of oxygen, closest to the atmosphere, has been recorded as: FeO / Fe₃O₄ / Fe₂O₃ —wustite / magnetite / hematite [1, 2]. The layer thickness is generally constant at (T > 700 °C), and its composition is approximately 95 % FeO, 4 % Fe₃O₄, and 1 % Fe₂O₃ [1, 2]. At temperatures (T = 570–800 °C), however, results can be found [16–22] that deviate both in terms of composition and thickness of the individual layers, demonstrating the complexity of the oxidation of iron and unalloyed steel in this temperature range [14]. The formation of a three-layer oxide layer can be explained simply using direct oxidation chemical reactions. The mechanism of formation of a three-layer oxide layer at the FeO/Fe₃O₄ and Fe₃O₄/Fe₂O₃ phase boundaries remains a subject of discussion due to its complexity, which is a consequence of specific transport processes through individual oxide layers [2, 15, 16]. When discussing carbon oxidation, it is necessary to take into account the microstructure of the steel at the heating temperature and the temperature range of stability for CO₂ and CO. Carbon in unalloyed hypoeutectoid steel at temperatures T > AC1 is present only in solid solution dissolved in Fe-α (ferrite) and Fe-γ (austenite). Ferrite is stable at T < AC3, and austenite is stable at T > AC1. Carbon reacts with oxygen to form CO and CO₂ during heating of steel in air. The reactions of direct oxidation of carbon by oxygen (C + O₂ → CO₂ and 2C + O₂ → 2CO) intersect at approximately T ≈ 700 °C [1, 2]. The lines of both reactions show that at T > 700 °C, CO is more stable, or that CO is preferentially formed at T > AC1 [1, 2].
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