OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 27 No. 3 2025 This means that, relative to the Boudouard equilibrium (C + CO₂ ↔ 2CO), the reaction proceeds from left to right at T > 700 °C) [1, 2, 12, 17], indicating a tendency towards decarburization at T > AC1. Since an increase in temperature favors the preferential oxidation of carbon to form CO, the equilibrium concentration of CO in the resulting gas mixture also increases [1, 2]. According to [20–22], it is important to know that gas mixtures (CO + CO₂) cause decarburization of steel if their composition lies below or to the right of the equilibrium line for this steel, while they cause carburization if their composition lies above or to the left of this line. During heating for quenching in air, decarburization of steel is initiated by the general oxidation of the steel surface due to the high partial pressure of oxygen. The oxidation and decarburization reactions occur simultaneously; therefore, various effects on both processes are intertwined, some directly proportional and some inversely proportional. As mentioned earlier, decarburization is visible only if the oxidation of the steel surface is slower than its decarburization, i.e., when the oxidation of carbon and the rate of carbon diffusion are greater than the oxidation rate. Visible decarburization depends on the oxidation potential of the atmosphere in the quenching furnace, which also determines the degree of surface oxidation. This is the reason for the large differences in visible decarburization that occur when heating in air or, for example, in a mixture of N₂ + 2% O₂ [8]. Visible decarburization is always greater when heated in low-oxygen atmospheres [8, 10]. It is also influenced by the adherence of the oxide layer to the steel surface (poor adherence of the oxide layer increases decarburization due to a decrease in the oxidation rate) and its permeability to gases (an impermeable oxide layer reduces decarburization), the carbon content of the steel, and the cooling rate after heating, while different alloying elements affect the kinetics of oxidation and decarburization. At low cooling rates, decarburization also occurs during cooling, especially in the two-phase (α + γ) region where the surface ferrite layer thickens due to slower oxidation. In hypoeutectoid unalloyed steels, the overall visible decarburization decreases with higher cooling rates. This is a consequence of the decrease and convergence of the temperatures Ar3 and Ar1 and the resulting decrease in ferrite content due to the increased pearlite content [1, 2]. At some point for each steel characteristic, the critical cooling rate is reached, Ar3 and Ar1 temperatures become equal. At this point, there is no longer any hypoeutectoid ferrite in the microstructure. If the cooling rate is thus high enough for the partially decarburized layer, then hypoeutectoid ferrite will no longer form in this layer either, and only pearlite will exist there. At even higher cooling rates, bainite or martensite is formed in the partially decarburized layer. Because of this, the partially decarburized layer is not fully visible at higher cooling rates and may disappear completely at a sufficiently high cooling rate, leaving only the fully decarburized surface layer visible. The metallographically determined decarburization depth is always smaller than the actual depth [1, 2]. In metallographic analysis, it is also necessary to take into account the limited ability of the human eye to detect small differences in the content of ferrite and pearlite or globular cementite in ferrite (spheroidized state), which further reduces the estimated thickness of the decarburized layer and increases the observation error. More accurate values for the decarburization depth are determined by microhardness measurements, but these are still not as accurate as the actual depths measured by chemical analysis [19, 22]. Thus, our experiments show a significant increase in carbon content by more than three times with longer equalizing periods during the carburizing process (Fig. 3). The observed increase in carbon content can be explained by carbon diffusion, which is influenced by both the absorption rate and the carburizing duration. This diffusion process facilitates the reaction with iron in the low alloy gear steel, resulting in the formation of a new carbon-based phase. A similar trend of increasing carbon content in the surface layer of Fe-C-Mn steels with longer carburizing periods has been observed [14–20]. The increase in thickness indicates successful diffusion of carbon into the surface layer during carburizing. As carbon diffuses into the steel, it combines with iron to form a new carbon-based phase, resulting in an increase in the layer thickness. This increased thickness is desirable, as it implies improved mechanical properties after quenching.
RkJQdWJsaXNoZXIy MTk0ODM1