OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 27 No. 3 2025 The effectiveness of heat treatment applied to carbon steels (C% < 0.25) remains limited because such treatment does not sufficiently improve surface properties (hardness, wear and impact resistance, fatigue, etc.) to meet the stringent surface requirements of contacting parts [1–10]. Among chemical-thermal treatment methods, carburization (using solid, gas, or liquid saturating media) is one of the most effective processing techniques. It aims to enrich the surface with carbon in the atomic state (from 0.7 to 0.9 wt.%) by diffusion into the austenite phase (at temperatures from 870 to 980 °C depending on the process), followed by quenching and tempering to enhance the mechanical properties of the surface according to the decreasing carbon gradient at a very limited depth, without affecting the core [1–5]. Taking this into account, it becomes possible to reduce the cost of the final product by using carbon steels (C% < 0.25) instead of expensive high-carbon alloy steels. On the other hand, the presence of carbon limits grain refinement on the steel surface, which suppresses the mobility of plastic deformation during solid-to-solid interaction [5–8]. Recently, various researchers have actively developed metal surface layers with gradient solid phases, using different technological methods of surface alloying with carbon, nitrogen, boron, etc. [6, 7]. In [7–13], studies were conducted on the influence of alloying elements such as Si, Ni, Cr, and Mo on the carburization characteristics of steels. A significant influence of these alloying elements on the carburization behavior of AISI 1018, 4820, 5120, and 8620 steels was demonstrated. The authors consider decarburization as the reverse process of carburization and conclude that alloying elements also significantly affect the decarburization of steel. Experimental results showed that Si promotes decarburization of ferrite, whereas Cr inhibits it in high-carbon steels. In [15], the influence of certain alloying elements on the decarburization of TRIP steel (transformationinduced plasticity) was studied. Experimental results revealed that increasing the content of Si and P accelerates decarburization. Decarburization is a process wherein carbon atoms diffuse outward from the material and react with furnace gas. Therefore, the effect of alloying elements on steel decarburization primarily affects the diffusion of carbon atoms. Many authors [1–8] investigated the thermodynamics and activity coefficients of carbon in FCC (facecentered cubic) Fe–Mn–C, Fe–Si–C, Fe–Ni–C, and other ternary alloys. Experimental results showed that Mn decreases the activity coefficient of carbon in austenite, which in turn reduces the diffusion coefficient of carbon. Conversely, the effect of Si on the diffusion coefficient of C in austenite is opposite to that of Mn. Although experimental methods can reveal the effect of alloying elements on carbon atom diffusion, elucidating the underlying atomic-level mechanisms remains a challenging practical task. In the practice of many enterprises, an easily implemented, simple, and inexpensivemethod of carburizing machine parts and mechanisms made of low-carbon steels is the use of a solid carburizer. In industry [1, 2], a two-stage process is traditionally applied: the first stage involves saturating parts with carbon using a solid carburizer followed by air cooling, and the second stage involves hardening and tempering. To reduce carburization period, the heating and equalizing temperature is set in the range of 900–1150 °C [10, 11]. A mandatory requirement for parts subjected to such processing is the allowance of 1–3 mm for subsequent machining in order to remove the decarburized layer. The addition of a large amount of carbon and other alloying elements to these steels leads to serious segregation of composition and surface decarburization of the products. It has been reported that composition segregation and decarburization have a negative effect on impact toughness, fatigue life, wear resistance, and other properties critical to the performance of medium carbon steels [4–7]. Decarburization reduces surface hardness and fatigue strength of steel, thereby shortening its service life [1, 2, 8–13]. Decarburization of the steel surface results in insufficient hardness in the surface region due to the reduction of carbon content, which greatly reduces fatigue life. Obviously, when steel is heated to high temperatures without a protective atmosphere, the surface layer reacts with oxygen, carbon dioxide, or steam in the furnace atmosphere, causing oxidation and decarburization simultaneously [1, 2]. Decarburization is a classic surface degradation phenomenon in the heat treatment of steels [14].
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