OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 27 No. 3 2025 a b c Fig. 5. Photo of the surface layer after heating for hardening at 950 °C and equalizing for 6 hours: a and b – decarburized layer at different magnifications; c – quenched martensite in the surface layer decarburization, and the ferrite structure becomes columnar, perpendicular to the decarburization surface. A partly decarburized layer appears in the sample at 850 °C, and the thickness of the fully decarburized layer decrease. The decarburized layer is composed of both complete and partial decarburization zones. Above 900 °C, the sample mainly shows a partly decarburized layer, because at this temperature the steel structure is completely austenitic, as ferrite and pearlite (a mixture of cementite and ferrite) are transformed into austenite. Carbon from austenite passes into the furnace atmosphere and interacts with furnace oxygen. As a result, the steel becomes depleted in carbon content. The average depth of the fully decarburized layer of each sample is shown in Fig. 7. As shown in Fig. 6, at a heating temperature below 1,000 °C, the thickness of the total decarburized layer increases slowly. However, above 1,000 °C, the thickness increases rapidly, showing exponential growth. The experiments showed the effect of heating and equalizing periods on the depth of the decarburized layer (Fig. 7). The depth of the decarburized layer was measured using an optical microscope, and the average decarburization depth of the samples is shown in Fig. 6. As can be seen in Fig. 6, the depth of the decarburized layer of the samples increased exponentially with the heating temperature, and parabolically with the equalizing period in the furnace at different temperatures (Fig. 7), with the growth rate gradually slowing down. To protect the surface of the cemented layer, it is necessary to apply a coating that will protect the surface from contact with the furnace atmosphere during heating for hardening (Fig. 8). It is known that the effective depth of the cemented layer is related to the hardness in depth after hardening, which ranges from 555 to 600 HV (Fig. 8). Fig. 6. Depth of decarburization as a function of heating temperature (equalizing for 50 min)
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