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The article presents the results of studying the kinetics of oxidation of Fe-Cr, Fe-Al and iron alloys at temperatures of 450–550 °C. The influence of preliminary oxidation of these alloys on the nitriding process has been studied. It has been established that alloying of Fe-Cr, Fe-Al alloys increases the amount of absorbed nitrogen, but decreases the overall depth of the nitrided layer. The duration of nitriding required to develop high hardness (over HV 1,000) depends on the composition of the solid solution. At a nitriding temperature of 520 °C, exposure is 10–15 minutes for steels of the first group, at least 3–4 h for steels of the second and third groups, and 5–6 h for steels of the fourth group. Studies have shown that the hardness of the layer is determined mainly by the composition of the solid solution; the amount and dispersion of the carbide phase have less effect. The hardness increases as a result of an increase in the hardening temperature and a decrease in the tempering temperature, which reduce the amount of the carbide phase, but increase the alloying of the solid solution. The hardness of the nitrided layer of high-speed steels P9, P18, having the same composition of the solid solution, is the same (HV 1,340) even despite the significant difference in the amount of the carbide phase. The hardness of the layer of steel 4Х5В2ФС (4Kh5V2FS), which contains more chromium in the solution, is HV 50–90 higher than the hardness of the layer on the steel 3Х2В8Ф (3Kh2V8F), which has 1.5–2 times more of the carbide phase. The behavior of steels with the same high chromium content (12 %), but different carbon content is characteristic. The hardness of the layer in steel 1Х13 (1Kh13), which has few carbides, is HV 100–180 higher than the hardness of the layer in steel Х12М (Kh12M), in which a significant portion of chromium is bound into carbides.

INFLUENCE OF PRE-OXIDATION ON THE NITRIDING PROCESS OF IRON ALLOYS