Carburizing of heat resistant steels in a controlled endothermic atmosphere

1. The optimal bending strength of high-alloy heat resistant steels 18Kh2N4VA, 14KhGSN2MF, 20Kh3MVF, and 20KhNV4MF after carburizing and heat treatment results from a carbon concentration of 0.9–1.1%; the optimal concentration is 0.8–0.9% for steels of the 18-25KhGT and 25KhGM types. 2. The highest contact fatigue strength of these steels is attained at a higher carbon concentration (1.3–1.4%) than for steels of the 12KhN3A type. 3. The highest heat resistance of the carburized steels investigated can be attained by increasing the surface carbon concentration to 1.4–1.5%. The heat resistance is highest for steel 20KhNV4MF, with a surface hardness of HRC 59–60 after tempering at 375–400°C. 4. In carburizing steels with a large amount of carbide-forming elements (chromium, tungsten, molybdenum, and vanadium) in a controlled endothermic atmosphere it should be kept in mind that these elements substantially increase the carbon concentration in γ-iron. With prolonged processing at elevated carbon potentials the amount of carbide phase in the surface increases greatly. Intensive carbide formation can be prevented by lowering the equilibrium concentration of carbon in γ-iron by 0.1–0.2%.