Carburizing in a nitrogen-based mixture with additives of pure methane

Conclusions

• 1. Use of a special furnace design with a built-in centrifugal fan that provides for circulation of the gaseous mixture at a rate of approximately 2 m/sec at a low gas-consumption rate makes it possible to conduct carburizing in a N₂−H₂−CH₄ mixture without the formation of soot on the surface of the components.

• 2. In saturating a commercial-iron foil up to 45 μm thick with carbon in a gaseous N₂−H₂−CH₄ mixture to a content that exceeds its solubility in austenite, a thin layer of cementite forms at the surface of the foil. A further extension of the carburizing time has virtually no effect on the thickness of the cementite layer and gives rise to the formation of a pyrocarbon film on the surface of the foil.

• 3. A carburized layer of appreciable thickness (about 1.0 mm after isothermal holding for 4 h at 930°C) and with a smoother concentration profile than that obtained with two-stage carburizing in other gaseous media forms during the carburizing of steel 18Kh2N4MA specimens in an N₂−H₂−CH₄ medium.

• 4. Control of the carburizing process is facilitated with the use of an N₂−H₂−CH₄ mixture. For a constant C_g value, control is, by nature, reduced to maintenance of the methane concentration in the mixture and to selection of the carburizing time.