Saturation of stainless and structural steels with hydrogen during nitriding

Conclusions

• 1. In the process of gas nitriding, all the stainless and structural steels tested are saturated with hydrogen; the concentration of diffusionally mobile hydrogen in samples 8 mm in diameter and 20 mm long varies from 5×10⁶ to 20×10⁻⁶ m³/100 g of metal. Most of the hydrogen is concentrated in a thin surface layer (∼0.05 mm) and amounts to 350×10⁻⁶ m³/100 g of metal, compared with (20–40)×10⁻⁶ m³/100 g of metal in the remainder of the case; hydrogen does not diffuse into the core.

• 2. Tempering for 10 h in a vacuum furnace or in dissociated ammonia not containing atomic hydrogen leads to a reduction of the hydrogen concentration in the case by a factor larger than 10, an increase in the corrosion resistance of nitrided stainless steels, and lower brittleness of the nitrided case.

• 3. Diffusionally mobile hydrogen has no effect on the mechanical properties of nitrided samples: The reduction in cross-sectional area and susceptibility to delayed fracture under constant static load remain unchanged in tests of samples with a sharp notch (r=0.1 mm).

• 4. The reduction of the hydrogen concentration leads to formation of a high-quality passive film on the surface of the sample, which improves the corrosion resistance of the steels.