Maeda, S., Kobayashi, A., Shimizu, Y., Kanayama, M. et al., "Development of γ′-Fe4N Phase Control Technology and Low-Carbon Alloy Steel for High-Strength Nitrided Gear," SAE Int. J. Mater. Manf. 8(3):749-756, 2015, doi:10.4271/2015-01-0519.
A new nitriding technology and material technology have been developed to increase the strength of microalloyed gears. The developed nitriding technology makes it possible to freely select the phase composition of the nitride compound layer by controlling the treatment atmosphere.The treatment environment is controlled to exclude sources of supply of [C], and H2 is applied as the carrier gas. This has made it possible to control the forward reaction that decomposes NH3, helping to enable the stable precipitation of γ′-phase, which offers excellent peeling resistance. A material optimized for the new nitriding technology was also developed. The new material is a low-carbon alloy steel that makes it possible to minimize the difference in hardness between the compound layer and the substrate directly below it, and is resistant to decline in internal hardness due to aging precipitation in the temperature range used in the nitriding treatment. The combination of these developed technologies has increased gear strength more than 50% against conventional gas-nitrocarburized products.This research also determined that the peeling resistance of the compound layer changed with the thickness of the compound layer and the ratio between γ′-phase and ε-phase in the compound layer. This relationship with the thickness of the compound layer indicated that it would be possible to realize equivalent surface fatigue strength to carburizing treatments by controlling thickness. As a result, it was possible to realize equivalent surface fatigue strength to carburizing treatments while maintaining the characteristic of low and stable deformation offered by nitriding treatments.