Steel nitriding by atmospheric-pressure plasma jet using N2/H2 mixture gas

•We have developed nitriding under the atmospheric pressure using plasma jet.•We newly used N2/H2 mixture gas as the operating gas.•We discovered that an optimal H2 mixture ratio exists in plasma-jet nitriding.•We found that Fe2O3 formation is enhanced when H2 ratio is low.•We observed the optical emission intensity of NH to be prominent in the jet plume.

We have succeeded in hardening surface of tool steel with a new plasma-assisted nitriding treatment, in which the pulsed-arc plasma jet is generated with N2/H2 mixture gas and the jet plume is sprayed onto steel surface under atmospheric pressure. We discovered that the thickness of the formed hardened layer is varied by H2 mixture ratio of the operating gas. Moreover, there exists an optimal H2 ratio for nitriding, the value of which is 1% under our experimental condition. From experimental results of optical emission spectroscopy for the jet plume and X-ray diffraction from treated steel surface, we consider that the production of NH radicals and surface oxidation are possible to relate to the mechanism governing the dependence of nitriding quality on H2 ratio.