The effect of hydrogen on the growth of the nitrided layer in r.f.-plasma-nitrided austenitic stainless steel AISI 316

Austenitic stainless steel AISI 316 has been nitrided by low-temperature r.f. plasmas containing various nitrogen–hydrogen gas mixtures, in order to study the effect of hydrogen on the growth of the ‘expanded austenite’ layer. The layers thus produced have been characterised using cross-sectional scanning electron microscopy, X-ray diffraction and instrumented microhardness measurements; the plasmas were studied by optical emission spectroscopy. The treated layer shows higher resistance to acid etching than the bulk material. Provided that the partial pressure of nitrogen is held constant, the addition of hydrogen at concentrations in the range 5–50% results in thicker nitrided layers and enhanced surface hardness compared with treatments in pure nitrogen. An excessive amount of hydrogen (∼75%), on the other hand, retards the nitriding process. Optical spectroscopy indicates that the addition of hydrogen does not increase the concentration of active nitriding species, although mass spectroscopy shows the presence of NH1–4. The beneficial effect of hydrogen is therefore due to the action of hydrogen atoms and molecules at the surface of the workpiece.