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 s tudy the effect of hydrogen on the growth of the 'expanded austenite' layer . The layers thus produced have been characterised using cross-sectional sc anning electron microscopy, X-ray diffraction and instrumented microhardnes s measurements; the plasmas were studied by optical emission spectroscopy. The treated layer shows higher resistance to acid etching than the bulk mat erial. Provided that the partial pressure of nitrogen is held constant, the addition of hydrogen at concentrations in the range 5-50% results in thick er nitrided layers and enhanced surface hardness compared with treatments i n pure nitrogen. An excessive amount of hydrogen (similar to 75%), on the o ther hand, retards the nitriding process. Optical spectroscopy indicates th at the addition of hydrogen does not increase the concentration of active n itriding species, although mass spectroscopy shows the presence of NH1-4. T he beneficial effect of hydrogen is therefore due to the action of hydrogen atoms and molecules at the surface of the workpiece. (C) 2000 Elsevier Sci ence S.A. All rights reserved.