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Institute ofScience and Technology ， 2200 − 2 Toyosawa， Fukuroi 437 − 8555 Plasma nitriding for hardening steel surface is achieved by spraying thermal pulsed − arc plasma jet under atmospheric − pressure N21H2 mixture ， The quality of nitriding is found to depend on ！ he H2 fiow rate ， which has an optimal value ． We prQpose a simple mechanism of this dependence． Moreover， the amount of H2 necessary for best nitriding can be decreased to l120 by changing the way ofH2 addition 、 Key Mords ： plasma nitriding ， case hardening ， pulsed −arc plasmajet ， atmospheric − pressure thermal plasma