INFLUENCE OF OXYGEN ADDITIONS DURING GAS NITRIDING ON THE STRUCTURE OF THE NITRIDED LAYERS

The influence of controlled oxygen additions during gas nitriding of s teels on the structure and built-up of the nitrided layers as well as on the nitriding reactions was investigated with metallurgical methods and Mossbauer spectroscopy. The results show that oxygen additions be low the oxidising threshold slightly improve the nitriding behaviour o f unalloyed steels. Furthermore, it is shown that for alloyed steel, w hich are tending to passivation starting form chromium concentrations of 3%, the nitriding can be considerably improved by oxygen additions. The passivation layers present for these steels are destroyed by exte rnal oxidation during the oxinitriding process. This external oxidatio n and the internal nitriding occurred parallel in the early stages of the oxinitriding.