Control of plasma nitriding using oxygen probes

It has been shown by experiments, that a recently developed vacuum-tight oxygen probe enables the direct control of the oxidising action of plasma nitriding atmospheres. In connection with this, the ‘oxygen-sensitivity’ of steels with different chromium content has been investigated according to the variation of the hydrogen and amount of oxygen in the nitriding atmosphere. Threshold oxygen partial pressures for the transition from nitriding to oxidising were evaluated. These critical partial pressures decrease with increasing chromium content. The oxygen probe measurements confirm the influence of further parameters like temperature, pumping time, leakage and history of the facility (time interval under atmospheric pressure) on the oxygen partial pressure and thus on the nitriding results. Measuring the oxidising potential by an oxygen probe offers new possibilities in controlling and optimising plasma nitriding in commercial facilities. This is of special importance for the treatment of high alloyed steels and non-ferrous materials like aluminium.