By analogy with carburising, electrochemical concentration cells with oxygen conducting solid electrolytes have proved their suitability for the control of oxynitriding atmospheres, both as oxygen probes and as nitriding probes using two cells. Their application to nitrocarburising atmospheres is currently under evaluation. The measurements and calculations described in the present paper make possible the development of computer aided control of gas nitriding atmospheres. Process control on the basis of probe signals provides a more precise and flexible approach to meeting the demands made on surface layers. It can prevent disturbances caused by uncontrolled oxygen intake andfacilitates a rational gas regime, thus contributing to process and product optimisation.

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The morphology of the nitride layer forming a t gas nitriding can decisively be influenced by the pretreatment of the surface. At a temperature of about 550 "C an activated surface very swiftly covers with iron nitrides. After passivating of the surface the nitride formation and the geometrical order of the nitride layer are determined by the heating rate. At high heating rates the formation of iron nitrides is localized to a few spots and the extension of the nitride layer proceeds by the growth of these spots. At low heating rates the surface is reactivated by the reducing gas before reaching the nitriding temperature. By further investigations is shown that nitriding of the surface starts with the formation of y'-nitrides.&-Nitrides occur in the further process of nitriding as a second layer on the y'-phase.Die Morphologie der sich bei der Gasnitrierung bildenden Nitridschicht kann durch die Vorbehandlung der Oberflache entscheidend beeinflu& werden. Eine aktivierte Oberflache bedeckt sich bei einer Temperatur von etwa 550 "C sehr rasch mit Eisennitriden. Nach einer passivierenden Behandlung entscheidet jedoch die Erwiirmungsgeschwindigkeit uber den dblauf der Nitridbildung und die geometrische Anordnung der Nitridschicht. Bei hohen Aufheizgeschwindigkeiten ist die Bildung von Eisennitriden auf wenige Stellen lokalisiert, und die Nitridschichtbildung lauft uber das Wachstum dieser Nitridinseln ab. Bei niedrigen Aufheizungsgeschwindigkeiten wird die Oberfliiche durch das reduzierende Gas noch vor Erreichen der Nitriertemperatur reaktiviert. I n weiteren Untersuchungen wird festgestellt, da13 sich auf der Oberfliiche zuerst ?'-Nitride bilden, auf die im Fortgang der Nitrierung die &-Nitride aufwachsen, wobei sich beide Phasen schichtartig ubereinander anordnen. Knowledge of the processes of covering the iron surface during gas nitridingIn the course of reaction between iron and ammonia the iron surface is covered by iron nitrides after exceeding the limit of nitrogen saturation. From the results obtained until now we can conclude that the first stages of nitride formation have a strong influence upon nitration as a whole and the resulting structure of material.According to KAPLINA and %JNKOVSKIJ ammonia decomposition occurs at certain adsorption sites of the metal surface; an electron exchange takes place and the nitrogen at the surface presents itself as a negatively charged ion in a chemisorbed state. The number of adsorption sites is determined by the periodicity of the crystal lattice. In relation to chemisorption there are a local dislocation of iron atoms and a vivid surface diffusion of nitrogen atoms. Therefore a spatial distribution being similar to that of the crystal lattice of iron nitrides can occur. At thermodynamically exposed sites these atomic orders are acting as nuclei for the incoherent nitride phase being formed in the further process of nitriding. Here we can conclude that the surface is not suddenly covered by a thin nitride film.

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