Thermodynamics, Kinetics and Microstructural Evolution of the Compound Layer; a Comparison of the States of Knowledge of Nitriding and Nitrocarburising

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    Abstract

    Ferritic nitrocarburising comprises a thermochemical surface treatment in which nitrogen and carbon are supplied simultaneously to a steel surface at a temperature between 823 and 853 K (550-580 ¢XC). Provided that the nitrogen and carbon activities imposed by the nitrocarburising agent on the steel surface are sufficiently high, a compound layer is formed at the surface of a ferritic steel, which consists predominantly of ? and/or ?' phases. In the region underneath the compound layer, the so-called diffusion zone develops. The compound layer has an interesting combination of wear and atmospheric corrosion performance. The diffusion zone brings about an improvement of the endurance limit as compared to an untreated component. Hence, nitrocarburising is perhaps the most versatile surface treatment for ferritic steel and has a potential for wide application. From the literature on nitrocarburising an incomplete, fragmentary picture emerges of the relation between nitrocarburising conditions, microstructure development and the properties/performance of nitrocarburised workpieces. This hinders modelling of the process-microstructure-property relations and thereby optimisation of the process conditions for tailoring a certain combination of properties. The present paper describes aspects of the thermodynamics, kinetics and microstructure evolution of the compound layer on pure iron during nitrocarburising, by comparing the current status of qualitative understanding with that for nitriding, which is largely quantitative. Iron was taken as a model case in order to assess the actual effect of the interstitial elements nitrogen and carbon provided by the nitriding and nitrocarburising gases.
    Original languageEnglish
    JournalHeat Treatment of Metals
    Volume2000.4
    Pages (from-to)92-102
    ISSN0305-4829
    Publication statusPublished - 2000

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