The microstructures and corrosion properties of ferritic and austenitic stainless steels have been investigated after plasma and gas nitriding at 200-450° C. Little difference was observed between the surface layers produced by the two nitriding techniques: that on ferritic X6Cr17 consisted of ferrite plus epsilon-carbide, that on the austenitic X5CrNi18-10 was the expanded austenite S phase. Pitting resistance of X5CrNi18-10 was improved by nitriding at up to 420° C, but deteriorated for higher nitriding temperatures. The presence of epsilon-carbides in the layer on X6Cr17 slightly reduced pitting resistance for nitriding temperatures up to 440° C. Annealing the nitrided specimens degraded corrosion resistance by redistributing the nitrogen in the surface layer and promoting chromium nitride precipitation.
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.
KurzfassungCharakteristisch für Warmarbeitswerkzeuge ist eine individuelle werkzeugspezifische Beanspruchung und ein daraus resultierendes breites Spektrum an spezifischen lebensdauerbegrenzenden Schädigungen. Da die Schädigung vor allem auf die oberflächennahen Randbereiche beschränkt ist, eröffnet eine Randschichtbehandlung zahlreiche Möglichkeiten der Verbesserung ihres Gebrauchsverhaltens. Besonders bewährt hat sich eine Nitrierung der Werkzeuge. Für einen Einsatz auf Warmarbeitswerkzeugen sind Nitrierschichten durch ihre hohe Anlassbeständigkeit und Warmfestigkeit hervorragend geeignet. Beispiele veranschaulichen das hohe Potenzial der Nitrierung für die Erhöhung der Standmenge von Werkzeugen für die Warmumformung. Sie dienen der Erläuterung wesentlicher Zusammenhänge zwischen dem Aufbau von Nitrierschichten und dem Gebrauchsverhalten der Werkzeuge.
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