Journal Published Online: 25 August 2017
Volume 6, Issue 4

Nitrogen Transport in Expanded Austenite Formed in Stainless Steels and CoCr Base Alloys



Expanded austenite is a very hard and wear resistant phase observed after nitrogen insertion into austenitic stainless steels and CoCr alloys and while maintaining their excellent corrosion resistance. Plasma nitriding, plasma immersion ion implantation (PIII), and low energy ion implantation (LEII), among others, are employed to form this phase in the temperature region of 350°C–450°C. The nitrogen supply rate does influence the transport with a transition from a supply limited to a diffusion limited mode is observed. This saturation shifts to higher current densities for higher process temperatures. In saturation, activation energies of 0.8 eV for the nitrogen diffusion is obtained for both classes of alloys. At elevated temperatures, the expanded phase starts to decay into CrN/Cr2N and a Cr-depleted matrix. Initially, a competition between forming these precipitates and long-range diffusion for reduction of concentration gradients is present. Without additional nitrogen insertion from the surface, the gradual decay of expanded austenite leads to very unusual nitrogen depth profiles. The phase transition is initiated from the surface, while the continuing nitrogen redistribution from the expanded phase towards the substrate dominates at the interface. For additional nitrogen insertion from the surface, a much more complex picture emerges. Whereas austenitic steels allow fast nitrogen diffusion with accelerating layer growth for longer times with faster transport through the formed Cr-free FeNi network, no additional nitrogen uptake occurs for CoCr alloys as the Cr-depleted Co matrix does not permit nitrogen transport. Thus, the layer thickness at lower temperatures can be larger than at higher temperatures in this case.

Author Information

Manova, D.
Leibniz-Institut für Oberflächenmodifizierung, 04318 Leipzig, Germany
Mändl, S.
Leibniz-Institut für Oberflächenmodifizierung, 04318 Leipzig, Germany
Pages: 25
Price: $25.00
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Stock #: MPC20160081
ISSN: 2379-1365
DOI: 10.1520/MPC20160081