Low-Temperature Gas-Phase Nitriding and Nitrocarburizing of 316L Austenitic Stainless Steel

<p>Low temperature paraequilibrium nitriding is an effective method to enhance surface hardness and corrosion resistance in austenitic stainless steels, provided that equilibrium nitride formation is suppressed. Following the standard double HCl “activation&rdquo; procedure developed by Swagelok Company to remove the passivating Cr2O3-rich native oxide, nitriding was done in a gas mixture of NH3/H2/N2. Three processing parameters (nitriding temperature, nitriding activity and duration) were controlled independently to understand both thermodynamic and kinetic aspects of the process.</p><p>Supersaturated nitrogen interstitials (7 ~ 25 at.%) were introduced into 316L stainless steel samples, which yielded a lattice expansion ranging from 1% to 10%. Room temperature ferromagnetism in expanded austenite in stainless steels was then induced due to the great increase in Fe-Fe interatomic distance. A combined of XRD, MFM and EBSD study revealed that the minimum lattice expansion required for ferromagnetism is ~ 5%. A nitrogen content of ~ 14 at.% was estimated (by AES) as the threshold required for the paramagnetic-to-ferromagnetic transition. The correlation of lattice parameter expansion and nitrogen content indicates that transition from paramagnetic austenite to ferromagnetic austenite played a role in the highly distorted lattice parameters of nitrogen-enriched expanded austenite.</p><p>Orientation-dependent nitrogen surface concentration and case depth were investigated using EBSD orientation mapping and AES cross-sectional line scans. In particular, <100>-oriented grains demonstrated a higher surface nitrogen concentration and a deeper case depth as compared to <111>-oriented grains. </p><p>Three different scenarios of low-temperature gas-phase nitrocarburizing processes were designed and compared. Dual-layered expanded austenites were obtained. The concentration depth profiles of nitrogen and carbon atoms can all be described as an outer layer of nitrogen-enriched region, with carbon atoms accumulated at the diffusion front of nitrogen. The total case depth obtained is mainly determined by the diffusion time of carbon. Grazing angle XRD and TEM were employed to study the precipitates formed after nitrocarburizing.</p>

Name thesis:degree_name

Doctor of Philosophy

Level thesis:degree_level

doctoral

Discipline thesis:degree_discipline

Materials Science and Engineering

Grantor dc:publisher

Case Western Reserve University School of Graduate Studies

Year dc:date

2013

Author dc:creator

• Wu, Dandan

Contributors dc:contributor

• Heuer, Arthur

• Materials Science

Statement dc:rights

• unrestricted
• This thesis or dissertation is protected by copyright: all rights reserved. It may not be copied or redistributed beyond the terms of applicable copyright laws.

Language dc:language

English

Repository record dc:identifier

http://rave.ohiolink.edu/etdc/view?acc_num=case1346900583

OAI identifier oai:identifier

oai:etd.ohiolink.edu:case1346900583