Hydrogen-Free Plasma Nitriding Process for Fabrication of Expanded Austenite Layer on AISI 316 Stainless Steel Surface.

The addition of hydrogen to nitrogen facilitates the formation of nitride phases in the plasma nitriding processes of stainless steels, though it also induces the deterioration of their mechanical properties. This study presents a hydrogen-free plasma nitriding process for fabricating a nitrogen-expanded austenite phase (γ) on an AISI 316 stainless steel surface. The steel substrate was nitrided in N-Ar plasma with various gas compositions discharged by radio frequency (RF) and direct current (DC) modes.

Quantitative Analysis of Nitrogen by Atom Probe Tomography Using Stoichiometric -FeN Consisting of N Isotope.

The nitrogen deficiency in steels measured by atom probe tomography (APT) is considered to arise from the obscurement of singly charged dimer nitrogen ions (N2+) by the iron-dominant peak (56Fe2+) at 28 Da. To verify this by quantifying the amount of N2+ ions, γ′-Fe4N consisting of the 15N isotope was prepared on iron substrates by plasma nitriding using a nitrogen isotopic gas (15N2). Although considerable amounts of 15N2+ were observed at 30 Da without overlap with any iron peak, the observed nitrogen concentrations of γ′-Fe4N were clearly lower than the stoichiometric composition (19–20 at%), using both pulsed voltage and pulsed laser atom probes.