**8. Conclusion**

The high-density plasma nitriding process is redesigned by using the plasma diagnosis to be working under the optimum conditions. In particular, OES (Optical Emissive-light Spectroscopy) is employed to search for the most suitable nitrogen– hydrogen gas flow rate to attain the higher yield of N2 <sup>+</sup> ions and NH radicals for low-temperature nitriding. AISI316 specimen is nitrided at 673 K and 623 K under

*Nitrogen Supersaturation of AISI316 Base Stainless Steels at 673 K and 623 K… DOI: http://dx.doi.org/10.5772/intechopen.102387*

the optimized conditions to describe the nitrogen supersaturation process by using multidimensional analysis.

Macroscopic evaluation on the formation of the nitrided layer is performed by XRD and SEM–EDX. The nitrogen supersaturation process with its higher content than 4 mass% induces the γ-lattice expansion and forms a plateau in the nitrogen content depth profile. This lattice expansion only occurs at the nitrogen diffusing zones so that the strains become incompatible between the nitrogen supersaturated and unsaturated zones. The unsaturated zones are plastically strained to distort their constituent grains.

EBSD is employed to make a mesoscopic evaluation on holding temperature and initial grain-size effects on the nitriding behavior in the nitrided layer and below the NFE. The plastic straining, the microstructure refining, and the two-phase structuring advance in synergy with nitrogen supersaturation and diffusion processes through the nitrided layer. This synergetic relation among these processes is observed in the heterogeneous nitriding process at 623 K. The microstructure refinement and two-phase structuring co-work, at the vicinity of localized nitrogen diffusion paths, with the nitrogen supersaturation and diffusion processes. This synergic working among four processes is the same as that in homogeneous nitriding. Through this localized nitrogen diffusion and supersaturation processes, the matrix microstructure far below NFE is also modified heterogeneously.

This heterogeneous nitriding turns to be homogeneous with increasing the holding temperature from 623 K to 673 K since the nitrogen diffusion rate is enhanced to reduce the localization behavior. To be noticed, the synergic process in the above is common to those heterogeneous and homogeneous nitriding processes. By refining the initial grain size of AISI316 before nitriding, this heterogeneous nitriding at 623 K also turns to be homogeneous even at 623 K. In addition, no localization in nitrogen supersaturation and diffusion occurs below NFE. The microstructure below NFE becomes the same as the original matrix of the original grain refined AISI316. This implies that the localization in nitrogen supersaturation and diffusion processes is suppressed by the enlargement of nitrogen diffusion paths.

Microscopic analysis with the use of STEM is employed to describe the microstructure refining and two-phase structuring in the nitrided AISI316 at 673 K. A single-crystal and poly-crystal like zones are formed at the vicinity of its surface. STEM analysis on the former reveals that the g-phase grain is much refined to have the size of 5 nm and the orientation of (111) by the synergic process in nitriding. The original coarse grain is nitrogen-supersaturated with its high nitrogen content and completely simple-sheared in (111)-direction by plastic straining to form very fine single-grain with no dislocations and nitrogen solutes left in its inside. STEM analysis discloses the two-phase structuring mechanism. After the phase mapping in EBSD analysis, this two-phase structure is only defined as a fine mixture of αand γ-grains. The nitrogen-rich zone with more compatibility to chromium forms one phase while the nitrogen-poor zone with less compatibility to iron and nickel becomes another phase. In addition, these two-phase poly-crystals formed in nearest neighboring to the γ-phase single crystals. Furthermore, the STEM analysis on the microstructure below NFE proves that the sheared polycrystalline grains by the plastic straining are formed even in the absence of nitrogen solute.

The above multidimensional analysis demonstrates that low-temperature plasma nitriding is driven by the synergic relation of plastic straining, microstructure refining, and two-phase structuring with the nitrogen supersaturation and zoneboundary diffusion processes. The homogeneous inner nitriding of initially finegrained AISI 316 is self-sustained by this synergic effect not to ignite the localization in nitrogen supersaturation even at 623 K. This self-sustainable nitriding is

attractive to make surface treatment of stainless steel medical parts. Especially, the nitrided AISI316 wire is straightforwardly utilized as a surgery wire by its uniform surface hardness and high loading capacity.

Instead of the conventional alloying design such as AISI316L and AISI316LN, this self-sustainable nitriding provides a new way of high nitrogen structured AISI316 to industrial and medical applications.
