**7. Discussion**

In HT-PN, the nitride layer of AISI316 was characterized by fine precipitation of ion and chromium nitride into the AISI316 matrix without its microstructure

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

modification [7–10]. In bulk HNSS, the AISI316 matrix was supersaturated by nitrogen solute with is low content to stabilize the g-phase structure [6]. LT-PN of AISI316 substrate is characterized by its multi-dimensional structure. In the macroscopic view, the homogeneously nitrided AISI316 plates and wires at 673 K and 623 K have a nitrogen supersaturated layer or HNSS layer with an average nitrogen content of 4 mass% and an average hardness of 1400 HV. The initial γ-phase structure changes to be γN-/αN, two-phase structure. Through the mesoscopic view, the nitrogen supersaturation process with two-phase structuring in the macroscopic view is further understood to investigate the synergic role among the nitrogen superstation, the plastic straining, the two-phase formation, the microstructure refinement, and the nitrogen boundary-diffusion. EBSD analysis on the heterogeneously nitrided matrix below NFE reveals that this synergic process advances to the localized grains near the nitrogen diffusing boundaries. This heterogeneous process is changed to be homogeneous by increasing the holding temperature and refining the initial grain size. In particular, the nitrided layer with the plateau of high nitrogen content by 4 mass%, the high hardness of 1400 HV, the fine twophase microstructure, and the high plastic strains advances into the depth by the assistance of nitrogen boundary diffusion network in LT-PN of AISI316 with refined grain size. Microscopic analysis reveals that the nitrided layer consists of the g-phase single crystals with the grain size of 5 nm and the g-phase Cr (N) – a-phase Fe/Ni (N) poly-crystals. This single crystal is yielded by plastic straining to have fine (111)-oriented grain boundaries in parallel to the easiest slipping directions. High KAM zones or highly plastic-strained zones in EBSD analysis correspond to these single crystals. Nitrogen solute and dislocations induced by plastic straining are not housed in these single crystals but they transport through these crystals to the depth of the matrix. Fine two-phase structure in IPF profile by EBSD analysis also corresponds to this fine poly-crystal of Cr (N) and Fe/Ni (N) zones. This fine phase separation by the chemical compatibility of nitrogen solute among Cr, Fe, and Ni proves that nitrogen supersaturation into AISI316 induces the phase separation from g-phase AISI316 to two distinct phases with a locally neighboring system of nitrogen-rich and nitrogen-poor zones. A very fee study [30] in the literature reports that phase-separation selectivity is enhanced by nitrogen doping. In the present nitrogen-induced phase separation, high nitrogen solute content with its average of 4 mass% disturbs locally with separation to nitrogen-rich and –poor phases. Remembering the IPF profile in **Figures 8, 10, 14**, and **20**, where this twophase structure forms a thick layer from the surface to the depth of nitrided AISI316. This local phase separation and nitrogen disturbance occur in a large volume with the advancement of nitrogen supersaturation and diffusion into the depth. The fine network of nitrogen zone- and grain-boundary diffusion paths in **Figures 16** and **17** sustains this large-scaled phase separation and nitrogen solute disturbance.

Localization of the nitrogen supersaturation and diffusion below NFE in **Figures 8** and **9**, and plastic straining effect on the formation of subgrains below NFE in **Figure 18**, also teaches the important role of synergic relation of plastic straining to nitrogen supersaturation and diffusion.

The mode-change from heterogeneous to homogeneous nitriding processes is controlled by refining the initial grain size of AISI316 substrates. In HT-NT, the nitriding behavior is sensitive to the holding temperature and chromium content; it has nothing to do with the microstructure. No studies were reported in the literature on LT-PN; This initial grain size effect on the inner nitriding is essential to understand the nitriding mechanism at low temperature. In the previous studies on HT-PN and LT-PN, the inner nitriding process is mainly dependent on the nitrogen body-diffusion process [7, 9]; hence, both HT-PN and LT-PN were thought to have nothing to do with the grain size in the microstructure. The mode-change by refining the initial grain size implies that the nitrogen solute at the lower holding temperature does not diffuse through the lattices in microstructure but diffuse through the zone and subgrain boundary network.

Assuming that each grain geometry with its size of D is modeled by a sphere with the diameter of D, its surface area is represented by πD2 . Let us calculate the number (N) of grains in a unit cubic cell with the edge length of L; e. g., N = (L/D)3 . Then, the grain boundary area (A) in the cubic cell is estimated by A = πD2 N = πL3 /D. When the initial grain size is refined from D0 to D1, the grain boundary extension rate (Er) is calculated by (D1/D0). In the present experiment, D0 = 15 μm and D1 = 1.5 μm; Er becomes 10, or the initial grain boundary extends to be 10 times larger than the original AISI316. This enlargement of the grain boundary diffusion network area is responsible for the initial grain effect on the mode change.

Once the homogeneous nitriding is triggered by this initial grain refining, this nitriding process is sustained by the synergic relation among the nitrogen supersaturation, the plastic straining, the two-phase structuring, the microstructure refining, and the nitrogen boundary diffusion. To be remembered in [15, 16, 22, 23], the textures in the intensely rolled stainless steels completely disappeared through LT-PN. The pre-existing refinement in the granular structure of substrates is a key point to make sustainable and homogeneous nitriding of stainless steels. Other factors, mechanically induced by pre-straining, have nothing to do with nitriding behavior. This suggests that pre-forging provides yield the refined surface specimen for this homogeneous and sustainable LT-PN.

The metallic parts and tools in the medical application require various engineering items to improve their performance in practical use. The nitrided austenitic stainless steels have superior chemical inertness and corrosion toughness to the bare AISI316 products in addition to their high hardness. As reported in [31, 32], the nitrided nickel-free and martensitic stainless steels have sufficient corrosion toughness even in NaCl and etching solutions. Furthermore, the nitrided layer of AISI316 has enough machinability to be finished by using the PCD (Poly-Crystalline Diamond) - chipped and CBN (cubic boron nitride) – chipped milling tools [33–35]. The NHSS layer of nitrided AISI316 products can be precisely finished to have functional surfaces and interfaces owing to this feature of nitriding.

AISI316 has many derivatives such as AISI316L and AISI316LN. The former is selected and used as a high-strength sheet for piercing and metal-forming to fuel injection orifices [36]. The latter is also utilized as a structural component in the design of experimental fusion reactors [37]. Instead of those traditional alloying designs on AISI316L, the nitrogen can be utilized to reduce the effect of carbon interstitials to the mechanical performance by the nitrided AISI316 layer. AISI316LN can be exchanged by the nitrided AISI316 to prevent the structural component surface from the severe damages by spontaneous emission of particles. NHSS-layered AISI316 works as a structural member to be working instead of AISI316L and AISI316LN. Further studies are necessary to describe the interstitial solute as an alloying element in the redesign of AISI316 derivatives in its family.
