**1. Introduction**

Stainless steel was invented in 1915 as non-rusting steel with high chromium and nickel contents by the alloying design [1]. Since then, various kinds of stainless steels have been developed to improve their features in suitable to each application; e.g., austenitic, martensitic, ferritic, and two-phase stainless steels are widely

utilized in the present society [2]. As one of the high-strength austenitic stainless steels, AISI316 plates, bars, and wires have been widely utilized in industries [3]. In addition, it has a family of industrial grades such as AISI316L, low carbon AISI316, to improve the corrosion toughness [4] and AISI316LN, nitrogen-bearing AISI316, to improve the erosion and wear toughness [5]. In the history to develop these austenitic stainless steels, nitrogen was highlighted as its effective alloying element to reduce the amount of nickel consumption in fabrication. In this development of HNSS (High Nitrogen Stainless Steels), the role of nitrogen solute contents in the mechanical properties of stainless steels has been studied in [6]. In parallel to these studies on HNSS, various processes were developed to make nitrogen supersaturation to the γ-phase steels beyond their maximum nitrogen solubility. However, the nitrogen content in HNSS is still limited by 1 mass%. An increase of the nitrogen solute content in HNSS or nitrogen supersaturation is still a challenge to significantly improve the mechanical and functional properties of HNSS.

In parallel to R & D on HNSS, the nitriding process is another route to utilize this nitrogen in the surface treatment and modification of Fe-Cr base alloys and stainless steels [7]. The gas and liquid nitriding processes were first employed to form the thick nitrided layer with the use of the ammonia gas, the chloride ion, and cyanic liquids. The plasma nitriding process was gradually selected as an environmentally benign route of surface treatment instead of those processes [7, 8]. This plasma nitriding process is classified into two categories on the dependence of holding temperature and duration [8]; e.g., high-temperature plasma nitriding (HT-PN) with nitride precipitation into the nitrided layer for hardening, and lowtemperature plasma nitriding (LT-PN) with the nitrogen supersaturation into the depth of matrix for hardening, strengthening and improvement of wear/corrosion toughness.

**Figure 1** depicts this categorizing on the plasma nitriding of AISI316 stainless steels. Above the master curve, the CrN (Chromium Nitride) or iron nitride precipitation governs the hardening process by HT-PN of AISI316 steels. Those nitrides precipitate in the ΑΙSI316 matrix to strengthen the stainless steel; their surface hardness increases but their nitrided layer thickness decreases with increasing the chromium content [7]. The nitriding process is governed by the nitrogen bodydiffusion mechanism, so that the nitrogen solute content exponentially decreases from the surface to the depth [9], the nitrogen content at the surface is limited by the maximum solubility of 0.3 mass% [7], and the square of nitrided layer thickness is proportional to the holding duration [7, 10]. The metal chromium content decreases by synthesis of CrN in the nitrided layer and results in significant loss of corrosion toughness, intrinsic to AISI316. On the other hand, no nitrides are synthesized in the nitrided layer by LT-PN; nitrogen solute supersaturates the AISI316 matrix to form a thicker nitrided layer than 50 μm, to harden this layer up to 1400 HV, and to modify the microstructure of matrix at 673 K for 14.4 ks (or 4 h) [11–15]. Remember in HT-PN that 1) thinner nitrided layer is only formed in case of the high chromium contents, 2) surface hardness is limited by 1200 HV, 3) no significant change of microstructure is observed in the nitrided layer, and 4) microstructure below NFE remains the same as the original AISI316 before nitriding. In particular, the LT-PN at 673 K is characterized by the nitrogen supersaturation into the austenitic stainless steels with higher nitrogen content than 4 mass% and without iron and chromium nitride precipitates [13–17]. As studied in [12, 13], this nitrided layer improved the corrosion toughness of original AISI316 stainless steels. No chromium content was reduced even after the nitriding process due to the nitrogen supersaturation. Hence, LT-PN becomes a candidate processing to modify the AISI316 product surfaces to a HNSS layer with high nitrogen solute content and to improve its mechanical and functional properties.

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

**Figure 1.** *Two categories in the plasma nitriding process on the dependence on the holding temperature and duration.*

In the present paper, 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 efficient nitriding. AISI316 specimen is nitrided respectively at 673 K and 623 K under the optimized conditions to describe the nitrogen supersaturation process by using the multidimensional analysis. Macroscopic evaluation on the formation of the nitrided layer is performed by XRD and SEM–EDX. EBSD is employed to make a mesoscopic evaluation of the 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 through the nitrided layer. This synergic relation among these processes is common to every nitriding behavior in LT-PN, irrespective of the holding temperature and initial granular structure. 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. STEM analysis directly prove that the microstructure refining process is driven by the shear localization in the plastic straining, two-phase structuring is induced by the microscopic disturbance of nitrogen content with its different chemical compatibility to iron, nickel, and chromium in AISI316, and that plastic straining still modifies the crystalline structure at the absence of nitrogen solutes even below NFE. 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 zone-boundary diffusion processes. The homogeneous inner nitriding of initially fine-grained 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 high surface hardness and high loading capacity.
