**6. Application of nitrided AISI316 to surgery wire**

A surgery wire requires high strength enough to apply the higher force for linkage of bones and tissues and for handling the miniature knife and tweezers. In addition, a surgery wire is demanded to have the high surface hardening enough to be free from damages and defects in mechanical troubles. Since the alloying elements for metallic medical parts and tools are strictly regulated, very few methods are useful to improve their mechanical properties and performance in practice. LT-PN is one of the most suitable methods to make surface treatment of AISI316 surgery wires without change of their constituent elements and element concentrations. In addition, the nitrogen supersaturated layer or NHSS layer has chemical inertness and sufficient corrosion toughness. No deterioration is expected to occur even by chemical polishing and pasteurization.

**Figure 19** depicts the experimental procedure from the preparation of finegrained AISI316 (FG-AISI317) wire to its uniaxial tensile testing. The FG-AISI316 wires with the average grain size of 1.5 μm, the diameter of 2.6 mm, and the length of 200 mm were prepared for LT-PN at 623 K for 14.4 ks. Other plasma processing conditions were the same as stated in Section 2. After nitriding, the side surface of wires is homogeneously nitrogen supersaturated to have a thick nitrided layer.

### **Figure 19.**

*An experimental procedure from the preparation of bare FG-AISI316 wire to the uniaxial tensile testing.*

Mesoscopic analysis with the use of SEM–EDX is performed to verify the homogeneous nitriding behavior. The uniaxial tensile testing system (AUTOGRAPH AGS-X 10 kN; Shimazu Co., Ltd., Tokyo, Japan).) is employed to measure the applied stress to stroke relationship both for bare and nitrided FG-AISI316 wires. This uniaxial loading was terminated when the maximum applied load reached 6 kN before fatal ductile fracture for microstructure analysis.

The FG-AISI316 wire was uniformly nitrided to have the nitrogen supersaturated layer with the thickness of 40 μm; the un-nitrided FGSS316 matrix is continuously capped by this nitrided surface layer. Since this nitrided layer thickness is nearly equal to 40 μm in the nitrided FGSS316 plates at 623 K in **Figure 14**, the nitrogen supersaturation process advances from the circumferential surface of wire to the depth in a similar manner to low-temperature nitriding in the FG-AISI316 plates.

**Figure 20** depicts the nitrogen solute mapping and IFP mapping in the circumferential and longitudinal cross-sections of nitrided FG-AISI316 wire, after uniaxial tensile loading. With respect to the nitriding mapping in both cross-sections, the nitrogen solute distribution is nearly the same as as-nitrided FG-AISI316 as compared between **Figures 12, 20a** and **c**. **Figure 20b** and **d** prove that the super-fine grained crystalline state with the two-phase structure in the nitrided FG-AISI316 is sustained during the uniaxial loading. To be noticed, the original equiaxed AISI316-matrix grains are elongated to be fibrous in the longitudinal direction as shown in **Figure 20d**. In addition, the FG-AISI316 matrix in the circumferential cross-section has bundle structure where each bundle of fibrous grains has specific crystallographic orientation as depicted in **Figure 20b**.

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

**Figure 20.**

*Microstructure of the nitrided FG-AISI316 after uniaxial tensile testing. a) Nitrogen mapping in the lateral cross-section, b) its IPF profile, c) nitrogen mapping in the longitudinal cross-section, and d) its IPF profile.*

This change of crystallographic structure in the FG-AISI316 matrix after uniaxial tensile loading, reveals that each original FG-AISI316 grain is elongated to fibrous grain by tensile loading and its crystallographic orientation is gradually rotated and aligned to the loading directions under the mechanical constraint by the nitrided layer. In particular, these fibrous grains are gradually sheared under this constraint to form a bundle structure with nearly the same crystallographic orientation. Let us investigate the effect of this crystallographic change in matrix on the mechanical properties of nitrided wire before and after uniaxial tensile loading.

**Figure 21** compares the hardness profile on the lateral cross-section of nitrided wire before and after loading. The nitrided layer before uniaxial loading has a hardness of 1400 HV in agreement with the average hardness of the lateral cross-section of the nitrided FGSS316 plate in **Figure 13a**. This hardness abruptly decreases from 1400 HV to 400 HV at the nitriding front end of 40 μm; this hardness of 400 HV becomes constant toward the center of the inner matrix. After uniaxial loading, the matrix hardness remains the same as 400 V; the work hardening is not enhanced by this uniaxial tensile loading to accumulate the inner strains. On the other hand, the high hardness in the nitrided layer is further enhanced to be 1600 HV. This increase of hardness in the nitrided layer by uniaxial loading only corresponds to the further microstructure evolution and phase transformation from γN-phase to αN-phase in the nitrided layer. That is, the microstructure change is locally induced in the nitrided layer by the applied plastic straining during the uniaxial loading.

The uniaxially applied stress (σapp) to stroke (δ) relationship is compared between the bare and nitrided FG-AISI316 wires in **Figure 22**. The stiffness (K) is defined as the average change of the applied stress to the measured stroke up to δ = 1 mm; e.g., K = σapp/δ. In the original FG-AISI316 wire, its stiffness becomes K0 = 500 MPa/mm before nitriding; while the nitrided FG-AISI316 wire has a slightly greater stiffness (KN) than K0; e.g., KN = 580 MPa/mm. The present nitrided wire is presumed as a composite of the nitrided layer with stiffness K1 and

**Figure 21.**

*Comparison of the hardness profile on the lateral cross-section of nitrided FG-AISI316 wires before and after the uniaxial tensile testing.*

### **Figure 22.**

*Comparison of the applied stress to stroke relationship between the bare and nitrided FG-AISI316 wires in the uniaxial tensile testing.*

the FG-AISI316 matrix with K0. After the rule of thumb for the stiffness of composite materials [29], KN is simply estimated by KN = (1 f) K0 + f K1, where f is the area fraction of the nitrided layer on the cross-section of the wire. In the present case, this f is only 6% because of the nitrided layer thickness of 40 μm of the wire lateral cross-section with a diameter of 2.6 mm. Assuming that K1 = (1600 HV/ 400 HV) K0, KN = 1.19 K0 590 MPa. This implies that the FG-AISI316 matrix in the nitrided wire is elastically constrained by the nitrided surface layer with higher hardness and stiffness similar to the lateral composite material.
