**4. Nanostructuring by low temperature plasma nitriding**

Lower temperature plasma nitriding process of austenitic stainless steels than 700 K is governed by the nitrogen supersaturation with nitrogen interstitial occupation of octahedral vacancy sites of fcc-structured supercells as well as the nitrogen diffusion through refined grain boundaries and slipping lines [13–15]. FGSS316 plates and wires are employed to describe the nano-structuring process with grain size refinement by this plasma nitriding.

#### **4.1 Low temperature plasma nitriding**

High density RF (Radio Frequency) – DC (Direct Current) discharging plasma nitriding system is utilized to generate the nitrogen-hydrogen plasmas. **Figure 8** illustrates a typical hollow cathode device for homogeneously nitriding a single FGSS316 wire. RF-nitrogen/hydrogen plasma is ignited to surround the wire surface by a cylindrical plasma sheath. The activated nitrogen atoms (N\*) and ions (N+ ) as well as the NH radicals are enriched in this sheath to increase the nitrogen ion density up to 1 × 1018 ions/m3 . Under this plasma processing condition, the nitrogen solute diffuses into the depth of FGSS316 matrix in wire to form the nitrided layer.

The inner nitriding process with nitrogen supersaturation is described by the multi-dimensional relation in **Figure 9**. When some nitrogen interstitial atoms diffuse and supersaturate the fcc-structured lattices in **Figure 9a**, their original lattice constant (Λ0) increases to Λ by occupation of nitrogen interstitials into the octahedral vacancy sites in them. Other nitrogen atoms diffuse to further depth through the grain boundaries. Then, the nitrogen supersaturated (NS) zone expands with the elastic strain (εe) while unsaturated (US) zone does not deform; the strain incompatibility occurs on the boundary between NS and US zones. The misfit distortion (βmisfit) is induced along this zone boundary as depicted in **Figure 9b**; e.g., βmisfit = ωspin + εp, where ωspin is a spin tensor to rotate the NS zone and to generate the crystallographic misorientation into a current zone, and εp, a plastic strain tensor to compensate for the strain incompatibility across the zone boundary. Nitrogen solutes further diffuse into the depth of grains through these zone boundaries, as shown in **Figure 9c**.

Through this multi-dimensional inner nitriding, high elastic strain energy density in NS zones drives the phase transformation from austenite to martensite. The spin rotation of zones and sub-grains advances with nitrogen diffusion and

#### **Figure 8.**

*Experimental setup for low temperature plasma nitriding of steel wires with use of the hollow cathode device. The same DC-bias was applied to the hollow and the FGSS316 wire.*

**63**

**Figure 9.**

*temperature plasma nitriding.*

*Micro-/Nano-Structuring in Stainless Steels by Metal Forming and Materials Processing*

supersaturation to refine the crystallographic structure. NS zones accompany with

*Multi-dimensional relation in the nitrogen interstitial atom diffusion and supersaturation in the low* 

This theoretical model is experimentally demonstrated in the following. As stated in [13–15], the inner nitriding advances homogeneously from the surface to the nitriding front end; it is rather difficult to experimentally describe each fundamental process separately from other processes in **Figure 9**. A normal-grained AISI316 plate is employed as a work material and nitrided at 623 K for 14.4 ks to decelerate the nitrogen diffusion rate and to describe the synergetic relation among the nitrogen supersaturation, the phase transformation, the plastic straining, the

**Figure 10** depicts the homogeneous and heterogeneous inner nitriding processes in the nitrided AISI316 at 623 K for 14.4 ks. Under this nitriding condition, the high nitrogen solute content, [N], around 5 mass% is present down to the nitriding front end (NFE) at the depth of 30 μm. To be noted, [N] remains to be 1 mass % even below NFE, as shown in **Figure 10a**. This implies that homogeneous nitriding advances to NFE and changes to be heterogeneous by localization in nitrogen diffusion below NFE. In fact, the phase mapping as well as the crystallographic structure changes drastically across

this NFE, which was indicated by the gray two-dots chain line in **Figure 10**.

the plastic strain distribution along the NS-US zone boundaries.

grain size refinement and the local nitrogen diffusion.

**4.2 Microstructure evolution by inner nitriding in depth**

*DOI: http://dx.doi.org/10.5772/intechopen.91281*

*Micro-/Nano-Structuring in Stainless Steels by Metal Forming and Materials Processing DOI: http://dx.doi.org/10.5772/intechopen.91281*

#### **Figure 9.**

*Electron Crystallography*

size refinement by this plasma nitriding.

**4.1 Low temperature plasma nitriding**

density up to 1 × 1018 ions/m3

shown in **Figure 9c**.

**4. Nanostructuring by low temperature plasma nitriding**

Lower temperature plasma nitriding process of austenitic stainless steels than 700 K is governed by the nitrogen supersaturation with nitrogen interstitial occupation of octahedral vacancy sites of fcc-structured supercells as well as the nitrogen diffusion through refined grain boundaries and slipping lines [13–15]. FGSS316 plates and wires are employed to describe the nano-structuring process with grain

High density RF (Radio Frequency) – DC (Direct Current) discharging plasma nitriding system is utilized to generate the nitrogen-hydrogen plasmas. **Figure 8** illustrates a typical hollow cathode device for homogeneously nitriding a single FGSS316 wire. RF-nitrogen/hydrogen plasma is ignited to surround the wire surface by a cylindrical plasma sheath. The activated nitrogen atoms (N\*) and ions (N+

as well as the NH radicals are enriched in this sheath to increase the nitrogen ion

solute diffuses into the depth of FGSS316 matrix in wire to form the nitrided layer. The inner nitriding process with nitrogen supersaturation is described by the multi-dimensional relation in **Figure 9**. When some nitrogen interstitial atoms diffuse and supersaturate the fcc-structured lattices in **Figure 9a**, their original lattice constant (Λ0) increases to Λ by occupation of nitrogen interstitials into the octahedral vacancy sites in them. Other nitrogen atoms diffuse to further depth through the grain boundaries. Then, the nitrogen supersaturated (NS) zone expands with the elastic strain (εe) while unsaturated (US) zone does not deform; the strain incompatibility occurs on the boundary between NS and US zones. The misfit distortion (βmisfit) is induced along this zone boundary as depicted in **Figure 9b**; e.g., βmisfit = ωspin + εp, where ωspin is a spin tensor to rotate the NS zone and to generate the crystallographic misorientation into a current zone, and εp, a plastic strain tensor to compensate for the strain incompatibility across the zone boundary. Nitrogen solutes further diffuse into the depth of grains through these zone boundaries, as

Through this multi-dimensional inner nitriding, high elastic strain energy density in NS zones drives the phase transformation from austenite to martensite. The spin rotation of zones and sub-grains advances with nitrogen diffusion and

*Experimental setup for low temperature plasma nitriding of steel wires with use of the hollow cathode device.* 

*The same DC-bias was applied to the hollow and the FGSS316 wire.*

. Under this plasma processing condition, the nitrogen

)

**62**

**Figure 8.**

*Multi-dimensional relation in the nitrogen interstitial atom diffusion and supersaturation in the low temperature plasma nitriding.*

supersaturation to refine the crystallographic structure. NS zones accompany with the plastic strain distribution along the NS-US zone boundaries.

This theoretical model is experimentally demonstrated in the following. As stated in [13–15], the inner nitriding advances homogeneously from the surface to the nitriding front end; it is rather difficult to experimentally describe each fundamental process separately from other processes in **Figure 9**. A normal-grained AISI316 plate is employed as a work material and nitrided at 623 K for 14.4 ks to decelerate the nitrogen diffusion rate and to describe the synergetic relation among the nitrogen supersaturation, the phase transformation, the plastic straining, the grain size refinement and the local nitrogen diffusion.

#### **4.2 Microstructure evolution by inner nitriding in depth**

**Figure 10** depicts the homogeneous and heterogeneous inner nitriding processes in the nitrided AISI316 at 623 K for 14.4 ks. Under this nitriding condition, the high nitrogen solute content, [N], around 5 mass% is present down to the nitriding front end (NFE) at the depth of 30 μm. To be noted, [N] remains to be 1 mass % even below NFE, as shown in **Figure 10a**. This implies that homogeneous nitriding advances to NFE and changes to be heterogeneous by localization in nitrogen diffusion below NFE. In fact, the phase mapping as well as the crystallographic structure changes drastically across this NFE, which was indicated by the gray two-dots chain line in **Figure 10**.

**Figure 10.**

*Experimental demonstration on the inner nitriding process. (a) Nitrogen content depth profile, (b) IPF mapping, phase mapping and KAM distribution, and (c) inner nitriding mechanism.*

Due to EBSD analysis, the synergetic relation is described by the inverse pole figure, the phase mapping and the plastic strain distribution below NFE in **Figure 10b**. The coarse grains near the grain boundary were only nitrided to change their microstructure; e.g., the austenitic and martensitic zones exclusively distribute in an A-grain. The retained austenitic zones in this grain had higher plastic strains (or higher KAM) and different crystallographic orientations from the martensitic zones, the plastic strains of which were much lower than these austenitic ones. This formation of γ- and α'-zone mixture with plastic straining and crystallographic rotation just corresponds to the multi-dimensional inner nitriding process. As depicted in **Figure 10c**, the nitrogen diffusion localizes across NFE so that the nitrogen selectively diffuses along the original grain boundary below NFE. This nitrogen main stream branches into the neighboring grains to this gran boundary. In each grain, the nitrogen further diffuses through the zone boundaries to drive the phase transformation, the plastic straining, and the refinement of zone sizes. Next, more precise EBSD analysis is made on this A-grain.

As compared between **Figure 11a** and **b**, little plastic strains were detected in the α'-zone of A-grain while the surrounding γ-zones were much plastically strained.

**65**

nitriding.

**Figure 11.**

*(b) KAM distribution, and (c) IPF mapping.*

*Micro-/Nano-Structuring in Stainless Steels by Metal Forming and Materials Processing*

This proves that some of NS-zones massively transform to α'-zones since the elastic strain energy density reaches to the critical level and that plastic strains are induced only in the neighboring γ-zones to α'-zones to compensate for misfit strain between NS-zones and surrounding unsaturated γ-zones. Comparing **Figure 11b** and **c**, these highly strained zones are just corresponding to crystallographically refined zones. This assures that spin rotations are induced together with the plastic strains by misfit distortion on the distributed zone boundaries to refine each zone size. Owing to this synergetic mechanism, the refined zones have mutual boundaries with high misorientation angles. This grain size refinement reflects on the high strength and hardness of the nitrided layer by the low temperature plasma

*Synergetic relation in the inner nitriding at 623 K, locally observed in the grain-A. (a) Phase mapping,* 

As had been discussed in [29, 30], the initial grain size of AISI316 has influence on the inner nitriding behavior; homogeneity in nitriding process is enhanced with reduction of initial grain size. This finding suggested that fine-grained AISI316 or FGSS316 structural components and parts could be homogeneously nitrided on the surface with a little effect of heterogeneous nitriding on the microstructure below NFE. In the following, a FGSS316 wire is nitrided at 623 K for 14.4 ks and its microstructure is precisely analyzed to describe the formation of nitrided layer as well as the microstructure evolution at the surface and below NFE during this low temperature nitriding. **Figure 12** depicts the nitrogen solute mapping, phase mapping, and crystallographic structure on the lateral and longitudinal crosssections of nitrided FGSS316 wire, respectively. The wire surface is surrounded by two-phase fine-grained layer with the thickness of 30 μm and the average nitrogen content of 5 mass% as seen in **Figure 12a** and **d**. The retained austenitic phase grains with larger size than nano-structured grains are present in the inside of nitrided layer as seen in **Figure 12b**, **c**, **e**, and **f**, respectively. This reveals that inner nitriding process advances almost homogeneously but that its synergetic relation is retarded in some parts to leave the nitrogen supersaturated γ-grains without phase

**4.3 Microstructure evolution in FGSS316 wires**

*DOI: http://dx.doi.org/10.5772/intechopen.91281*

*Micro-/Nano-Structuring in Stainless Steels by Metal Forming and Materials Processing DOI: http://dx.doi.org/10.5772/intechopen.91281*

#### **Figure 11.**

*Electron Crystallography*

**64**

**Figure 10.**

Due to EBSD analysis, the synergetic relation is described by the inverse pole figure, the phase mapping and the plastic strain distribution below NFE in **Figure 10b**. The coarse grains near the grain boundary were only nitrided to change their microstructure; e.g., the austenitic and martensitic zones exclusively distribute in an A-grain. The retained austenitic zones in this grain had higher plastic strains (or higher KAM) and different crystallographic orientations from the martensitic zones, the plastic strains of which were much lower than these austenitic ones. This formation of γ- and α'-zone mixture with plastic straining and crystallographic rotation just corresponds to the multi-dimensional inner nitriding process. As depicted in **Figure 10c**, the nitrogen diffusion localizes across NFE so that the nitrogen selectively diffuses along the original grain boundary below NFE. This nitrogen main stream branches into the neighboring grains to this gran boundary. In each grain, the nitrogen further diffuses through the zone boundaries to drive the phase transformation, the plastic straining, and the refinement of zone sizes. Next,

*Experimental demonstration on the inner nitriding process. (a) Nitrogen content depth profile, (b) IPF* 

*mapping, phase mapping and KAM distribution, and (c) inner nitriding mechanism.*

As compared between **Figure 11a** and **b**, little plastic strains were detected in the α'-zone of A-grain while the surrounding γ-zones were much plastically strained.

more precise EBSD analysis is made on this A-grain.

*Synergetic relation in the inner nitriding at 623 K, locally observed in the grain-A. (a) Phase mapping, (b) KAM distribution, and (c) IPF mapping.*

This proves that some of NS-zones massively transform to α'-zones since the elastic strain energy density reaches to the critical level and that plastic strains are induced only in the neighboring γ-zones to α'-zones to compensate for misfit strain between NS-zones and surrounding unsaturated γ-zones. Comparing **Figure 11b** and **c**, these highly strained zones are just corresponding to crystallographically refined zones. This assures that spin rotations are induced together with the plastic strains by misfit distortion on the distributed zone boundaries to refine each zone size. Owing to this synergetic mechanism, the refined zones have mutual boundaries with high misorientation angles. This grain size refinement reflects on the high strength and hardness of the nitrided layer by the low temperature plasma nitriding.

#### **4.3 Microstructure evolution in FGSS316 wires**

As had been discussed in [29, 30], the initial grain size of AISI316 has influence on the inner nitriding behavior; homogeneity in nitriding process is enhanced with reduction of initial grain size. This finding suggested that fine-grained AISI316 or FGSS316 structural components and parts could be homogeneously nitrided on the surface with a little effect of heterogeneous nitriding on the microstructure below NFE. In the following, a FGSS316 wire is nitrided at 623 K for 14.4 ks and its microstructure is precisely analyzed to describe the formation of nitrided layer as well as the microstructure evolution at the surface and below NFE during this low temperature nitriding. **Figure 12** depicts the nitrogen solute mapping, phase mapping, and crystallographic structure on the lateral and longitudinal crosssections of nitrided FGSS316 wire, respectively. The wire surface is surrounded by two-phase fine-grained layer with the thickness of 30 μm and the average nitrogen content of 5 mass% as seen in **Figure 12a** and **d**. The retained austenitic phase grains with larger size than nano-structured grains are present in the inside of nitrided layer as seen in **Figure 12b**, **c**, **e**, and **f**, respectively. This reveals that inner nitriding process advances almost homogeneously but that its synergetic relation is retarded in some parts to leave the nitrogen supersaturated γ-grains without phase

#### **Figure 12.**

*Microstructure and nitrogen mapping across NFE on the lateral and longitudinal cross-sections of nitrided FGSS316 wire at 623 K for 14.4 ks. (a) Nitrogen mapping, (b) phase mapping, and (c) IPF mapping in the nitrided layer, on the lateral cross-section. (d) Nitrogen mapping (e) phase mapping in the nitrided layer, and (f) IPF mapping in the nitrided layer, on the longitudinal cross-section.*

transformation and grain size refinement. In the following section, these retained γ-zones are employed as a marker to investigate the effect of uniaxial loading on the nitrided surface layer.

Let us analyze the effect of inner nitriding on the FGSS316 matrix below NFE. **Figure 13a** shows the initial microstructure of FGSS316 wire. This microstructure of FGSS316 before nitriding has equiaxial crystallographic structure with the average grain size of 2 μm. **Figure 13b** and **c** shows the crystallographic microstructure after nitriding on the lateral and longitudinal cross-sections of wire, respectively. The original microstructure is modified to form the skewed linear zones with finer grains as pointed by "b" in **Figure 13c**. These nitrogen-supersaturated zones consist of the transformed α'-zones and their surrounding plastic-strained. Just as discussed in **Figures 10** and **11**, the inner nitriding process advanced heterogeneously in the depth below NFE even when nitriding the FGSS316 wire.

### **4.4 Controlled crystallographic structure by plasma nitriding**

Different from the metal forming, the plastic strains are induced along the network of subgrain and zone boundaries, which are newly generated in the inside of original matrix by nitrogen diffusion and supersaturation. Since this network also works as new nitrogen diffusion paths, further nitrogen diffusion and supersaturation advances into the depth of original grains. This concurrent co-working of nitrogen diffusion and supersaturation sustains the synergetic relation among the phase transformation, the plastic straining, and the grain size refinement. In particular, fine zone network results in refined nano-sized crystallographic structure after plasma nitriding.

**67**

*Micro-/Nano-Structuring in Stainless Steels by Metal Forming and Materials Processing*

**5. Formation of bundle structures in nitrided wire by uniaxial loading**

*Comparison of the crystallographic structure before and after plasma nitriding. (a) Initial microstructure of FGSS316 wire, (b) microstructure of FGSS316 matrix on its lateral cross-section, and (c) microstructure of* 

The nitrided FGSS316 wire is further uniaxially loaded in tensile. The austenitic grains are continuously linked between its nitrided layer and inner matrix. This nitrided layer had influence on the mechanical response of wire since the same elasto-plastic strains are applied to these two regions. Precise microstructure analysis is also made to describe the microstructure evolution of this nitrided wire

The uniaxial tensile loading test was performed by using the precision universal testing machine AUTOGRAPH AGS-X 10 kN (SHIMADZU Co. Ltd.). This uniaxial loading was terminated when the maximum applied load reached 6 kN before fatal ductile fracture for microstructure analyses. The applied load and stroke were in situ monitored by the load cell and linear scaler, respectively. The bare FGSS316 wire without nitrided layer has an ultimate strength (σU) of 1.18 GPa at the stroke

A normal FGSS316 wire elongates in the tensile direction at room temperature (RT) without significant change of microstructure except for the formation and coalescence of voids near the fatal ductile fracture of wire [31]. Microstructure of high carbon steel wire is sensitive to drawing process since it has a multi-

dimensional microstructure, where each austenitic grain boundary houses pearlite blocks and each block consists of pearlite colony with the same lamellar structure of cementite (or θ-phase) and lamellar ferrite [32]. However, little microstructure

*DOI: http://dx.doi.org/10.5772/intechopen.91281*

during the uniaxial loading.

**Figure 13.**

**5.1 Uniaxial loading procedure**

*FGSS316 matrix on its longitudinal cross-section.*

(δ) of 4.9 mm, or, at the nominal strain (ε) of 0.17.

**5.2 Microstructure evolution during uniaxial loading at RT**

*Micro-/Nano-Structuring in Stainless Steels by Metal Forming and Materials Processing DOI: http://dx.doi.org/10.5772/intechopen.91281*

**Figure 13.**

*Electron Crystallography*

nitrided surface layer.

**Figure 12.**

transformation and grain size refinement. In the following section, these retained γ-zones are employed as a marker to investigate the effect of uniaxial loading on the

*Microstructure and nitrogen mapping across NFE on the lateral and longitudinal cross-sections of nitrided FGSS316 wire at 623 K for 14.4 ks. (a) Nitrogen mapping, (b) phase mapping, and (c) IPF mapping in the nitrided layer, on the lateral cross-section. (d) Nitrogen mapping (e) phase mapping in the nitrided layer, and* 

Let us analyze the effect of inner nitriding on the FGSS316 matrix below NFE. **Figure 13a** shows the initial microstructure of FGSS316 wire. This microstructure of FGSS316 before nitriding has equiaxial crystallographic structure with the average grain size of 2 μm. **Figure 13b** and **c** shows the crystallographic microstructure after nitriding on the lateral and longitudinal cross-sections of wire, respectively. The original microstructure is modified to form the skewed linear zones with finer grains as pointed by "b" in **Figure 13c**. These nitrogen-supersaturated zones consist of the transformed α'-zones and their surrounding plastic-strained. Just as discussed in **Figures 10** and **11**, the inner nitriding process advanced heterogeneously in the

Different from the metal forming, the plastic strains are induced along the network of subgrain and zone boundaries, which are newly generated in the inside of original matrix by nitrogen diffusion and supersaturation. Since this network also works as new nitrogen diffusion paths, further nitrogen diffusion and supersaturation advances into the depth of original grains. This concurrent co-working of nitrogen diffusion and supersaturation sustains the synergetic relation among the phase transformation, the plastic straining, and the grain size refinement. In particular, fine zone network results in refined nano-sized crystallographic struc-

depth below NFE even when nitriding the FGSS316 wire.

*(f) IPF mapping in the nitrided layer, on the longitudinal cross-section.*

**4.4 Controlled crystallographic structure by plasma nitriding**

**66**

ture after plasma nitriding.

*Comparison of the crystallographic structure before and after plasma nitriding. (a) Initial microstructure of FGSS316 wire, (b) microstructure of FGSS316 matrix on its lateral cross-section, and (c) microstructure of FGSS316 matrix on its longitudinal cross-section.*
