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

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 during the uniaxial loading.

## **5.1 Uniaxial loading procedure**

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

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

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 multidimensional 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

change occurs at the RT in its uniaxial tensile loading. A composite wire also has no microstructure evolution before pop-out of fibers from core matrix [33]. Let us investigate the microstructure at the nitrided layer and in the matrix of wire, respectively, and describe the effect of posterior elasto-plastic straining to nitriding on the microstructure evolution in wire.

**Figure 14** depicts the plastic strain distribution and phase mapping on the lateral and longitudinal cross-sections of nitrided FGSS316 wire, respectively, after uniaxial loading. The retained austenite regions with larger size than fine grained two-phase nitrided layer in **Figure 12** completely disappear at the nitrided layer. Every nitrided layer has homogeneous two-phase microstructure with fine grain sizes. Compared with the plastic strain distribution before uniaxial loading, the whole nitrided layer is uniformly subjected to high plastic straining. This reveals that the applied plastic strains by uniaxial loading drives the synergetic process of inner nitriding at the retained austenitic zones and induces the phase transformation from the retained γ-phase to γ−/α'-phase fine mixture.

The microstructure as well as the phase in the matrix below NFE is modified by this uniaxial loading. Nearly full austenitic phase of matrix before loading changes to mixture of γ-fibers and transformed α'-fibers. As shown in **Figure 14b** and **d**, these α'-fibers with its lateral size of 0.5 μm are aligned along the loading direction to form a bundle structure together with γ-fibers. The volume fraction of these α'-bundles increases from the vicinity of NFE to the depth in matrix. This suggests that the nitrided FGSS316 wire fractures of α'-bundles in ductile at its center when this fraction reaches to the critical maximum.

Let us compare the above phase mapping in matrix with the plastic strain distribution in **Figure 14**. The high plastic straining zones are just corresponding to the α'-fiber zones. This proves that this γ to α'-phase transformation during the uniaxial loading is induced by the high plastic straining of γ-zones. That is, the highly strained γ-grains are plastically strained and aligned along the loading direction to form the γ-fibers. During this microstructure evolution, some of γ-fibers transforms massively to α'-fibers.

The phase transformation and formation of bundle microstructure in the above reflects on the crystallographic structure after uniaxial loading. **Figure 15** depicts the IPF mapping at the nitrided layer and in the matrix on the lateral and

#### **Figure 14.**

*Plastic straining and phase mapping on the lateral and longitudinal cross-sections of nitrided FGS316 wire after uniaxial loading. (a) Plastic strain lateral distribution, (b) phase mapping in lateral, (c) plastic strain longitudinal distribution, and (d) phase mapping in longitudinal.*

**69**

**6. Discussion**

**Figure 15.**

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

longitudinal cross-sections of wire, respectively. The γ-phase matrix before uniaxial loading has crystallographic structure with the average grain size of 2 μm and without preferred orientations. After loading, these γ- and α'-grains are aligned along the loading direction respectively to form α'- and γ-bundles. In particular, most of α'-bundles have unique (111) directions. This might be because the transformed bundles are aligned in the tensile directions under the constrained conditions by the nitrided layer, surrounding the matrix. Each initial γ-grain is forced to deform elasto-plastically only in the tensile direction and to form γ-fibers and γ-bundles since no plastic deformation is allowed in the lateral direction under the confinement of nitrided layer. Some of strained γ-fibers makes massive transformation to α'-fibers; these α'-fibers are assembled into a single α'-bundle with the preferred orientation to (111). After [26, 34], the easiest crystallographic orientation to form

*Nitrogen mapping and crystallographic structure on the lateral and longitudinal cross-sections of nitrided FGSS316 wire after uniaxial loading. (a) Nitrogen mapping in lateral, (b) lateral IPF mapping in ND,* 

*(c) nitrogen mapping in longitudinal, and (d) longitudinal IPF mapping in TD.*

this martensitic fibers and bundles is thought to be (111) tensile direction.

suggests that elasto-plastic compatibility is preserved across this interface.

The polycrystalline materials are generally described by the grain boundary characteristics and crystallographic orientation of each constituent grain as well as its grain size [35]. Each grain boundary energy is determined by the misfit orientation angle between adjacent grains. The compatible grain boundary has low energy enough to stack some amount of dislocations; while the incompatible one has high

**Figure 15** reveals that the nitrided layer surrounding the matrix has homogeneous super fine-grained two phase structure after uniaxial loading. No cracks and defects are seen on the nitrided layer surface and in the inside of layer; the fatal fracture of this nitrided FGSS316 wire occurs as the ductile fracture of matrix as explained before. The fine continuous interface between the nitrided layer and the matrix also

*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 15.**

*Electron Crystallography*

on the microstructure evolution in wire.

tion from the retained γ-phase to γ−/α'-phase fine mixture.

this fraction reaches to the critical maximum.

transforms massively to α'-fibers.

change occurs at the RT in its uniaxial tensile loading. A composite wire also has no microstructure evolution before pop-out of fibers from core matrix [33]. Let us investigate the microstructure at the nitrided layer and in the matrix of wire, respectively, and describe the effect of posterior elasto-plastic straining to nitriding

**Figure 14** depicts the plastic strain distribution and phase mapping on the lateral and longitudinal cross-sections of nitrided FGSS316 wire, respectively, after uniaxial loading. The retained austenite regions with larger size than fine grained two-phase nitrided layer in **Figure 12** completely disappear at the nitrided layer. Every nitrided layer has homogeneous two-phase microstructure with fine grain sizes. Compared with the plastic strain distribution before uniaxial loading, the whole nitrided layer is uniformly subjected to high plastic straining. This reveals that the applied plastic strains by uniaxial loading drives the synergetic process of inner nitriding at the retained austenitic zones and induces the phase transforma-

The microstructure as well as the phase in the matrix below NFE is modified by this uniaxial loading. Nearly full austenitic phase of matrix before loading changes to mixture of γ-fibers and transformed α'-fibers. As shown in **Figure 14b** and **d**, these α'-fibers with its lateral size of 0.5 μm are aligned along the loading direction to form a bundle structure together with γ-fibers. The volume fraction of these α'-bundles increases from the vicinity of NFE to the depth in matrix. This suggests that the nitrided FGSS316 wire fractures of α'-bundles in ductile at its center when

Let us compare the above phase mapping in matrix with the plastic strain distribution in **Figure 14**. The high plastic straining zones are just corresponding to the α'-fiber zones. This proves that this γ to α'-phase transformation during the uniaxial loading is induced by the high plastic straining of γ-zones. That is, the highly strained γ-grains are plastically strained and aligned along the loading direction to form the γ-fibers. During this microstructure evolution, some of γ-fibers

The phase transformation and formation of bundle microstructure in the above reflects on the crystallographic structure after uniaxial loading. **Figure 15** depicts the IPF mapping at the nitrided layer and in the matrix on the lateral and

*Plastic straining and phase mapping on the lateral and longitudinal cross-sections of nitrided FGS316 wire after uniaxial loading. (a) Plastic strain lateral distribution, (b) phase mapping in lateral, (c) plastic strain* 

*longitudinal distribution, and (d) phase mapping in longitudinal.*

**68**

**Figure 14.**

*Nitrogen mapping and crystallographic structure on the lateral and longitudinal cross-sections of nitrided FGSS316 wire after uniaxial loading. (a) Nitrogen mapping in lateral, (b) lateral IPF mapping in ND, (c) nitrogen mapping in longitudinal, and (d) longitudinal IPF mapping in TD.*

longitudinal cross-sections of wire, respectively. The γ-phase matrix before uniaxial loading has crystallographic structure with the average grain size of 2 μm and without preferred orientations. After loading, these γ- and α'-grains are aligned along the loading direction respectively to form α'- and γ-bundles. In particular, most of α'-bundles have unique (111) directions. This might be because the transformed bundles are aligned in the tensile directions under the constrained conditions by the nitrided layer, surrounding the matrix. Each initial γ-grain is forced to deform elasto-plastically only in the tensile direction and to form γ-fibers and γ-bundles since no plastic deformation is allowed in the lateral direction under the confinement of nitrided layer. Some of strained γ-fibers makes massive transformation to α'-fibers; these α'-fibers are assembled into a single α'-bundle with the preferred orientation to (111). After [26, 34], the easiest crystallographic orientation to form this martensitic fibers and bundles is thought to be (111) tensile direction.

**Figure 15** reveals that the nitrided layer surrounding the matrix has homogeneous super fine-grained two phase structure after uniaxial loading. No cracks and defects are seen on the nitrided layer surface and in the inside of layer; the fatal fracture of this nitrided FGSS316 wire occurs as the ductile fracture of matrix as explained before. The fine continuous interface between the nitrided layer and the matrix also suggests that elasto-plastic compatibility is preserved across this interface.

#### **6. Discussion**

The polycrystalline materials are generally described by the grain boundary characteristics and crystallographic orientation of each constituent grain as well as its grain size [35]. Each grain boundary energy is determined by the misfit orientation angle between adjacent grains. The compatible grain boundary has low energy enough to stack some amount of dislocations; while the incompatible one has high

energy enough to interact with dislocations [36]. In the metal forming of these polycrystalline materials or in the nitrogen supersaturation process, the dislocations as well as the slipping lines and planes interact with their grains and grain boundaries. In intense rolling, the grains are sheared and deformed to align their crystallographic orientation along the rolling direction and to form the textured microstructure. The applied elastoplastic distortion by rolling works to induce the phase transformation during rolling by high elastic strain energy density, to shear and elongate the grains by the plastic strains, and to spin the grain orientations toward the preferred one along the rolling direction. As reported in [37], the grain size is reduced down to sub-microns by repetitive rolling; due to small misorientation angles between adjacent grains, they are easy to agglomerate into larger grains by heat treatment.

In piercing, the grains near the shearing plane are affected by the applied elastoplastic distortion [38]. The increase of elastic strain energy density during piercing induces the phase transformation. The grains are sheared and fractured along the shearing plain. The original crystallographic structure of work materials is changed to align along the shearing plane by severe spinning with piercing. Although the grains are refined at the vicinity of shearing plain, the misorientation angles among them are small enough to be identified as nearly the same grain. Even in other metal forming processes than two in the above, their mechanical interactions of elastoplastic distortion with crystallographic structure is described by the strain-induced phase transformation, the shear deformation, the grain size refinement as well as the elastic recovery from the elasto-plastically strained state in unloading.

On the other hand, no elastoplastic distortion was directly applied to granular structure by the low temperature plasma nitriding. Instead of this direct straining, a large elastic distortion is induced into the nitrogen supersaturated zones in the work materials by lattice expansion. This distortion reaches to 10%, enough to drive the phase transformation in zones as well as the plastic distortion to compensate for the misfit on the zone boundaries between the nitrogen supersaturated and unsaturated ones. The symmetric component of this distortion works as a shearing strain tensor to form new slipping lines network across the original grain boundary. The asymmetric one drives spin-rotation in each zone to form the zone boundaries with high misorientation angles and to significantly refine the original grain size of work materials. Since those newly built-up zone boundaries play as a nitrogen diffusion path, this process advances concurrently with the nitrogen solute diffusion from the surface to the depth of materials. Since the zone size ranges in the nanometer order, the stainless steel work materials are covered by two-phase, nano-grained, nitrogen steel surface layer. The smallest zone size is determined by the mechanical balancing between the elastic straining in each material supercell and the slip-line formation surrounding it in the nitrogen supersaturation [39]. Precise TEM analyses down to the atomic scale are useful to describe the nitrogen interstitial atom distribution in the supercell [40].

Even when the rolled and pierced steel specimens are uniaxially loaded, their microstructure never changes themselves at room temperature before fracture. Their uniaxial stress-strain curves are determined by the original microstructure of as-rolled and as-pierced materials since no mechanical interaction occurs between the controlled grains by previous metal forming and the applied strain by uniaxial loading. On the other hand, the in situ microstructure evolution takes place during the uniaxial loading of the nitrided FGSS316 wire. The nitrogen supersaturated layer works a double role under this uniaxial tensile loading. The original FGSS316 matrix inside the wire is elastically supported by this nitrided layer. Since the austenitic grains are continuously linked with those in the nitrided layer, the matrix inside the wire is intensely elongated to change its microstructure to two-phase fibrous grains by the uniaxial loading. The microstructure in the as-nitrided layer is also affected by uniaxially applied plastic strains. The retained austenitic grains

**71**

**Figure 16.**

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

with large size in the nitrided layer change themselves to two-phase, fine grains, which are the same as homogeneously nitrided nanostructure before uniaxial loading. This in situ refinement of granular structure in the nitrided layer reflects on the hardness profile. The original hardness of as-nitrided layer is 1400 HV; this is further enhanced up to 1600 HV after uniaxial loading. This proves that nitrogen could diffuse locally to the retained austenitic zones along the slip-lines by externally applied plastic straining and drive the nitrogen supersaturation process in them for refinement of their microstructure. Owing to the elastic constraint by the nitrided layer, the work-hardening process during the uniaxial loading is enhanced in the wire matrix to attain higher ultimate stress (σU); e.g., σU = 1.23 GPa in the

This in situ microstructure evolution by uniaxial loading posterior to nitriding, suggests further possibility of crystallographic control to improve the mechanical properties of metallic works, takes place in the nitrided layer and in the matrix inside without mutual interactions. The microstructure evolution of low temperature nitrided members and parts in the above must be enhanced during warm and hot processing. After recent work on the high carbon steel wire during drawing [41, 42], significant reduction of lamellar ferrite distance as well as free carbon dissociation from the cementite lamellar structure in the perlite colony and block are responsible for high strengthening of high carbon steel wires. This implies that further carbon supersaturation is a key process to drive the in situ evolution to the preferred crystallographic microstructure to higher strength of wire. Owing to the equivalent role between carbon and nitrogen solutes to be working as an interstitial atom in steel [43], local nitrogen mobility from nitrogen supersaturated zone with high nitrogen content to NS-zones with lower nitrogen content could drive the in situ nitrogen alloying process of wire matrix during warm-/hot-drawing and rolling. In this nitriding a priori to metal forming, FGSS316 wire was first nitrided and then uniaxially loaded. How about the plasma nitriding of the rolled AISI304 plate? As shown in **Figure 3**, the nearly full-martensitic phase of rolled AISI304 plate

*Crystallographic structure on the cross-section of the rolled AISI304 plate after plasma nitriding at 673 K for 14.4 ks. (a) IPF mapping in the normal direction, (b) KAM distribution, and (c) phase mapping.*

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

nitrided FGSS316 wire at δ = 5.7 mm or at ε = 0.19.

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

with large size in the nitrided layer change themselves to two-phase, fine grains, which are the same as homogeneously nitrided nanostructure before uniaxial loading. This in situ refinement of granular structure in the nitrided layer reflects on the hardness profile. The original hardness of as-nitrided layer is 1400 HV; this is further enhanced up to 1600 HV after uniaxial loading. This proves that nitrogen could diffuse locally to the retained austenitic zones along the slip-lines by externally applied plastic straining and drive the nitrogen supersaturation process in them for refinement of their microstructure. Owing to the elastic constraint by the nitrided layer, the work-hardening process during the uniaxial loading is enhanced in the wire matrix to attain higher ultimate stress (σU); e.g., σU = 1.23 GPa in the nitrided FGSS316 wire at δ = 5.7 mm or at ε = 0.19.

This in situ microstructure evolution by uniaxial loading posterior to nitriding, suggests further possibility of crystallographic control to improve the mechanical properties of metallic works, takes place in the nitrided layer and in the matrix inside without mutual interactions. The microstructure evolution of low temperature nitrided members and parts in the above must be enhanced during warm and hot processing. After recent work on the high carbon steel wire during drawing [41, 42], significant reduction of lamellar ferrite distance as well as free carbon dissociation from the cementite lamellar structure in the perlite colony and block are responsible for high strengthening of high carbon steel wires. This implies that further carbon supersaturation is a key process to drive the in situ evolution to the preferred crystallographic microstructure to higher strength of wire. Owing to the equivalent role between carbon and nitrogen solutes to be working as an interstitial atom in steel [43], local nitrogen mobility from nitrogen supersaturated zone with high nitrogen content to NS-zones with lower nitrogen content could drive the in situ nitrogen alloying process of wire matrix during warm-/hot-drawing and rolling.

In this nitriding a priori to metal forming, FGSS316 wire was first nitrided and then uniaxially loaded. How about the plasma nitriding of the rolled AISI304 plate? As shown in **Figure 3**, the nearly full-martensitic phase of rolled AISI304 plate

#### **Figure 16.**

*Crystallographic structure on the cross-section of the rolled AISI304 plate after plasma nitriding at 673 K for 14.4 ks. (a) IPF mapping in the normal direction, (b) KAM distribution, and (c) phase mapping.*

*Electron Crystallography*

energy enough to interact with dislocations [36]. In the metal forming of these polycrystalline materials or in the nitrogen supersaturation process, the dislocations as well as the slipping lines and planes interact with their grains and grain boundaries. In intense rolling, the grains are sheared and deformed to align their crystallographic orientation along the rolling direction and to form the textured microstructure. The applied elastoplastic distortion by rolling works to induce the phase transformation during rolling by high elastic strain energy density, to shear and elongate the grains by the plastic strains, and to spin the grain orientations toward the preferred one along the rolling direction. As reported in [37], the grain size is reduced down to sub-microns by repetitive rolling; due to small misorientation angles between adjacent grains, they are easy to agglomerate into larger grains by heat treatment. In piercing, the grains near the shearing plane are affected by the applied elastoplastic distortion [38]. The increase of elastic strain energy density during piercing induces the phase transformation. The grains are sheared and fractured along the shearing plain. The original crystallographic structure of work materials is changed to align along the shearing plane by severe spinning with piercing. Although the grains are refined at the vicinity of shearing plain, the misorientation angles among them are small enough to be identified as nearly the same grain. Even in other metal forming processes than two in the above, their mechanical interactions of elastoplastic distortion with crystallographic structure is described by the strain-induced phase transformation, the shear deformation, the grain size refinement as well as

the elastic recovery from the elasto-plastically strained state in unloading.

On the other hand, no elastoplastic distortion was directly applied to granular structure by the low temperature plasma nitriding. Instead of this direct straining, a large elastic distortion is induced into the nitrogen supersaturated zones in the work materials by lattice expansion. This distortion reaches to 10%, enough to drive the phase transformation in zones as well as the plastic distortion to compensate for the misfit on the zone boundaries between the nitrogen supersaturated and unsaturated ones. The symmetric component of this distortion works as a shearing strain tensor to form new slipping lines network across the original grain boundary. The asymmetric one drives spin-rotation in each zone to form the zone boundaries with high misorientation angles and to significantly refine the original grain size of work materials. Since those newly built-up zone boundaries play as a nitrogen diffusion path, this process advances concurrently with the nitrogen solute diffusion from the surface to the depth of materials. Since the zone size ranges in the nanometer order, the stainless steel work materials are covered by two-phase, nano-grained, nitrogen steel surface layer. The smallest zone size is determined by the mechanical balancing between the elastic straining in each material supercell and the slip-line formation surrounding it in the nitrogen supersaturation [39]. Precise TEM analyses down to the atomic scale are useful to describe the nitrogen interstitial atom distribution in the supercell [40]. Even when the rolled and pierced steel specimens are uniaxially loaded, their microstructure never changes themselves at room temperature before fracture. Their uniaxial stress-strain curves are determined by the original microstructure of as-rolled and as-pierced materials since no mechanical interaction occurs between the controlled grains by previous metal forming and the applied strain by uniaxial loading. On the other hand, the in situ microstructure evolution takes place during the uniaxial loading of the nitrided FGSS316 wire. The nitrogen supersaturated layer works a double role under this uniaxial tensile loading. The original FGSS316 matrix inside the wire is elastically supported by this nitrided layer. Since the austenitic grains are continuously linked with those in the nitrided layer, the matrix inside the wire is intensely elongated to change its microstructure to two-phase fibrous grains by the uniaxial loading. The microstructure in the as-nitrided layer is also affected by uniaxially applied plastic strains. The retained austenitic grains

**70**

changed to a mixture of nitrided austenitic and martensitic phases. Since the original martensitic and austenitic peak positions shift to the low angle of 2θ and their peak widths become significantly broad, this mixture composes of the fine grained austenitic and martensitic zones with nitrogen supersaturation.

EBSD was also employed to describe this microstructure change of rolled AISI304 plate after nitriding. As shown in **Figure 16a**, the textured structure of rolled AISI304 completely disappeared and changed to fine-grained structure without preferred crystallographic orientation. This change is driven by high plastic straining in **Figure 16b**; every original grains with and without textures by rolling is plastically strained and spin-rotated by the nitrogen supersaturation to form homogeneous fine-grained structure. As depicted in **Figure 16c**, this fine microstructure consists of two phase with the fraction of martensite by 70%. This dramatic crystallographic structure evolution proves that posterior nitriding to metal forming is useful to further control the microstructure of stainless steels.
