**4. Mesoscopic characterization on the plasma nitrided AISI316**

EBSD was employed to describe the microstructure change before and after nitriding and to analyze the effects of holding temperature and initial grain size on nitriding behavior. In the EBSD analysis, three items are employed to disclose the mesoscopic view on the nitriding behavior; e.g., the phase mapping, the KAM (Kernel Angle Misorientation) distribution, and the IPF (Inverse Pole Figure) profile. As shown in **Figure 5**, the nitrided AISI316 is composed of the nitrogen supersaturated γ-phase (γN) and α-phase (αN). The phase mapping on the cross-section of the nitrided layer depicts that nitrogen supersaturation induces the lattice

expansion and transforms γ- to α-zones and that plastic straining occurs in the nearest neighboring γ-zones to each lattice expanding zones. The dislocations are generated to compensate for the mismatched strains between the unsaturated γphase zones and the lattice strained α/γ-zones [22, 23].

KAM profile represents the equivalent plastic strain distribution which is induced by the lattice expansion during the nitrogen supersaturation after [24]. IPF provides information on the grain refinement and subgrain formation with crystallographic spin-rotation.

**Figure 8** depicts the phase mapping, the KAM distribution, and IPF profile on the cross-section of nitrided AISI316 at 623 K. As had been discussed in [22, 23, 25], most of the microstructure above NFE at the depth of 30 μm from the surface has two-phase structure, high plastic strains, and fine grain structure. Due to relatively homogeneous nitrogen supersaturation, the plastic straining and microstructure refinement processes co-work with the nitrogen supersaturation and zoneboundary diffusion processes. However, this synergic nitriding process commences to localize by itself near NFE and completely localizes below NFE. Since the α-phase is continuously formed along the a-path, the nitrogen atoms diffuse along this apath to the depth of AISI316 matrix. This zone-boundary diffusion assists the further nitrogen diffusion into the neighboring grains such as A- and B-grains. As seen in **Figure 8**, the phase transformation, the plastic straining, and the grain size refinement advance even in A- and B-grains with this nitrogen supersaturation and diffusion processes.

Let us describe this local nitrogen supersaturation into A- and B-grains. **Figure 9** depicts the phase mapping, the strain distribution, and the grain refining in these grains. Compared between **Figure 9a** and **b**, the α-phase zones in A-grain are formed in the absence of plastic strains, and, the high KAM zones correspond to the γ-phase. Since these plastic strains are induced by the lattice expansion in the nitrogen supersaturated γ-phase, these lines and zones with high KAM

**Figure 8.**

*EBSD analysis on the cross-section of the nitrided AISI316 at 623 K for 14.4 ks. a) Phase mapping, b) KAM distribution, and c) inverse pole figure in the ND direction.*

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

**Figure 9.**

*EBSD analysis on the cross-section below NFD for the nitrided AISI316 at 623 K for 14.4 ks. a) Phase mapping, b) KAM distribution, and c) inverse pole figure in the ND direction.*

correspond to the nitrogen diffusing paths into A-grain. Most of these diffusing paths terminate intermediately so that nitrogen supersaturation and microstructure refinement processes also stop in the inside of A-grain. Comparing **Figure 9b** and **c**, high KAM zones in **Figure 9b** is just corresponding to fine-grained zones in **Figure 9c**. This demonstrates that the grain-size refinement is induced by the plastic straining.

**Figure 9** reveals that the nitrogen supersaturation and zone-boundary diffusion processes localize below NFE. These processes co-work with the plastic straining, the two-phase structuring, and microstructure refinement in local to modify the crystallographic structure far below NFE. This heterogeneous nitrogen supersaturation process must be changed to be more homogeneous by controlling the external and internal nitriding conditions.

First, the holding temperature is increased from 623 K to 673 K to investigate its effect on this mode change. **Figure 10** depicts the phase mapping, the plastic strain distribution, and the IPF profile on the cross0section of nitrided AIS316 at 673 K for 14.4 ks.

Although the a-path for zone-boundary nitrogen diffusion is formed below NFE together with plastic strains and refined microstructures, the homogeneous nitrogen supersaturation process advances above NFE. The two-phase structuring in **Figure 10a**, the plastic straining in **Figure 10b**, and microstructure refining in **Figure 10c** co-work in synergy and co-terminates at d = 60 μm. The heterogeneous nitrogen supersaturation is suppressed to the local area below NFE. This proves that the mode-change from the heterogeneous nitriding to the homogeneous one is performed by simply increasing the holding temperature.

In addition to this external item, the initial grain size refinement is selected as an internal item to make the mode change. The intense rolling was employed to reduce the original AISI316 plate thickness by 90%. **Figure 11** shows the phase mapping, the plastic strain distribution, and the IPF profile on the surface of the fine-grained AISI316 (or GF-AISI316) plate. As well known in the rolling and stamping of AISI304 plates [26] and AISI316 bars [27], the transformation from γ-phase to αphase is induced into AISI316 by this intense rolling as shown in **Figure 11a**. The agreement between the α-phase zones and the high strained zones proves this strain-induced transformation in comparison to **Figure 11a** and **b**. The initial grain size with its average of 15 μm is reduced to 1.5 μm in **Figure 11c** by rolling.

**Figure 10.**

*EBSD analysis on the cross-section of the nitrided AISI316 at 673 K for 14.4 ks. a) Phase mapping, b) KAM distribution, and c) inverse pole figure in the ND direction.*

**Figure 11.**

*EBSD analysis on the surface of rolled AISI316 plate specimen before nitriding. a) Phase mapping, b) KAM distribution, and c) inverse pole figure in ND direction.*

This GF-AISI316 specimen was nitrided at 623 K for 14.4 ks to investigate the effect of initial grain size on the mode change in nitrogen supersaturation. **Figure 12** shows the SEM image and nitrogen mapping on the cross-section of nitrided GF-AISI316 at 623 K for 14.4 ks. The uniform nitrided layer with a thickness of 40 μm was formed with a fine microstructure above NFE. The microstructure below NFE is also homogeneous and looks to be the same as the microstructure before nitriding in **Figure 12**. The hardness depth profile and the nitrogen solute content depth profile across NFE are respectively depicted in **Figure 13**.

The hardness with its average of 1400 HV above NFE drastically decreases down to the matrix hardness of 250 HV just across NFE in **Figure 13a**. The nitrogen solute *Nitrogen Supersaturation of AISI316 Base Stainless Steels at 673 K and 623 K… DOI: http://dx.doi.org/10.5772/intechopen.102387*

**Figure 12.**

*SEM and nitrogen mapping on the cross-section of the rolled AISI316 specimen after nitriding at 623 K for 14.4 ks. a) SEM image on the cross-section, and b) nitrogen mapping to the depth.*

### **Figure 13.**

*Hardness and nitrogen content depth profiles of the rolled AISI316 specimen after nitriding at 623 K for 14.4 ks. a) Hardness depth profile, and b) nitrogen content depth profile.*

content depth profile with the average of 4 to 5 mass% also decreases down to zero across NFE in **Figure 13b**. These profiles reveal that the homogeneous nitrogen supersaturation advances to NFE and terminates at NFE. This is completely different from the heterogeneous nitrogen supersaturation process across NFE when using the AISI316 with the average grain size of 15 μm. Let us make EBSD analysis on the cross-section of nitrided FG-AISI316 with a comparison between **Figures 8** and **14**. As shown in **Figure 14a**, the two-phase structuring, the plastic straining, and the grain size refining take place only above NFE and no processes advance across NFE. The two-phase, highly strained, and grain-refined zone in **Figure 14a**–**c** is just equivalent to the nitrided layer. The phase map, the KAM distribution, and IPF profile below NFE in **Figure 14** are the same as those in **Figure 11**. That is, no nitrogen supersaturation takes place below NFE.

This comparison of EBSD results among **Figures 8, 11**, and **14** reveals that the initial grain size refinement has a significant influence on the synergic relationship among the nitrogen supersaturation, the nitrogen zone-boundary diffusion, the plastic straining, the two-phase structuring, and the microstructure refining.

Let us reconsider the heterogeneous and homogeneous nitrogen supersaturation processes and their mode change. As depicted in **Figure 8**, the heterogeneous nitrogen supersaturation gradually turns to be homogeneous according to the

**Figure 14.**

*EBSD analysis on the surface of rolled AISI316 plate specimen after nitriding at 673 K for 14.4 ks. a) Phase mapping, b) KAM distribution, and c) inverse pole figure in ND direction.*

nitrogen boundary diffusion mechanism change from the localized path to the network. Under the localized boundary diffusion mechanism, the nitrogen supersaturation, the plastic straining, the grain refining, and the phase transformation only advance at the vicinity of zone boundaries. Neglecting this localization, the synergic relationship to drive the inner nitriding process by heterogeneous nitrogen supersaturation is the same as that by homogeneous nitrogen supersaturation. Hence, if the localized nitrogen boundary diffusion is revised by the zone-boundary diffusion in a network, this heterogeneous nitrogen supersaturation process could be controlled to change itself to a homogeneous process. Both the holding temperature increase and the initial grain size refinement work as an external and internal trigger to enhance the nitrogen zone boundary diffusion mechanism.

In the case of the higher holding temperature, the nitrogen boundary diffusion rate is enhanced in similar manner to the increase of body diffusion rate in the traditional plasma nitriding processes such as ion- and radical-plasma nitriding. Although those HT-PN processes required a higher temperature than 773 K enough to sustain the diffusion path, the holding temperature of 673 K is enough to drive the homogeneous nitriding.

The HT-PN processes have no initial grain-size refinement effect on their nitriding behavior. On the other hand, the heterogeneous nitriding even at 623 K completely changes to be homogeneous when using the fine-grained AISI316 substrates. This mode change proves that grain-boundary diffusion works as a network to supply a sufficient amount of nitrogen solutes enough to sustain the synergic nitriding process at every spot above NFE. As demonstrated in **Figures 12**–**14**, NFE works just as a front end for homogeneous nitrogen supersaturation and diffusion.

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