**3. Macroscopic characterization on the plasma nitrided AISI316**

XRD and SEM–EDX were utilized to describe the nitrogen supersaturation and nitrided layer formation in LT-PN of the AISI316 at 673 K and 623 K or 14.4 ks. **Figure 5** compares the three XRD diagrams of un-nitrided and nitrided AISI316 specimens.

The bare AISI316 is characterized by γ (111) peak at 2θ = 44.3° and γ (200) at 2θ = 51.7°, respectively. These peaks shift to γ<sup>N</sup> peaks in the shallow 2θ directions in both nitrided AISI316 specimens at 673 K and 623 K. This peak shift proves that

**Figure 5.**

*XRD diagrams for plasma nitrided AISI316 specimens at 673 K and 623 K for 14.4 ks.*


**Table 1.**

*Estimate the induced lattice strain by the nitrogen supersaturation into the nitrided AISI316 specimen at 673 K and 623 K.*

nitrogen interstitial occupies the vacancy sites of austenitic lattice in AISI316 and the lattice expands by itself. That is, the nitrogen supersaturation to AISI316 is first defined by this lattice expansion in **Figure 5**. After theoretical study in [21], this lattice expansion is induced by the occupation of interstitial nitrogen atoms to the octahedral vacancy sites. Let us estimate the lattice strains by this nitrogen supersaturation. **Table 1** summarizes the peak shift and lattice strain induced by the

**Figure 6.** *SEM image and EDX mapping on the cross-section of nitrided AISI316 specimen at 673 K for 14.4 ks.*

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

### **Figure 7.** *Nitrogen solute content depth profiles for the nitrided AISI316 at 673 K and 623 K for 14.4 ks.*


### **Table 2.**

*Nitrogen content analysis on the nitrided layer of AISI316 at 623 K and 673 K for 14.4 ks.*

nitriding at 673 K and 623 K, respectively. The nitrogen supersaturation induces the lattice strain of 7% at 673 K and 10% at 623 K in the nitrided layer, respectively.

SEM–EDX was utilized to describe the nitrogen solute distribution in the depth of nitrided AISI316 at 673 K and 623 K, respectively. SEM image and nitrogen mapping on its cross-section in **Figure 6** show that a thick nitrided layer with a thickness of 60 μm is formed to have uniform nitrogen solute content. To be noticed, the microstructure below NFE is slightly modified by the nitriding. The nitrogen content depth profiles are also depicted in **Figure 7**. Irrespective of the holding temperature, these profiles have a plateau with the constant nitrogen content of 4 mass% till NFE, as listed in **Table 2**.

This nitrogen content decreases at the vicinity of NFE, but a significant nitrogen content of 0.5 mass% is present even below NFE. The role of non-zero nitrogen content blow NFE is considered in the mesoscopic evaluation on the inner nitriding behavior.
