**3.1 Austenitic stainless steels**

The behavior of nitriding at high temperatures, above 500°C, and at low temperatures, below 420°C, mainly affects the corrosion resistance of the nitrided surface [14]. Austenitic stainless steels cannot be nitrided conventionally at temperatures close to 500–550°C due to intense precipitation of CrN and Cr2N chromium nitrides in the diffusion zone [42–45]. The precipitation of these nitrides increases the surface hardness but greatly decreases the corrosion resistance due to chromium removal from the solid solution in the matrix. Nitriding must be carried out below 430°C in order to avoid precipitation of nitrides. In this low-temperature nitriding process, generally between 380°C and 420°C, the diffusion kinetics of the chromium substitutional element is significantly reduced, which inhibits the formation of chromium nitrides. The increasing diffusion of nitrogen in the austenite generates a supersaturated solid solution that expands the CFC crystalline lattice and forms the metastable phase called Expanded Austenite—(γN) [46–48]. The formation of expanded austenite promotes an increase in surface hardness without compromising corrosion resistance [20, 49, 50].

**Figure 8** shows the microstructures obtained by Bruno et al. [50] for AISI 316 L steel after nitriding at temperatures of 550°C (a) and 380°C (b). When this steel is nitrided at 550°C, the nitrided surface becomes dark and severely etched, which denotes the loss of corrosion resistance in this region under the action of Villela's reagent. When nitriding is carried out at 380°C, the nitrided surface appears as a white layer, and the non-nitrided matrix shows a microstructure very similar to a

### **Figure 8.**

*Images of the nitrided surfaces after nitriding at (a) 380°C and (b) 550°C. Bruno et al. [50].*

typical austenitic steel microstructure. In this low-temperature nitriding condition, the Marble metallographic reagent does not etch the nitride layer, only the matrix, which indicates a better corrosion resistance of the nitrided surface. This corrosion resistance behavior against metallographic reagents results from the formation mechanism of expanded phases on the nitrided surface. The X-ray diffractograms in **Figure 9** show the non-nitrided condition after nitriding at 380°C (a) and 550°C (b). In the non-nitrided condition, only FCC austenite peaks are present. When nitriding at 380°C, FCC austenite peaks are shifted to the left and become broader, resulting from residual compression stresses and distortion of the crystalline lattice caused by nitrogen supersaturation. When expanded austenite is formed on the surface of the nitride specimen, the corrosion resistance is maintained or even improved compared to the non-nitrided specimen. On the other hand, when nitriding is carried out at 550°C, several CrN and Cr2N diffraction peaks show up. Chromium nitride precipitation induces depletion of the Cr content of the metallic matrix and is responsible for the loss of corrosion resistance of the nitrided surface.

The expansion of the FCC crystal lattice and the increase of the lattice parameter, which occurs when expanded austenite is formed, are shown in **Figure 9**. Expanded austenite peaks are shifted to the left, and the volume variation is close to 10% [20]. Strain-free FCC austenite has a lattice parameter equal to 0.359 nm (ICDD® Card 00–033-0397). After plasma nitriding, the lattice parameter in expanded austenite increases to 0.375 nm, corresponding to a calculated nitrogen content at a supersaturation equal to 34.6% atomic or approximately 8.5% by mass. These estimations do not consider the contribution of the residual stresses in shifting the diffraction peaks to the left) [51, 52]. Expanded austenite is responsible for the increased surface hardness up to 7 times over the original hardness, as shown in **Figure 10** [20].

The interstitial supersaturation of the matrix may be due to nitrogen diffusion in nitriding or carbon diffusion [25–54] upon plasma carburizing. The surface treatment may comprise both nitrogen and carbon diffusion, and the plasma treatment is called nitrocarburizing or just carbon for plasma carburizing. These treatments may be carried out at low temperatures, below 430°C for nitrocarburizing and below 500°C for carburizing, avoiding carbide or nitride precipitation. **Figure 11** shows the microstructures after (a) nitriding, consisting of a monolayer of austenite expanded by nitrogen (γN); (b) nitrocarburization consisting of a double layer

**Figure 9.** *XRD spectra for AISI 316L steel before and after nitriding [50].*

composed of austenite expanded by nitrogen (γN) in the outer region and carbon expanded austenite (γC) between the first layer and the matrix; and (c) carburizing consisting of a carbon-expanded monolayer of austenite (γC).

In nitrocarburizing and carburizing, colossal interstitial supersaturation leads to expansion of the crystalline lattice, generating the expanded phases "γN" and "γC." **Table 2** shows the expansion characteristics of the austenite FCC crystalline lattice under each condition and the dissolved nitrogen content in the supersaturated condition. The volume expansion of the FCC lattice, indicated by the ratio Δ*a*/ *a*, is responsible for the hardening and the generation of residual compressive stresses.

### **Figure 10.**

*Surface hardening due to the formation of expanded austenite after plasma nitriding AISI 316L stainless steel at 400°C [20].*

### **Figure 11.**

*Microstructures of austenitic stainless steel after (a) nitriding, (b) nitrocarburizing, and (c) carburizing at 400°C. scanning electron microscopy [54].*


### **Table 2.**

*Lattice parameters, lattice expansion, and calculated interstitial content at supersaturation for the expanded austenite layers in AISI 316L steel.*

**Figure 12.**

*Carbon pickup and compressive residual stresses on the surface of AISI 316L stainless steel after low temperature carburizing to colossal carbon enrichment [53].*

**Figure 12** shows the relationship between carbon supersaturation in the FCC crystalline lattice and residual stresses in the case-hardened surface of AISI 316 L austenitic stainless steel. Both parameters gradually decrease toward the nucleus [53] due to supersaturation and the generation of residual compressive stresses on the treated surfaces. **Figure 13** shows the high potential for surface hardening for the three types of treatment [54].

### **Figure 13.**

*Maximum hardness on AISI 316L austenitic stainless steel surface upon plasma nitriding, nitrocarburizing, and carburizing at 400°C [54].*

## **3.2 Martensitic stainless steels**

Martensitic stainless steels behave similarly to austenitic stainless steels concerning the formation mechanisms of the nitrided surface at different process temperatures. **Figure 14** shows the microstructures of AISI 420 steel after plasma

**Figure 14.** *Microstructures of nitrided surfaces at (a) 550°C and (b) 380°C [33].*
