**3.3 Precipitation hardening stainless steels**

The plasma nitriding process for PH precipitation-hardening stainless steels should preferably be carried out at temperatures equal to or below the aging temperature of the steel part. PH steels are aged at different temperatures, specified according to the final desired mechanical properties. A proper selection of the nitriding temperature allows for reaching the desired surface hardness without compromising the quenched pus tempered hardness achieved during aging. The plasma nitriding treatment of PH steel components may be carried out at lower or higher temperatures depending on the application and the operating conditions.

**Figure 22(a)** shows the nitrided layer and the resulting hardening of aged 17–4PH steel after 4 h at 550°C plasma nitriding treatment. For this condition, the nitrided layer is composed of a diffusion zone formed by the precipitation of iron and chromium nitrides. The precipitation of these nitrides promotes an intense surface hardening, capable of raising the surface hardness to values close to 1300 Vickers [34]. **Figure 22(b)** shows that the 35.3 HRC hardness of the substrate, previously aged for 4 h at 552°C, condition H1025 (AMS 5643 2013), practically remained unchanged after nitriding at 550°C/4 h with a measured value of 34.4 HRC [34]. **Figure 23** shows a gentle hardness profile and a nitriding depth close to 0.65 mm.

Nitriding of PH steels can also be performed at lower temperatures so as not to affect corrosion resistance [63]. **Figure 24** shows the nitrided layer of 17–4PH steel after plasma nitriding at 400°C using the active screen technique. It is observed that the nitrided layer is white and not etched by Villela's reagent, unlike the nitrided layer at 550°C, which is dark and severely etched by Villela's reagent. This difference in behavior is related to the nitriding mechanisms. As hardening in nitriding at 550°C occurs with the precipitation of chromium nitrides, corrosion resistance decreases as the matrix is depleted in chromium. When plasma nitriding is carried out at 400°C, hardening occurs by forming a nitrogen supersaturated layer of expanded martensite (α<sup>0</sup> N) without nitrides precipitation, reaching 1130 Vickers.

*Nitrided layer (a) and hardening characteristic (b) of 17–4PH steel after plasma nitriding in DC-plasma at 550°C [34].*

**Figure 23.** *Transverse hardening profile of 17–4PH steel after DC plasma nitriding at 550°C [34].*

### **3.4 Ferritic and duplex stainless steels**

Low-temperature plasma nitriding of ferritic and duplex stainless steels is still being developed and is not commercially available yet. However, many reports

**Figure 24.**

*Hardness variation after plasma nitriding of 17–4PH stainless steel α*<sup>0</sup> *<sup>N</sup> expanded martensite layer and Vickers hardness before and after ASPN at 400°C [63].*

promise good results for use in most different components and applications. When ferritic stainless steels are nitrided at low temperatures, precipitation of chromium nitrides is avoided [35, 64]. **Figure 25** shows the microstructure of a plasma nitrided AISI 410S stainless steel with a layer of expanded ferrite (αN) containing Fe3N iron nitrides. Shifted to the left, expanded ferrite (αN) peaks appear on the X-ray

**Figure 26.** *XR diffraction pattern of the surface after plasma nitriding AISI 410S stainless steel at 400°C [64].*

**Figure 27.** *Surface hardness and hardness profile of an AISI 410S stainless steel after plasma nitriding at 400°C [64].*

diffraction pattern of the nitrided layer, **Figure 26**. Vertical dashed lines indicate the positions of ferrite peaks in the matrix. Besides, Fe3N peaks were also detected. The absence of chromium nitrides in the nitrided layer grants the corrosion resistance of the nitrided surface. **Figure 27** shows the hardening obtained in the nitriding by comparing the maximum surface hardness obtained and the transversal hardening profile of the nitrided surface.

Corrosion resistance testing carried out by immersion in 3% FeCl3 aqueous solution, for 88 h, at room temperature showed a better performance of the nitrided specimens concerning the non-nitrided ones, **Figure 28**. When the steel is nitrided at a low temperature (N400°C), the corrosion properties are not changed compared to the non-nitrided condition. However, high-temperature nitriding (N530°C)

### **Figure 28.**

*Mass loss and corrosion rates of non-nitrided and plasma nitrided AISI 410S stainless steel during immersion in 3% FeCl3 aqueous solution for 88 h at room temperature [65].*

### **Figure 29.**

*Microstructure of 2205 duplex stainless steel, after low-temperature plasma nitriding. Expanded ferrite and expanded austenite [67].*

promotes a significant loss of corrosion resistance compared to the other two conditions [65].

Duplex stainless steels' microstructure is composed of austenite and ferrite in approximately equal proportions. In this condition, low-temperature nitriding leads to the formation of expanded austenite (γN) and expanded ferrite (αN) on top of ferrite and austenite strings, respectively [36, 66, 67]. **Figure 29** shows the microstructure on the surface of type 2205 duplex stainless steel after plasma nitriding at 400°C. The austenite and ferrite bands and the formation of the respective expanded phases on the nitrided surface are observed in the photomicrograph [67]. The X-ray diffraction pattern in **Figure 30** shows the initial phases' peaks and shifted to the left, the respective peaks of the nitrogen-expanded phases. Fe3N iron nitrides were also detected [67]. Consequently, the formation of expanded austenite and expanded ferrite on the surface led to an intense hardening of the nitrided surface, as shown in **Figure 31**.

**Figure 30.** *XR-diffraction pattern of 400°C plasma nitrided 2205 duplex stainless steel [67].*

**Figure 31.**

*Hardness of ferrite and austenite and expanded ferrite and expanded austenite in 400°C plasma nitrided 2205 duplex stainless steel [36].*
