**5. Plasma diffusion surface treatment**

A plasma diffusion technology, which combines a plasma nitriding treatment with a subsequent solubilization treatment, was proposed in pioneering work by Pinedo et al. for solid-state alloying [68, 69]. A 1 mm thick sample of AISI 316 L austenitic stainless steel, plasma nitrided at 470°C for 12 h, in a 1N2:1H2 gas mixture, formed a 60 μm deep, 1290 HV hard nitrided layer composed of γ + CrN + Cr2N. After nitriding, the material was solubilized at 1150°C and cooled in water to promote the diffusion and homogeneous redistribution of nitrogen through the sheet's cross-section. After solubilizing, a 0.80 wt.%N, homogeneous nitrogen content was found throughout the thickness, consistent with Thermocalc© predictions. **Figure 34** shows the nitrogen enrichment profile on the nitrided surface, reaching a maximum content of 10 wt.%. **Figure 35** shows the hardness profile along the sheet thickness showing a homogeneous increase in hardness from 200 to 300 HV, achieving complete hardening of the cross-section.

**Figure 34.** *Compositional profile of nitrogen obtained by GDOES after plasma nitriding of AISI 316 L steel [69].*

**Figure 35.** *Transverse hardening of AISI 316 steel nitrided under plasma and solubilized at 1150°C [69].*

Recently [70] Berton et al. applied the same principle for an AISI 409 ferritic stainless steel. AISI 409 specimens 0.8N2:0.2H2 plasma nitrided at 510°C for 2 h and later solubilized at 1100°C for 1 h for nitrogen diffusion in the ferritic matrix. Once the steel was solubilized, it was subjected to quenching and tempering treatments to obtain a high surface hardness tempered martensite layer. **Figure 36** presents the nitrogen profile along the cross-section for the solubilized (NS) and tempered conditions from 950°C (Q950) and 1050°C (Q1050), showing a maximum effective enrichment of the order of 1% by mass. This enrichment after the complete quenching and tempering cycles promotes a maximum rise in surface hardness close to 4x the core hardness at transverse hardening depths of up to 600 μm, as shown in **Figure 37**.

### **Figure 36.**

*Compositional profile of nitrogen determined by WDS in the diffused condition (NS) and after quenching from 950°C (NS-Q950) and 1050°C (NSQ1050) [70].*

**Figure 37.**

*Transverse hardening profiles of AISI 409 steel under conditions; solubilized (NS) and after quenching at 950°C (Q950) and 1050°C (Q1050), and tempering for 1 h at 250 (T2), 450 (T4), and 650°C (T6). The untreated condition is shown for comparison [70].*
