**3. Low-temperature diffusion surface treatment**

Conventional gaseous or liquid nitriding processes are traditionally carried out at temperatures above 520°C. However, this process temperature is a limiting factor, considering that when the nitriding of stainless steels is conducted above 500°C, intense precipitation of chromium nitrides and carbides occurs in the diffusion zone, which, despite substantially increasing the hardness, greatly compromises corrosion resistance [14–18].

The diffusion temperature is the main control parameter to prevent chromium nitrides and chromium carbides precipitation. Precipitation of chromium carbides and nitrides requires substitutional diffusion, which only occurs at temperatures higher than 500°C. Zhang and Bell [14] and Ichii et al. [19] pioneered the study and development of stainless steel's nitrogen and carbon diffusion processes in low temperatures. The process temperature must be selected, not too low, to allow intense diffusion of the C and N interstitial elements but not high enough to permit substitutional diffusion. At these low temperatures, the chromium substitutional element's mobility is sufficiently reduced to inhibit the nucleation and growth of nitrides and/or carbides. Under these conditions, the matrix becomes continuously and increasingly enriched by the interstitial element, promoting a non-equilibrium saturation of the crystalline lattice and stabilizing expanded phases formed in the diffusion layer.

Interstitial supersaturation in the diffusion zone contributes to: (i) formation of interstitially supersaturated phases, (ii) intense interstitial hardening, as a consequence of the colossal amount of interstitial element and its stress fields, contributing to increasing the wear resistance without compromising corrosion resistance [20–24], and (iii) generation of residual compressive stresses in the expanded layer as a result of the restrictive effect of the diffusion-free substrate, which improves fatigue properties [24, 25].

When performing the X-Ray Diffraction of these supersaturated layers, it is observed that the matrix peaks are shifted to lower 2θ angles and show a greater FWHM - full-width at half-maximum height than the peaks of the unenriched matrix phase. This displacement and broadening of the peaks indicate elastic deformation due to the expansion of the crystalline lattice. Zhang and Bell [14] named this phase "Expanded Phase" due to the expansion of the lattice parameters of the crystalline unit cell. On the other hand, Ichii et al. [19] called this phase S-Phase due to the "shifting" to lower angles of the XRD peaks.

Bell and Chen [26] and Sun [27] presented a limit curve for precipitation of chromium nitrides and carbides as a function of temperature and time of plasma diffusion process for an austenitic AISI 316 L stainless steel. "Nitrogen Expanded Austenite (γN)" and "Carbon Expanded Austenite (γC)" are formed during nitriding or carburizing for temperatures and times below the limit curves shown in **Figure 7**.

Expanded phases obtained at diffusion temperatures between 350°C and 430°C are responsible for surface hardening [20, 21, 28–31]. This hardening can be obtained in all stainless steel families with the formation of different phases expanded by nitrogen and/or carbon [20, 32–36]. **Table 1** shows the different expanded phases formed by low-temperature plasma diffusion surface treatment and their hardening characteristics for different families of stainless steels.

**Figure 7.**

*Limit curves for precipitation of chromium nitrides or chromium carbides in austenite as a function of temperature and time of plasma diffusion process [26, 27].*


**Table 1.**

*Expanded phases formed during low-temperature nitriding of stainless steels.*

A passivating Cr2O3 film formed on the surface of the parts to be nitrided prevents nitrogen or carbon from entering stainless steel. Thus, the passive film's mechanical or chemical removal process should be employed before diffusion. The chemical removal of the passive film by acid pickling may compromise the surface finish of the parts or maybe potential damage to the operators' health or equipment. Currently, modern low-temperature gas nitriding processes still use acid pickling [37] for depassivation of the Cr2O3 layer during exposure of the parts'surface to atmospheres containing halides (NF3 or HCl) has been carried out in Lowtemperature gas carburizing [38]. Activation of the parts'surface by nickel plating to prevent repassivation by catalytic decomposition of NH3 gas [39] has also been used.

Activation of the surface by "sputtering" in H2, under high voltage and low pressure—[40], use not only the kinetic energy of the ions but also the reducing character of hydrogen [41], preserving the surface quality of the parts being nitrided.
