**2.3 Diffusivity of simultaneous nitrogen and carbon in austenitic stainless steel**

From diffusion experiments performed by Million et al, (1995), the interesting interactions of nitrogen and carbon are known, indicating that the presence of nitrogen enhances the activity of carbon and thus, its diffusion. It should, therefore, be possible to produce expanded austenite and to enhance the layer growth by simultaneous carbon and nitrogen

solubility of the latter in the substrate is sufficient then this diffusion layer can be out of equilibrium at low temperatures (ii) at higher temperatures the follow steps occur. The surface formation of nitride, carbide or carbonitride layers of the main element of the substrate and (iii) the subsurface precipitation of nitrides, carbides or carbonitrides of alloying elements in the substrate (e.g. Fe, Ti, Al, Cr, Mo, V). In addition to the law of thermodynamics, the formation of the various phases is also govern by the nitrogen and carbon surface activities, and therefore are related to the temperature of the process used

Tables 1. (a) and (b) summarize the possible nitriding and carburizing configuration as

**(a)** 

**(b)** 

To reduce further the potential of distortion and to avoid structural modifications of the substrate, and without repeating the quench and tempering treatments, these carburizing and nitriding treatments have evolved, in the past few years, towards lower temperature processes (350–450oC for austenitic stainless steels). This reduction in the treatment temperatures had to include specific treatments for removing oxide layers, which act as a

**2.3 Diffusivity of simultaneous nitrogen and carbon in austenitic stainless steel** 

From diffusion experiments performed by Million et al, (1995), the interesting interactions of nitrogen and carbon are known, indicating that the presence of nitrogen enhances the activity of carbon and thus, its diffusion. It should, therefore, be possible to produce expanded austenite and to enhance the layer growth by simultaneous carbon and nitrogen

Table 1. Possible configurations of (a) nitriding, (b) carburizing.

barrier to the diffusion of nitrogen and carbon.

(gaseous or plasma), and to the composition of the gas.

described by Hertz, et al. (2008).

implantation. Treatment of austenitic stainless steel in either nitrogen or methane plasma at 400 oC results in the formation of expanded austenite (Zhang et al, 1985 & Ueda et al, 2005). The different amounts of nitrogen or carbon in solid solution can be explained by the strength of the interaction between nitrogen or carbon and chromium. Williamson et al, (1994) noted that the strong interaction of nitrogen with chromium results in the trapping of nitrogen at chromium sites. This leads to a much higher supersaturation but reduced diffusivity in comparison to a methane treatment. However, the interaction is not really strong to form CrN. Carbon has a weaker interaction with chromium, so it diffuses inwards faster and a lower supersaturation is attained under similar treatment conditions. In both cases, nitrogen and carbon remain in solid solution, presumably on interstitial sites.
