**2.2 Diffusion in austenitic stainless steel**

The mechanisms of nitriding and carburizing involve the transfer of the diffusing species to the surface, the establishment of a diffusing species activity gradient which drives the diffusion process, and the diffusion for itself, may be accompanied by the formation of nitrides or carbides (on the surface or in the core). The diffusion of interstitial species into a metal can only proceed if it exists a chemical potential (or activity) gradient of those species between the surface and the core of the material.

The first step of a thermochemical treatment therefore leads to enrichment of the treated substrate surface with active species. This process makes it necessary to decompose or activate (thermally or in plasma) the gaseous atmosphere and to bring the active species to the surface, so that they can be initially absorbed and afterwards diffuse into the substrate.

The diffusion of the nitrogen and/or carbon elements successively leads to the following steps: (i) the formation of a diffusion layer enriched with the diffusing elements and if the

Low Temperature Thermochemical Treatments

(Bell. T & Sun, 1998).

of Austenitic Stainless Steel Without Impairing Its Corrosion Resistance 323

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.

**3. Thermochemical surface treatment to produce expanded austenite** 

As it has been known that the chemical composition of austenitic stainless steel makes them fully austenitic up to room temperature, and thus no phase transformation hardening takes place upon quenching. Consequently, surface treatments are an interesting alternative way to increase the surface hardness and improve the wear resistance. However, surface treatment of this steel has traditionally been considered bad practice (ASM , 1961), as it poses two main problems: the passive oxide film and the precipitation of chromium carbides (Sun et al, 1999). The passive chromium oxide film on austenitic stainless steel is stable under a wide range of conditions and isolates the substrate from the environment. This effect has been of interest for austenitic stainless steel components exposed to carburizing gas mixtures, either in service (Christ, 1998 & Yin, 2005) or for surface engineering purposes (Ueda et al, 2005). In the latter case, the oxide layer impairs diffusion of the hardening elements and, consequently, needs to be removed by applying some sort of surface activation process prior to the surface engineering treatment (Parascandola et al, 2001 & Sommers et al, 2004). Furthermore, traditional surface engineering treatments are conducted at high temperature, around 500–600 ºC in the case of nitriding, and 900–1000 ºC for carburizing (Zhang et al, 1985 & Ueda et al, 2005). At these temperatures, and with increasing availability of nitrogen and carbon from the hardening medium, profuse precipitation of chromium nitrides and carbides occurs, leading to a marked deterioration of the corrosion resistance of Austenitic stainless steel. However, low temperature thermochemical diffusion treatments with nitrogen and/or carbon have been reported to increase the surface hardness without affecting or even improving the corrosion resistance

The most popular technology used to achieve the aforementioned low temperature thermochemical treatments of stainless steels is plasma technology, namely plasma nitriding (Rie & Broszeit, 1995; Stinville et al, 2010), plasma carburizing (Sun, 2005, Tsujikawa et al, 2007) and plasma hybrid treatments (Sun, 2008; Li et al, 2010). Due to the formation of a native oxide film stainless steel surface when exposed to air or residual oxygen before and during the treatment process, it is rather difficult to facilitate nitrogen and carbon mass transfer from the treatment media to the component surface. However, during plasma processing, due to the sputtering effects of energetic ions, the oxide film can be removed easily and effective mass transfer is obtained. This makes the plasma technology unique for surface treatment of stainless steels. An alternative is using the more conventional gaseous processes like gas nitriding (Gemma et al, 2001) and gas carburizing (Ernst et al, 2007).

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

Tables 1. (a) and (b) summarize the possible nitriding and carburizing configuration as described by Hertz, et al. (2008).


**(b)** 

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

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 barrier to the diffusion of nitrogen and carbon.
