**2. Enlarging application of Austenitic Stainless Steel**

Starting from the mid of 1980's, investigations have been performed to improve surface hardness of ASS and thus enlarging their possibility of wider application, but led significant loss of its corrosion resistance. This tendency occur due to the sensitivity effect. Sensitization is a common problem in austenitic steel where precipitation of chromium carbides (Cr23C6) occurs at the grain boundaries at elevated temperatures, typically between 450 to 850oC; diffusional reaction in forming chromium nitride/carbide leads to the depletion of Cr in the austenitic solid solution and consequently unable to produce Cr2O3 passive layer to make stainless feature. As a result, it reduces the corrosion resistance property of the stainless

Low Temperature Thermochemical Treatments

of Austenitic Stainless Steel Without Impairing Its Corrosion Resistance 319

Fig. 2. The steps to process specification starting from required part properties.

Thermochemical treatments, sometimes referred to as case hardening or cementation, are based on the modification of the chemical composition of the substrate material. These treatments can be succeeded by a change in the structure through heat treatment. The formal definition available in BS EN 10052:1994 reads as follows (British standard, 1994):

*Thermochemical treatment*: Heat treatment carried out in a medium suitably chosen to produce a

*Diffusion treatment*: Heat treatment or operation intended to cause the diffusion towards the interior of the ferrous product of elements previously introduced into the surface (for

The two major low temperature thermochemical processes developed for austenitic stainless steels are nitriding and carburizing (Lewis et al, 1993; Bell. T, 2002). The former is normally carried out at temperatures below 450ºC and the later below 500ºC. The purpose of using low temperatures is to suppress the formation of chromium nitrides and carbides in the alloyed layers, such that chromium is retained in solid solution for corrosion protection (Sun et al, 1999; Thaiwatthana et al, 2002). Hardening of the nitrided layer and the carburised layer is due to the incorporation of nitrogen and carbon respectively in the austenite lattice, forming a structure termed expanded austenite, which is supersaturated with nitrogen and carbon respectively (Lewis et al, 1999; Thaiwatthana et al, 2002). More recently, a hybrid process has also been developed, which combines the nitriding and carburizing actions in a single process cycle by introducing nitrogen and carbon simultaneously into the austenite lattice to form a hardened zone comprising a nitrogen expanded austenite layer on top of a carbon expanded austenite layer (Tsujikawa et al, 2005; Sun et al, 2008; Li et al, 2010). There exist some synergetic effects between nitrogen and carbon: under similar processing conditions, the hybrid treated layer is thicker, harder and possesses better corrosion

From these definitions it becomes clear that two main factors will govern the process, namely: the exchange or absorption reaction with the medium, and the diffusion in the metal (ASM, 1977). As it is illustrated in Fig. 3, the medium will determine the way in which

change in the chemical composition of the base metal by exchange with the medium.

**2.1 Thermochemical and diffusion surface engineering treatments** 

In the case of diffusion treatment, the definition in that same standard is:

example, following carburizing, boriding or nitriding).

resistance than the individual nitrided layer and carburised layer.

steel. This phenomenon causes reduction in ductility, toughness and aqueous corrosion resistance (Clark & Varney, 1962).

The efforts have been made in the past decades to modify the surfaces of these materials to improve their surface hardness, wear resistance as well as corrosion resistance which is shown in Fig. 1.

Fig. 1. Enlarging application of Austenitic Stainless Steel (Czerwiec, 2010).

Bell et al. (T. Bell, 2001) suggested that a low temperature nitriding can eliminate the formation of chromium nitrides but at the expense of strengthening effects made by CrN precipitates. Alternatively, the strengthening effect will be replaced by supersaturation of interstitial species in austenite matrix which leads to the hardening of the surface region several tens micro meter thick. This precipitation-free nitride layer not only exhibits high hardness but also possesses good corrosion resistance due to the availability of retaining chromium in solid solution for corrosion protection.

In relation with the functional properties of a part, such as fatigue and static strength, or wear and corrosion resistance, are the basis for specifying the proper process and steel as illustrated in Fig. 2. (T. Bell, 2005). The functional part properties that essentially depend on the compound layer are wear resistance, tribological properties, corrosion resistance and general surface appearance. Both abrasive and adhesive wear resistance increase with hardness and with minimised porosity of the compound layer. Porosity can be positive in lubricated machinery parts as the pores act as lubricant reservoirs. The compound layer depth has to be deep enough not to be worn away. The diffusion layer (depth, hardness and residual stress) determines surface fatigue resistance and resistance to surface contact loads.

steel. This phenomenon causes reduction in ductility, toughness and aqueous corrosion

The efforts have been made in the past decades to modify the surfaces of these materials to improve their surface hardness, wear resistance as well as corrosion resistance which is

Fig. 1. Enlarging application of Austenitic Stainless Steel (Czerwiec, 2010).

chromium in solid solution for corrosion protection.

Bell et al. (T. Bell, 2001) suggested that a low temperature nitriding can eliminate the formation of chromium nitrides but at the expense of strengthening effects made by CrN precipitates. Alternatively, the strengthening effect will be replaced by supersaturation of interstitial species in austenite matrix which leads to the hardening of the surface region several tens micro meter thick. This precipitation-free nitride layer not only exhibits high hardness but also possesses good corrosion resistance due to the availability of retaining

In relation with the functional properties of a part, such as fatigue and static strength, or wear and corrosion resistance, are the basis for specifying the proper process and steel as illustrated in Fig. 2. (T. Bell, 2005). The functional part properties that essentially depend on the compound layer are wear resistance, tribological properties, corrosion resistance and general surface appearance. Both abrasive and adhesive wear resistance increase with hardness and with minimised porosity of the compound layer. Porosity can be positive in lubricated machinery parts as the pores act as lubricant reservoirs. The compound layer depth has to be deep enough not to be worn away. The diffusion layer (depth, hardness and residual stress) determines surface fatigue resistance and resistance to surface contact loads.

resistance (Clark & Varney, 1962).

shown in Fig. 1.

Fig. 2. The steps to process specification starting from required part properties.
