**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 (Bell. T & Sun, 1998).

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).

Low Temperature Thermochemical Treatments

solution by the Cr "trap sites".

2006).

nitrogen counterparts, show some advantage.

of Austenitic Stainless Steel Without Impairing Its Corrosion Resistance 325

Christiansen et al., 2004). Expanded austenite without nitrides/carbides is obtained when high amounts of atomic nitrogen and/or carbon are dissolved in stainless steel at temperature below 450 oC for nitrogen and about 550 oC for carbon. The nitrogen/carbon atoms are presumed to reside in the octahedral interstices of the f.c.c. lattice (Christiansen et al, 2004). Long range order among the nitrogen/carbon atoms has so far not been confirmed with X-ray diffraction techniques. Typically, nitrogen contents in expanded austenite range from 20 to 30 at% N; carbon contents range from 5 to 12 at% C (Sun et al, 1999 & Blawert et al, 2001). In terms of N:Cr ratio the homogenity range of nitrogen-expanded austenite spans from approximately 1:1 to 3:1 (Christiansen & Somers, 2005). Expanded austenite is metastable and tends to develop chromium nitrides/carbides (Li et al, 1999; Jirásková, et al., 1999; Christiansen & Somers, 2005,). The high interstitial content of C/N is obtained because of the relatively strong affinity of Cr atoms for N and (to a lesser extent) C atoms, leading to anticipated short range ordering of Cr and N/C. Due to the low mobility of Cr atoms as compared to interstitial N/C atoms at lower treatment temperatures, chromium nitrides/carbides do not precipitate until after long exposure times and N/C is kept in solid

The improvement in wear resistance is perhaps the most outstanding feature of EA. The degree of improvement depends on the sliding conditions, but volume losses between one and two orders of magnitude lower than the untreated ASS are commonly reported for dry sliding (Thaiwatthana et al, 2002). This improvement is attributed to the increased surface hardness, with a typical ratio 4:1 compared to the untreated ASS (Qu et al, 2007). The EA layer prevents the surface from undergoing plastic deformation, and changes the wear mechanism from adhesion and abrasion, to a mild oxidational wear regime (Qu et al, 2007). However, under heavier loads, deformation of the subsurface occurs and leads to catastrophic failure, through propagation of subsurface cracks and spallation of the EA layer (Sun & Bell, 2002). In this way, the carbon EA layers, being thicker and tougher than their

With regard to corrosion, the results vary significantly depending on the testing conditions. Surprisingly, most researchers found that low temperature nitriding and/or carburizing do not harm the corrosion resistance of ASS, or even improve it. No conclusive explanation has been found for this improved corrosion behaviour, although it is clear that the benefit stands as long as nitrogen and carbon remain in solution and EA is free of precipitates (Li & Dong, 2003). In NaCl solutions, it is generally reported that EA remains passive under similar or wider range of potential compared to the untreated ASS, carbon EA showing a marginal advantage over nitrogen EA (Martin et al, 2002). Similar or slightly higher initial current densities have usually been measured on EA, together with the absence of pitting potential, in contrast to what is usual for ASS (Aoki & Kitano, 2002). Regarding repassivation, the evidence indicates that the passive film heals slower on EA than on ASS (Dong et al,

**3.1 The influence of process variables and composition of expanded austenite** 

The depth profiles for thermochemically hardened stainless steels typically show a trend of increasing depth with higher temperatures and longer process durations. The very hard layer of nitrogen-expanded austenite exhibits a relatively shallow depth with an abrupt transition to the softer substrate material. The high hardness values associated with nitrided

These have proven feasible and industrially acceptable for performing low temperature nitriding and carburizing of stainless steels, provided that the component surface is activated before the gaseous process by special chemical treatments and the oxide film formed during the gaseous process is disrupted by introducing certain special gas components (Gemma et al, 2001).

Fluidized bed as one method of thermochemical surface treatments could employed as the expanded austenite (EA) layer formation on source of interest. To obtain the structure, thickness and and quality of the alloyed zone of γN and γC can be controlled by the processing parameters, such as temperature, time and gas composition in the fluidized bed. The duplex surface layer by combined carburizing and nitriding of 316L steel should be thick and mildly dropping hardness profile. Focusing in the concentration of hybrid process in terms of surface morphology, elemental profiles/structural characteristics, hardness and tribological properties, and corrosion behavior were placed in this presentation.

The use of fluidized bed furnace in heat treating operation has been introduced by Reynoldson which offers several advantages, including faster treatment time, precise control of treatment parameters, despite its economic benefits of low investment and operational cost (Reynoldson, 1995; Haruman & Sun, 2005). The schematic picture of fluidized bed furnace is shown on Fig. 4. Recent work has shown that low temperature nitriding of austenitic stainless steel is possible in a fluidized bed furnace (Haruman & Sun, 2005).

Fig. 4. Schematic picture of Fluidized bed furnace.

It is nowadays widely accepted that hard, wear and corrosion resistant surface layers can be produced on Austenitic stainless steel by means low temperature nitriding and/or carburizing in a number of different media (salt bath, gas or plasma), each medium having its own strengths and weaknesses (Bell, 2002). In order to retain the corrosion resistance of austenitic stainless steel, these processes are typically conducted at temperatures below 450 ºC and 500 ºC, for nitriding and carburizing respectively. The result is a layer of precipitation free austenite, supersaturated with nitrogen and/or carbon, which is usually referred to as S-phase or expanded austenite (Sun et al, 1999; Li, 2001; Li, et al., 2002; Christiansen, 2006).

Expanded austenite is the microstructural feature which responsible for the highly demanded combination of excellent corrosion and wear performances. Expanded austenite X (X = N, C) hitherto also called S-phase (Ichii et al, 1986; Thaiwatthana et al, 2002;

These have proven feasible and industrially acceptable for performing low temperature nitriding and carburizing of stainless steels, provided that the component surface is activated before the gaseous process by special chemical treatments and the oxide film formed during the gaseous process is disrupted by introducing certain special gas

Fluidized bed as one method of thermochemical surface treatments could employed as the expanded austenite (EA) layer formation on source of interest. To obtain the structure, thickness and and quality of the alloyed zone of γN and γC can be controlled by the processing parameters, such as temperature, time and gas composition in the fluidized bed. The duplex surface layer by combined carburizing and nitriding of 316L steel should be thick and mildly dropping hardness profile. Focusing in the concentration of hybrid process in terms of surface morphology, elemental profiles/structural characteristics, hardness and

The use of fluidized bed furnace in heat treating operation has been introduced by Reynoldson which offers several advantages, including faster treatment time, precise control of treatment parameters, despite its economic benefits of low investment and operational cost (Reynoldson, 1995; Haruman & Sun, 2005). The schematic picture of fluidized bed furnace is shown on Fig. 4. Recent work has shown that low temperature nitriding of austenitic stainless steel is possible in a fluidized bed furnace (Haruman & Sun, 2005).

It is nowadays widely accepted that hard, wear and corrosion resistant surface layers can be produced on Austenitic stainless steel by means low temperature nitriding and/or carburizing in a number of different media (salt bath, gas or plasma), each medium having its own strengths and weaknesses (Bell, 2002). In order to retain the corrosion resistance of austenitic stainless steel, these processes are typically conducted at temperatures below 450 ºC and 500 ºC, for nitriding and carburizing respectively. The result is a layer of precipitation free austenite, supersaturated with nitrogen and/or carbon, which is usually referred to as S-phase or expanded austenite (Sun et al, 1999; Li, 2001; Li, et al., 2002;

Expanded austenite is the microstructural feature which responsible for the highly demanded combination of excellent corrosion and wear performances. Expanded austenite X (X = N, C) hitherto also called S-phase (Ichii et al, 1986; Thaiwatthana et al, 2002;

tribological properties, and corrosion behavior were placed in this presentation.

components (Gemma et al, 2001).

Fig. 4. Schematic picture of Fluidized bed furnace.

Christiansen, 2006).

Christiansen et al., 2004). Expanded austenite without nitrides/carbides is obtained when high amounts of atomic nitrogen and/or carbon are dissolved in stainless steel at temperature below 450 oC for nitrogen and about 550 oC for carbon. The nitrogen/carbon atoms are presumed to reside in the octahedral interstices of the f.c.c. lattice (Christiansen et al, 2004). Long range order among the nitrogen/carbon atoms has so far not been confirmed with X-ray diffraction techniques. Typically, nitrogen contents in expanded austenite range from 20 to 30 at% N; carbon contents range from 5 to 12 at% C (Sun et al, 1999 & Blawert et al, 2001). In terms of N:Cr ratio the homogenity range of nitrogen-expanded austenite spans from approximately 1:1 to 3:1 (Christiansen & Somers, 2005). Expanded austenite is metastable and tends to develop chromium nitrides/carbides (Li et al, 1999; Jirásková, et al., 1999; Christiansen & Somers, 2005,). The high interstitial content of C/N is obtained because of the relatively strong affinity of Cr atoms for N and (to a lesser extent) C atoms, leading to anticipated short range ordering of Cr and N/C. Due to the low mobility of Cr atoms as compared to interstitial N/C atoms at lower treatment temperatures, chromium nitrides/carbides do not precipitate until after long exposure times and N/C is kept in solid solution by the Cr "trap sites".

The improvement in wear resistance is perhaps the most outstanding feature of EA. The degree of improvement depends on the sliding conditions, but volume losses between one and two orders of magnitude lower than the untreated ASS are commonly reported for dry sliding (Thaiwatthana et al, 2002). This improvement is attributed to the increased surface hardness, with a typical ratio 4:1 compared to the untreated ASS (Qu et al, 2007). The EA layer prevents the surface from undergoing plastic deformation, and changes the wear mechanism from adhesion and abrasion, to a mild oxidational wear regime (Qu et al, 2007). However, under heavier loads, deformation of the subsurface occurs and leads to catastrophic failure, through propagation of subsurface cracks and spallation of the EA layer (Sun & Bell, 2002). In this way, the carbon EA layers, being thicker and tougher than their nitrogen counterparts, show some advantage.

With regard to corrosion, the results vary significantly depending on the testing conditions. Surprisingly, most researchers found that low temperature nitriding and/or carburizing do not harm the corrosion resistance of ASS, or even improve it. No conclusive explanation has been found for this improved corrosion behaviour, although it is clear that the benefit stands as long as nitrogen and carbon remain in solution and EA is free of precipitates (Li & Dong, 2003). In NaCl solutions, it is generally reported that EA remains passive under similar or wider range of potential compared to the untreated ASS, carbon EA showing a marginal advantage over nitrogen EA (Martin et al, 2002). Similar or slightly higher initial current densities have usually been measured on EA, together with the absence of pitting potential, in contrast to what is usual for ASS (Aoki & Kitano, 2002). Regarding repassivation, the evidence indicates that the passive film heals slower on EA than on ASS (Dong et al, 2006).
