**2.1 Thermochemical and diffusion surface engineering treatments**

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

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

*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 example, following carburizing, boriding or nitriding).

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 resistance than the individual nitrided layer and carburised layer.

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

Low Temperature Thermochemical Treatments

Bell, 1991).

coefficients.

diffusion zone are respectively:

phase with the slowest diffusion properties.

**2.2 Diffusion in austenitic stainless steel** 

between the surface and the core of the material.

of Austenitic Stainless Steel Without Impairing Its Corrosion Resistance 321

1972), and treatments as long as 72 hours are common practice in industry. On the other hand, thermochemical treatments produce smooth case-core interfaces, which are beneficial for not only the wear and fatigue performance, but also the load bearing capacity (Sun &

During nitriding, ammonia (NH3) in the furnace atmosphere decomposes into hydrogen and nitrogen at the surface, enabling nitrogen atoms to be adsorbed at the steel surface and to diffuse further into the steel as illustrated in Fig. 3. In nitrocarburizing it is additionally

The flux of nitrogen and carbon from the gas to the steel surface is proportional to the

 *dmN /dt(surface) = k1 [ cN (gas) – cN (surf) ]* (5)

Here *m* denotes mass, *t* time, *c* concentration per volume unit and *k1* and *k2* are reaction rate

The transfer of nitrogen and carbon from the surface further into the steel is controlled by diffusion. Diffusion rates follow Fick's first law, which for the compound layer and

*dm/dt(diff zone) = – DDiff dc/dx* (8)

The slowest of the three stages controls the nitrogen and carbon transfer rates. For a compound layer consisting of alternating ε-γ'-ε layers, the rate will be determined by the

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

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

*dmC /dt(surface) = k2 [ cC (gas) – cC (surf) ]* (6)

 *dm/dt(comp layer) = – DComp dc/dx* (7)

 *dm/dt(surface) = dm/dt(comp layer) = dm/dt(diff zone)* (9)

necessary to have a carbonaceous gas transferring carbon to the steel surface.

concentration differences between the gas and the surface:

Balance of mass requires that all three mass transfer rates are equal:

the diffusing elements are delivered to the metal surface. A number of different media are available (solid, liquid, gas and plasma), and a detailed account of the media used for carburizing will be given in a following section.

Fig. 3. Concentrations and concentration gradients of nitrogen and carbon. (Christiansen & Somers, 2005)

Once in the metal, the transport of the absorbed substance takes place by diffusion, and follows Fick's laws:

$$\mathbf{J} \equiv -\mathbf{D} \parallel d\mathbf{C} / d\mathbf{x} \llcorner \tag{1}$$

$$\mathbf{J} = -\mathbf{D} \left[ \, \left\| \mathbf{\mathcal{C}} \right\| \, \left\| \mathbf{\mathcal{S}} \right\| \right] \tag{2}$$

$$\mathfrak{BC} \mid \mathfrak{dt} = \left[ \begin{array}{c} \mathfrak{BC} \end{array} \right] / \mathfrak{dc}^2 \mid \mathfrak{dc}^2 \end{array} \tag{3}$$

where *J* is the flux of diffusing substance, *D* is the diffusion coefficient, and *∂C / ∂x* is the concentration gradient (ASM, 1977). Therefore, the transport of the substance in solution is driven by its concentration gradient and the diffusion coefficient which, at the same time, depends on the temperature, the chemical composition and phase structure of the substrate. For a given alloy, kept at constant temperature in a medium with a consistent concentration of the substance of interest, the case depth will only depend on the time, according to equation (9):

$$\mathbf{x} = \mathbf{a} \text{ (Dt)}\prime = \mathbf{K}t\prime\prime\tag{4}$$

where *x* is the case depth, a is a constant, *D* is the element diffusivity, *t* is the treatment time and *K* is a factor determined by a and *D* (ASM, 1977). Higher treatment temperatures yield the same case depth in shorter time, although there are technical limitations related with life of the furnaces, and metallurgical considerations regarding the side effects of keeping the substrate material at high temperatures (Parrish & Harper, 1994). Consequently, diffusion treatments are slower when compared to other surface deposition techniques (Hurricks,

the diffusing elements are delivered to the metal surface. A number of different media are available (solid, liquid, gas and plasma), and a detailed account of the media used for

Fig. 3. Concentrations and concentration gradients of nitrogen and carbon. (Christiansen &

Once in the metal, the transport of the absorbed substance takes place by diffusion, and

 ∂C / ∂t = [ ∂2C / ∂x2 ] / ∂x2 (3) where *J* is the flux of diffusing substance, *D* is the diffusion coefficient, and *∂C / ∂x* is the concentration gradient (ASM, 1977). Therefore, the transport of the substance in solution is driven by its concentration gradient and the diffusion coefficient which, at the same time, depends on the temperature, the chemical composition and phase structure of the substrate. For a given alloy, kept at constant temperature in a medium with a consistent concentration of the substance of interest, the case depth will only depend on the time, according to

where *x* is the case depth, a is a constant, *D* is the element diffusivity, *t* is the treatment time and *K* is a factor determined by a and *D* (ASM, 1977). Higher treatment temperatures yield the same case depth in shorter time, although there are technical limitations related with life of the furnaces, and metallurgical considerations regarding the side effects of keeping the substrate material at high temperatures (Parrish & Harper, 1994). Consequently, diffusion treatments are slower when compared to other surface deposition techniques (Hurricks,

 *J = – D [ dC/ dx]* (1)

J = – D [ ∂C / ∂x] (2)

 *x = a (Dt)½ = Kt½* (4)

carburizing will be given in a following section.

Somers, 2005)

equation (9):

follows Fick's laws:

1972), and treatments as long as 72 hours are common practice in industry. On the other hand, thermochemical treatments produce smooth case-core interfaces, which are beneficial for not only the wear and fatigue performance, but also the load bearing capacity (Sun & Bell, 1991).

During nitriding, ammonia (NH3) in the furnace atmosphere decomposes into hydrogen and nitrogen at the surface, enabling nitrogen atoms to be adsorbed at the steel surface and to diffuse further into the steel as illustrated in Fig. 3. In nitrocarburizing it is additionally necessary to have a carbonaceous gas transferring carbon to the steel surface.

The flux of nitrogen and carbon from the gas to the steel surface is proportional to the concentration differences between the gas and the surface:

$$\text{d}m\_N/\text{d}\text{t}\_{\text{(surforec)}} = k\_1 \left\{ \text{c}\_{N \text{ (gas)}} - \text{c}\_{N \text{ (surf)}} \right\} \tag{5}$$

$$\text{Am}\_{\text{C}}/\text{dt}\_{\text{(surfacc)}} = k\_2 \left\{ \text{c}\_{\text{C} \text{ (gas)}} - \text{c}\_{\text{C} \text{ (surf)}} \right\} \tag{6}$$

Here *m* denotes mass, *t* time, *c* concentration per volume unit and *k1* and *k2* are reaction rate coefficients.

The transfer of nitrogen and carbon from the surface further into the steel is controlled by diffusion. Diffusion rates follow Fick's first law, which for the compound layer and diffusion zone are respectively:

$$\text{dm/dt}\_{\text{(comp layer)}} = -\text{D}\_{\text{Comp}} \,\text{dc/dx} \tag{7}$$

$$dm\% \text{dt}\_{(diff\\_zure)} = -\text{D}\_{\text{D\\_ff}} \text{dc}\% \text{dx} \tag{8}$$

Balance of mass requires that all three mass transfer rates are equal:

$$\text{dm/dt}\_{\text{(surface)}} = \text{dm/dt}\_{\text{(conv 1 layer)}} = \text{dm/dt}\_{\text{(diff 2 zone)}} \tag{9}$$

The slowest of the three stages controls the nitrogen and carbon transfer rates. For a compound layer consisting of alternating ε-γ'-ε layers, the rate will be determined by the phase with the slowest diffusion properties.
