**5.3 Wear property**

332 Corrosion Resistance

(a)

(b)

Such distributions of nitrogen and carbon in the surface layer are likely to produce some beneficial influences upon the properties of hybrid treated 316L steel. Figure 6b shows results of nitrogen concentration on hybrid dual-stages obtained from energy dispersive Xray (EDS) analysis. According to these curves, it can be clearly seen that the surface hybrid layer contains very high amount of nitrogen, and nitrogen concentration is gradually reducing from surface to the core with distance increasing due to a low diffusion rate in the case of samples at low temperature. However, some carbon remains in the sub-surface layer. Fig. 10 summarizes the phase compositions in the treated specimens as determined by XRD from the specimen treated at 450°C for 8h. As confirmed by XRD analysis in Fig. 10, the nitriding treated surface layer comprises mainly the S phase or the expanded austenite. For the hybrid process, consisting of dual layers (Figs. 7d & 7e), revealed another thin interfacial layer. This interfacial layer is believed to be due to the accumulation of carbon as has also been reported in literature (Sun, 2006). One interesting aspect of the diffraction displayed in Fig. 10 regards the variation of the (200) diffraction line width in relation with 2θ angle. This behaviour can be explained by the lattice distortion caused by the greater amount of nitrogen in the interstitial sites and/or only by crystallographic orientation present in this

Fig. 9. Carbon profiles (a) and nitrogen profiles (b) along the depth.

phase. The XRD analysis did not show any peak from nitride or carbide phase.

In accordance with the findings for plasma nitriding (Lewis, 1993; Rie, 1995), the S phase layer produced in this fluidized bed furnace process has minimal chromium nitride/carbide precipitation. Comparing the diffractograms for the nitrided samples with the untreated material, it clearly shows that Bragg reflections (peaks) are shifted to lower 2θ angles. It was The wear properties of the low temperature surface treatment specimens as weight loss under dry sliding friction are presented in Fig. 11 along with an untreated specimen for comparison purpose. The results suggest that the fluidized bed thermochemical-treated specimens have excellent wear resistance. The 8N specimen has the highest wear resistance compared to the values of 4C-4N, 8(C+N), and 8C specimens.

Fig. 11. Wear under dry sliding condition.

Low Temperature Thermochemical Treatments

densities where the re-passivation behavior start to occur.

**316SS 316L** 

Fig. 12. Anodic polarization curves measured in 3.0%NaCl.



0

250

E, mV/SCE

500

750

1000

carburizing-CT.

**6. Conclusion** 

450

of Austenitic Stainless Steel Without Impairing Its Corrosion Resistance 335

dissolution current density increased slowly and gradually with applied potential (Y. Sun & E. Haruman, 2008). Although in the first 250 mV/SCE scan of CT show small increases in current

**NCT**

Although, the corrosion behaviour of low temperature nitrided, carburized and hybrid stainless steel thermochemical treatment have been investigated by several investigators (Zhang et al, 1985; Rie et al, 1995; Sun et al, 1999), the reason for improvement of corrosion resistance due to nitrogen and carbon supersaturation in austenite has not been fully understood. A possible mechanism is that supersaturation of nitrogen and carbon promotes the improvement of the passivation ability of austenite, and this effect seems to give beneficial with increasing degree of supersaturation (Munther et al, 2004). Thus, the improvement of corrosion resistance for hybrid-NCT treated material may be attributed to the extremely large supersaturation of the upper part of nitrogen-enriched layer with both nitrogen and carbon. This would be contribute to the observed higher hardness and better corrosion resistance as compared to those achieved by individual nitriding-NT and

1E-07 0.00001 0.001 0.1 10 1000 Current Density (mA/cm2

)

**NT** 

**CT**

**untreated**

The thermochemical treatments of AISI 316L stainless steel in a fluidized bed process at

C demonstrate that it is possible to produce hard layer of an expanded austenite phase without precipitation of chromium carbide/nitride. For nitriding and carburizing treatments the expanded layers consisted of a single layer γN or γC phase while specimens treated by nitrocarburizing or hybrid process gave dual layers consisting of γN at the surface and γC ahead of γN. The layer produced in fluidized bed process is not uniform in thickness under the same treatment conditions. The nitriding treatment produced 8.35 µm

The highest hardness for 8N specimen is considered to be responsible for best wear resistance property among the treated specimens used in this investigation. However the findings suggest that nitriding, hybrid, nitrocarburizing and carburizing the austenitic stainless steel at 4500C using a fluidized bed furnace can improve surface hardness and wear resistance of austenitic stainless steel. It is to be noted that at the initial stage of sliding, all the specimens in Fig. 11 gave accelerated weight loss and then leveling off after certain period. It is presumed that at the initial stage of sliding, the 600g load of the sliding mate material was encountered by the asperities of substrate surface, which effectively caused high load sliding and thus more wear loss. The eventual dropping off may be related to smoothening of the asperities at the wear surface, which produced more contact area for the sliding load of 600g and hence reduced or constant wear rate.

The work hardening effect may also cause this tendency together with possibilities of surface oxide or carbide/nitride formation at a certain period of sliding, thus leading to an equilibrium condition of constant wear rate. However, no evidence is available to explain the exact reasons of these wear phenomena.

#### **5.4 Corrosion properties**

Corrosion tests using the electrochemical technique demonstrated that the precipitation free carburized and nitrided layers have very good corrosion resistance in the corrosive environments.

The most subtantial improvement in properties of austenitic stainless steels by the hybrid process lies in corrosion resistance as evaluated by electrochemical testing (Li & Bell, 2004). Fig. 12 shows the anodic polarization curves measured for several specimens in 3.0% NaCl solution. As expected, both individual nitriding and carburizing reduce the current density of the steel in the anodic region, indicating improved corrosion resistance. After the hybrid treatment, the anodic polarization curve is shifted towards lower current density by several orders of magnitude as compared to that for the untreated and individually nitrided and carburised steel. This registers an improvement in corrosion resistance by several orders of magnitude and signifies the excellent corrosion resistance of the hybrid treated surface. The much enhanced corrosion resistance observed for the hybrid treated surfaces may be attributed to the extremely large supersaturation of the upper part of the nitrogen-enriched layer with both nitrogen and carbon (see Fig. 9). This would contribute to the observed higher hardness and better corrosion resistance as compared to those achieved by individual nitriding and carburizing.

The treatment conditions are the same as those in Fig. 7. The electrochemical test results for Hybrid-NCT, Nitriding-NT, Carburizing-CT were described in Fig. 12. The NT and CT showing that the current density of treated stainless steel were decreased in the anodic region which indicating positive effect regarding the improvement of corrosion resistance compared to the substrate. After Hybrid-NCT treatment, the anodic polarization curved is shifted towards lower current density which explain that the corrosion rate was decreased and the polarization current measurement gave 0.00003 mA/cm2 and demonstrate an improvement in corrosion resistance as compared to that untreated and individually nitrided and carburized steel, while passivation current of NCT is the lowest followed by CT, NT and untreated respectively. This trend also similar to the maximum potential passivation behaviour since the dissolution current density increased slowly and gradually with applied potential (Y. Sun & E. Haruman, 2008). Although in the first 250 mV/SCE scan of CT show small increases in current densities where the re-passivation behavior start to occur.

Fig. 12. Anodic polarization curves measured in 3.0%NaCl.

Although, the corrosion behaviour of low temperature nitrided, carburized and hybrid stainless steel thermochemical treatment have been investigated by several investigators (Zhang et al, 1985; Rie et al, 1995; Sun et al, 1999), the reason for improvement of corrosion resistance due to nitrogen and carbon supersaturation in austenite has not been fully understood. A possible mechanism is that supersaturation of nitrogen and carbon promotes the improvement of the passivation ability of austenite, and this effect seems to give beneficial with increasing degree of supersaturation (Munther et al, 2004). Thus, the improvement of corrosion resistance for hybrid-NCT treated material may be attributed to the extremely large supersaturation of the upper part of nitrogen-enriched layer with both nitrogen and carbon. This would be contribute to the observed higher hardness and better corrosion resistance as compared to those achieved by individual nitriding-NT and carburizing-CT.
