**5.2 Elemental profile of carbon and nitrogen**

The carbon profile investigated on 4C-4N specimen using EDS FE-SEM is depicted in Fig. 9. It is found that higher carbon at the deeper layer which indicates that carbon pushed-ahead by the incoming nitrogen atom and the dissolved carbon is accumulated at the front of the nitride layer which has also been reported in the literature (Lewis et al, 1993).

Elemental analysis of the Hybrid specimen CN gave more carbon beyond the nitrided layer, but some carbon was also observed at the surface. The variations of chemical concentration in the hybrid nitrocarburized layer were also measured with EDS. These figures shows the typical nitrogen and carbon profiles produced in treated 316L steel. It can be seen from these Figures that there are two features in the nitrogen and the carbon profiles. Firstly, the nitrogen profile on the surface of the treated layer is similar to that of nitrided 316L steel. Secondly, the maximum nitrogen concentration is on the surface and the maximum carbon concentration appears beneath as if carbon was 'pushed' to the middle of the layer by nitrogen.

Low Temperature Thermochemical Treatments

and lower defect density in γC layers as compared to γN layers.

Fig. 10. Comparison of XRD patterns of treated specimens.

compared to the values of 4C-4N, 8(C+N), and 8C specimens.

untreated 8C 8N 8(C+N) 4C-4N

Fig. 11. Wear under dry sliding condition.

**5.3 Wear property** 

of Austenitic Stainless Steel Without Impairing Its Corrosion Resistance 333

caused primarily by the dissolution of nitrogen which causes a dilation of the fcc lattice (hence the name expanded austenite), although residual stress and stacking faults also play a role in this respect (Somers, 2005). The X-ray diffraction pattern of carburised AISI 316L is shown in Fig. 10. γC is identified as the only phase present in the surface adjacent region, i.e. within the information depth for the probing X-ray beam. A marked difference is observed as compared to nitrided AISI 316L; a smaller shift of the austenite peaks to lower 2θ, which indicates a substantially lower content of the interstitially dissolved atoms, provided that nitrogen and carbon induce a similar distortion in the fcc lattice. The asymmetrical (200) austenite peak in Fig. 10 indicates a depth-gradient of the carbon content in the near surface zone. The distinct peaks for the carburised sample indicate a smooth concentration gradient

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

**Linear Wear test Results**

0 10 20 30 40 50 60 **Time [minutes]**

0 0.2 0.4 0.6 0.8 1

**Cummulative** 

**weight loss [mg]**

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

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 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 caused primarily by the dissolution of nitrogen which causes a dilation of the fcc lattice (hence the name expanded austenite), although residual stress and stacking faults also play a role in this respect (Somers, 2005). The X-ray diffraction pattern of carburised AISI 316L is shown in Fig. 10. γC is identified as the only phase present in the surface adjacent region, i.e. within the information depth for the probing X-ray beam. A marked difference is observed as compared to nitrided AISI 316L; a smaller shift of the austenite peaks to lower 2θ, which indicates a substantially lower content of the interstitially dissolved atoms, provided that nitrogen and carbon induce a similar distortion in the fcc lattice. The asymmetrical (200) austenite peak in Fig. 10 indicates a depth-gradient of the carbon content in the near surface zone. The distinct peaks for the carburised sample indicate a smooth concentration gradient and lower defect density in γC layers as compared to γN layers.

Fig. 10. Comparison of XRD patterns of treated specimens.
