**5.1 Layer morphology and hardness profile**

Hardened layers with different morphologies were observed as a result of the various treatment conditions and the thicknesses of the layers produced in different conditions are shown in Table 2. The layer thicknesses are found to be different at different treatments, and their growths against time in Fig. 6. show that layer thickness increases with processing time.

Fig. 6. Thickness of treated layers measured from micrographs.

Micrographs in Fig. 7 show that the morphology of the layer changed with treatment conditions. The two specimens processed under combined treatment conditions, 8(C+N), and 4C–4N produced duplex layers irrespective of whether they were processed simultaneously (Fig. 7b) or sequentially (Fig. 7d). The processed layer thicknesses in Table 2. show that the nitrided specimen, 8N, has a thickness between 3.26 to 8.35 m and the carburized specimen, 8C, is in between 1.00 to 3.92 m.

Furthermore, the nitrided-only 8N specimens have deeper layers than combined processed specimens. The depth of the simultaneously carburized and nitrided specimen, 8(C+N), reaches only 50% that of the nitrided specimen, and the thickness of 4C–4N specimen had only about 45% compared to the nitrided-only 8N specimen after being processed for the same duration of 8 h due to the half nitriding duration.

For a similar treatment duration, the Plasma process is reported by Tsujiwaka on 2005 which is produce about 18 m thick layer which is much higher compared to that of the present conventional nitriding treatment in fluidized bed furnace. In plasma process the native oxide layer is removed mostly by bombardment of the plasma gas which is completely absent in conventional fluidized process. This is one of the reasons why convention fluidized bed treatment produced small layer thickness compared to the corresponding plasma nitriding. Previous investigation revealed that nitriding at 450°C became effective after treatment for 6 h where a continuous nitrided layer was produced (Sun, 2006). This is due to the fact that the incubation time phenomena which may be related to the nature of

Low Temperature Thermochemical Treatments

Fig. 8. Depth profiles of Microhardness.

**5.2 Elemental profile of carbon and nitrogen** 

4C–4N specimen.

**Hardness Hv0.03**

nitrogen.

of Austenitic Stainless Steel Without Impairing Its Corrosion Resistance 331

Fig. 8 shows the hardness depth profiles of the treated specimens. The carburized 8C specimen developed a maximum hardness of about 500 Hv, which is much lower than the hardnesses of 1230 to 1588 Hv for other three nitrided and nitrocarburised specimens. The nitrided layer of the 8N specimen produced a hard layer of 1588 Hv with an abrupt layer– core interface, while the 8C carburizing produced a gradually decreased hardness profile. Two combined carburized and nitrided specimens, 8(C + N) and 4C–4N developed a similar tendency to bulge in hardness profiles at inner carburized layer as shown in Fig. 7. The most gradual decrease in hardness from 1230 Hv level to substrate hardness was displayed by the

**Depth profile of Microhardness**

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

0 10 20 30 40 **Distance from the surface[µm]**

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

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

nitride layer which has also been reported in the literature (Lewis et al, 1993).

the fluidized bed process, where the disruption of the native oxide film to cause the nitriding reactions, has to be by thermal dissociation. The higher the temperature, the faster is the dissociation of the oxide film and thus the shorter the incubation time. According to this hypothesis it is understood why a very thin discontinuous layer was formed after a 2 h treatment time ; the layer was about 8 µm thick after 8 h nitriding, which gave a bright appearance and was resistant to the etchant used to reveal the microstructure of the substrate. The treated layers for 4C-4N consist actually of two separate zones with a somewhat diffused interface which was clearly observed under microscope, but not revealed in the micrographs. The outer zone is γN and the inner zone is γC. Conversely, the nitrocarburised sample shows a distinct separation of the γN and γC layers, with the γC layer closest to the austenite substrate (as indicated in the micrograph in Fig. 7b). The X-ray diffractograms of carburised and nitrided AISI 316L show that two different types of expanded austenite are present (Fig. 6).

(a) (b)

Fig. 7. SEM micrographs of 450°C treated specimens: (a) Nitrided 8 h, (b) Nitrocarburised 8 h, (c) Carburised 8 h, (d) Hybrid 4 h Carburised followed by 4 h Nitrided.

the fluidized bed process, where the disruption of the native oxide film to cause the nitriding reactions, has to be by thermal dissociation. The higher the temperature, the faster is the dissociation of the oxide film and thus the shorter the incubation time. According to this hypothesis it is understood why a very thin discontinuous layer was formed after a 2 h treatment time ; the layer was about 8 µm thick after 8 h nitriding, which gave a bright appearance and was resistant to the etchant used to reveal the microstructure of the substrate. The treated layers for 4C-4N consist actually of two separate zones with a somewhat diffused interface which was clearly observed under microscope, but not revealed in the micrographs. The outer zone is γN and the inner zone is γC. Conversely, the nitrocarburised sample shows a distinct separation of the γN and γC layers, with the γC layer closest to the austenite substrate (as indicated in the micrograph in Fig. 7b). The X-ray diffractograms of carburised and nitrided AISI 316L show that two different types of

(a) (b)

(c) (d) Fig. 7. SEM micrographs of 450°C treated specimens: (a) Nitrided 8 h, (b) Nitrocarburised 8 h,

(c) Carburised 8 h, (d) Hybrid 4 h Carburised followed by 4 h Nitrided.

expanded austenite are present (Fig. 6).

Fig. 8 shows the hardness depth profiles of the treated specimens. The carburized 8C specimen developed a maximum hardness of about 500 Hv, which is much lower than the hardnesses of 1230 to 1588 Hv for other three nitrided and nitrocarburised specimens. The nitrided layer of the 8N specimen produced a hard layer of 1588 Hv with an abrupt layer– core interface, while the 8C carburizing produced a gradually decreased hardness profile. Two combined carburized and nitrided specimens, 8(C + N) and 4C–4N developed a similar tendency to bulge in hardness profiles at inner carburized layer as shown in Fig. 7. The most gradual decrease in hardness from 1230 Hv level to substrate hardness was displayed by the 4C–4N specimen.

Fig. 8. Depth profiles of Microhardness.
