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Following on from this, analysis could take place regarding the amount by which the microstructure of each sample had expanded due to carburisation. To start with only the polished sides of the samples were considered. This is the data which should confirm how effective the carburisation process was in relation to the different materials. For this analysis comparisons were drawn by creating a series of overlay graphs, which included the graphs from the following sample types:

 

• Untreated Unpolished

• Untreated Polished

• Treated Unpolished

• Treated Polished

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Again the K22 treated samples were the first to be examined. This comparison of the lattice expansion of the polished sides between the treated and non-treated samples was the most critical data gathered from the XRD investigations.

 

By examining the percentage lattice expansions in Table 6 it is clear to see that the nickel based alloys did not react well to either the K22 or K33 carburisation process, with little carbon diffusion appearing to be present in either sample. In the case of the Nikrothal, when subject to both treatments, the atomic spacing within the crystal lattice actually decreased in some planes, meaning that the carbon diffusion process was not successful. This reduction in spacing is actually more common of an annealing process.

 

However when comparing the nickel based Inconel 617 samples, the K22 treatment showed to be slightly more effective than the K33 treatment[YC1] [C2] , giving greater percentage lattice expansions ranging from 0.03% to 0.45%. This was unexpected and could possibly be a mix up of samples by the company.

 

The iron based stainless steels all reacted as expected to the K22 carburisation treatment; with all showing some amount of expansion. The alloys which appear to have absorbed the most carbon were: Stainless Steel 254 and Stainless Steel 832. However due to the unexpected missing 3rd peak on the Stainless Steel 254 graph seen in Figure 20 further investigation is required.

 

Comparison of these results to the surface roughness testing, micro-hardness testing and optical microscopy should allow for a determination of whether the SS 254 sample has reacted in accordance to the XRD results.

Examining Table 7 which ranks the materials in terms of the magnitudes of the lattice expansions, it can be seen that the ranking of expansions of the lattice in {111} and {200} planes is the same.

 

It was noted however that the exact position of the peak referring to the {200} plane in SS same sample the 353 was not possible to ascertain due its asymmetrical nature.

 

It is expected for this ranking to closely match the RST surface roughness rankings, as it is thought that a high lattice expansion would cause greater movements within the microstructure, leading to the creation of a rougher surface.

 

Bearing this in mind, the iron based alloys which expanded the least on the polished face were Stainless Steel 2343 and 353, would expect to show a smoother surface from RST usually, however if carbides have formed in the process it may affect the roughness results.

 

It was also noticed that a greater expansion tended to occur within the {200} plane when compared to the {111} and{220} plane. This showed that the expansion was non-homogeneous and was also consistent with results found in previous investigations. It is known that when an FCC structure is subjected to stress, the shift experienced by the {200} plane is always the largest due to the induced stacking faults.