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The second analysis involved the change in atomic spacing’s between the polished and unpolished faces of the carburised samples. This expansion was again expected to very small relative to the expansion due to carburisation, but it does allow for examination of the effect which surface roughness and plastic deformation had on the effectiveness of the carburisation process.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

It was immediately noticed by analysing the overlay graphs (Figure 14 – Figure 22) comparing the treated unpolished samples to the treated polished samples;  that the peaks appeared to be broader on the unpolished face. It was also noted that in the majority of cases, there was a slightly larger lattice expansion on the polished side. This can be seen in Table 5.

 

The reason for this being that the higher plastic deformation found on the unpolished, rougher side allows for a higher solubility on that face of the sample. This greater solubility allows for a greater amount of carbon to be absorbed and thus high residual stresses caused by the interstitials, as referred to by Maistro. And higher compressive stresses act to prevent lattice expansion.

 

A graph representing the typical solubility of carbon solutes within steel is shown schematically below in Figure 13.

The more steeper gradient of decay after the initial diffusion of carbon in the non-polished side of a sample is due to the greater amount of carbon that has got to be diffused within a similar depth of the modified layer when compared to the polished side. This higher initial concentration followed by sudden decay is the way which the carbon is distributed in such cases, and is shown in the improvised Figure 13.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The steeper interstitial content gradient is a steeper residual stress gradient on the non-polished face. This non-homogenous carbon concentration is the explanation for the wider peaks witnessed on the non-polished face of the samples. This was only witnessed in the stainless steel samples.

 

The first examination involved looking at the samples which had undergone the K22 carburisation treatment.

The first thing to note was that the unpolished face of the Stainless Steel 2343 sample appears to have absorbed more carbon than the polished side which contradicts with the results of the other 8 samples. All of the other stainless steel samples appear to show a slightly greater lattice expansion on the polished surface, but these differences are again of no large significance.

 

When solely analysing the samples which had undergone the standard K22 carburisation process, the nickel based Inconel 617 alloy is shown to have the greatest expansion from the non-polished to the polished face.

The 353 sample once again showed a large expansion in one plane, also suggesting that it is particularly sensitive to surface contamination.

 

When looking at the nickel based samples which had undergone the lengthier K33 carburisation treatment, the expansion from the non-polished to polished side was very similar to that of the K22 treated samples, with nothing noteworthy regarding the difference in the treatments being witnessed.

 

The lattice expansion in the {200} plane of SS 353 could not be identified due to the wide and asymmetrical nature of the  peakintensity spike, meaning an accurate estimation of the positon of the peak was not possible.

 

he 3rd peak on the {220} plane within Stainless Steel 254 was also not visible on the softwareXRD pattern. If another peak was present at an unexpected angle then it could suggest that a possible phase change to a phase other than austenite had taken place during the carburisation process.? But as the peak is simply not clearly visible, the structure can still be confirmed as an FCC one, possibly with a change in texturing.