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Untreated Polished Vs Un-Polished Expansion

 

In terms of the investigation briefly looking at the effect which polishing the samples had on its microstructure, there was little to note. The peak shifts, corresponding to atomic spacing changes, were of little significance, the majority being of a percentage change of less than 1% which was as expected.

 

Treated Polished Vs Un-Polished Expansion

 

Examining both sides of the samples after carburisation allowed for an understanding of how the surface finish affected a materials ability to absorb carbon. From the results it was clear that a polished surface has less compressive stress allowing the increase in lattice expansion with the expansion being in the region of approximately 0.1% to 1% greater on the polished sample side compared to unpolished side with the majority of the samples.

 

When comparing the K22 and K33 treated nickel samples, the expansion between the polished and unpolished sides showed very little variation.

 

Interestingly the polished side of sample SS 254 after carburisation did not show a third visible peak and therefore a lattice expansion in the {220} plane could not be calculated. It was noted that lattice expansion on the {200} plane was the largest, showing that the lattice expansion was non-homogenous.

 

Polished Treated Vs Polished Untreated Expansion

 

Measuring the expansion of the crystal lattice gave an indication of how much carbon had been dissolved into the matrix of the crystal. When looking only at the polished side of the samples it was shown that the iron based alloys were far more reciprocating to the carburisation process than the nickel samples, with the average lattice expansion ranging from 2.4% to 6%. The samples which appeared to expand the most were the SS 254, SS 832 alloys.

 

The ratio of the lattice in expansion in the {111} plane and {200} plane appeared to follow a similar pattern between all the samples, and should allow for comparative results with the RST data and carburised layer depth information gathered from the optical microscope.

 

The nickel samples did not seem to react well to either the K22 or the K33 carburisation treatment, with the lattice of the Inconel 617 expanding by approximately 1% and the Nikrothal showing the completely unexpected results of contracting.

 

Unpolished Treated Vs Unpolished Untreated Expansion

 

The results of scanningfrom the unpolished side of the treated materials acted to confirm the results found on the polished side. The percentage lattice expansion increase between the treated and untreated samples were slightly lower (≈1.5%) than seen on polished sides. This was true for all iron based alloys apart from SS 2343.

Once again the stainless steels reacted better to the treatment than the nickel based alloys. The Nikrothal sample again also contracted, suggesting that there may have been an issue during the carburisation process or with the homogeneity of the carbon dissolution.

 

XRD Lattice Expansion Conclusion

 

Overall the XRD results regarding lattice expansion suggest that in the nickel alloy samples the K22 treatment was slightly more effective [YC1] than that K33 treatment, however by rather insignificant margins. This analysis was not helped by the fact that one of the K22 samples (Inconel 617) was producing completely unexpected results by reacting better than the K33 treatment. However the The nickel based samples seemingly poor ability to absorb carbon seems to confirm the results of previous S-phase investigations, where the formation of the S-Phase was found to be significantly slower in nickel based samples compared to iron based samples.

 

Other than that it showed that the polishing process resulted in a slightly greater lattice expansion and confirmed that the iron based alloys are more susceptible to carbon absorption than the nickel based alloys. The sample which expanded the most were the SS 254 and SS 832 samples. Further investigations will reveal if this larger expansion corresponds to more substantial carbon diffusion and thus a harder sample.