Surface Engineering by Expanded Austenite
XPS Analysis
XPS Results
The alloys which were selected for analysis were:
These were selected to allow for a comparison between a stainless steel sample which appeared to react well to the carburising treatment, in terms of expansion, and a nickel based sample. SS 2343 was chosen in particular due to its expansion and the potential of carbides present.
It was planned to also run sample SS 353 to confirm the identity of any carbides that were present that could not be seen via the XRD analysis. However due a technical fault with the equipment it was unfortunately not possible to carry out the XPS analysis on the SS 353 sample.
Inconel 617 (K33) Atomic Concentrations
Information regarding the atomic concentrations of each element were taken from the surface and S-phase layer.
Comparison of Elemental Compositions Between XPS and Literature of Inconel 617
This higher chromium content in the S-phase could back up the thought that the formation of the chromium carbide ‘Tongbaite’ had occurred. ‘Tongbaite’ has the chemical formula Cr3C2; this meaning 3 parts chromium 2 parts carbon. Adjacent to the surface the ratio of chromium to carbon is very similar, being approximately 7:4. This suggests that the formation of the Tongbaite carbide could have occurred.
Studying the graphs of Figures 60 & 61 showed no unexpected results. There was a higher oxygen content found at the surface, confirming from the existence of the protective oxide layer and/or surface contamination. Also present at the surface was a higher carbon content, which exponentially decayed with the sample depth, confirming that the carburising treatment did form a layer of expanded austenite.
Stainless Steel 2343 Atomic Concentrations
Comparison of Elemental Compositions Between XPS and Literature of SS 2343
The element compositions were as expected in the Stainless Steel 2343 sample. The iron and carbon content shown in Figures 61 & 62 also provide further justification for presence of Hagg carbides in the treated SS 2343 samples. A Hagg carbide is one which is 5 parts iron, 2 parts carbon, a ratio of 2.5:1. The approximate iron - carbon ratio given by the XPS data is 3:1, which is close enough to suggest that the formation of Hagg carbides are possible. Oce again there is a high oxygen and carbon content at the surface, confirming that the carburisation treatment has been successful in case hardening the sample by expanded austenite whilst also preserving the protective, passive layer. However a percentage of the carbon at the surface could also be caused be surface contamination.
XPS Analysis
Comparing the iron based sample that acted as expected (SS 2343) and the nickel sample, it is clear that Inconel 617 was not able to absorb as much carbon into its lattice. This can be seen by comparing the relative carbon contents in the S-phase layer; 19% for SS 2343 compared to 10% for Inconel 617 (K33). But as it is thought that there is a formation of Hagg carbides in SS 2343, it cannot be determined what percentage is interstitial carbon and what percentage is made of carbide formation.
Looking at the surfaces, the carbon content of nickel alloy is actually higher than that of the iron alloy; 24% and 18% respectively (This value is taken from the point where the oxygen content reaches approximately zero, signifying the end of any surface contamination) This suggests that carbon atoms were not able to penetrate as deep within the surface of the nickel based sample, so were not able to diffuse within the lattice. This would lead to the layer of expanded austenite being much thinner in comparison to the iron based sample, as was proved in the ‘carburised layer depth’ section.
XPS Summary
The element concentrations measured in Inconel 617 made it possible that the chromium carbide ‘Tongbaite’ could have formed in the alloy.
Similarly the element concentrations in SS 2343 also seem to have confirmed that the correct conditions are present for the formation of Hagg carbides. In terms of both the surface carbon concentration and the carbon concentration within the S-phase, the iron based sample was able to absorb far more carbon than the nickel sample, which was also expected.
