strathSEEA

XRD Phase Identification Results

 

 

Precipitates Found Post Carburisation

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

By looking at Table 10, the main carbides that appeared to be present were Hagg/iron carbides and chromium carbides, however further XRD investigations out-with this report are required to fully confirm the existence of these additional compounds. Hagg carbides are a type of iron carbide with a particular ratio of 5 parts iron and 2 parts carbon. Carbide precipitation in stainless steels (the process by which carbon or chromium is drawn out of the material matrix to react with the atmosphere) can be detrimental to the properties of stainless steel. Chromium carbides in particular are harmful to the effectiveness of stainless steels, as their formation draws chromium away from the surface solid solution and into chromium carbide precipitates the crystalline lattice, reducing the effectiveness of the passive layer and the corrosion resistive properties of the material. This loss of chromium-oxide and increase in chromium-carbide is known as sensitisation. The formation of iron carbides is relatively less harmful.

 

For stainless steels which had a chromium weight percentage of approximately 20% or greater (all samples except R350 and 2343) some form of chromium carbide was thought to be present, with the exception of SS 353 which shall be discussed hereafter. It was also noted that the Hagg carbides appeared to be present in the materials which contained the highest iron content (SS 2343 and SS R350). It is common for the formation of chromium carbides to be seen more regularly than Hagg/iron carbides as chromium has a stronger affinity to carbon than iron does.

 

However in the case of SS 353 it was not possible to identify the formation of any carbides from the XRD analysis. This perhaps could be contributed to the unusually wide, asymmetrical position of the 2nd austenite peak, which could be covering carbide peaks of lesser intensities. This unusual peak shape could be contributed to a number of reasons, such as stacking faults or precipitates forming and causing abnormal expansion magnitudes. XPS or SEM analysis is required to determine if any carbides are present.

 

Unlike chromium carbides, iron or Hagg carbides are not necessarily detrimental to the properties of stainless steel. As no chromium depletion is occurring, the corrosion resistance of the material is not affected. And the precipitation of the carbides can even strengthen the material via precipitation hardening at the grain boundaries.

 

The formation of carbides has been found to relax “the stresses originated from supersaturation solid solution.” This is because the formation of carbides uses atoms that would otherwise take the form of interstitials, causing distortion within the lattice and resulting in residual stresses. With atoms instead forming these precipitates, they are not causing stresses as interstitials within the microstructure. The lower the interstitial count, the smoother the surface tends to be.

When looking at the nickel based samples, no Hagg carbides were found. This suggests that nickel acts to supress the precipitation of Hagg carbides. This was also suspected in a paper investigating nitriding austenitic steel by G Maistro, L Nyborg, S Vezzù and Y Cao. This appears to be confirmed when looking at the two iron based samples with the highest nickel content, SS 353 and SS 254, as Hagg carbides appear in neither of these samples.

 

 

XRD Phase Identification Summary

 

The most prominent conclusion that could be drawn from the XRD phase identification process was that an expanded austenite structure was confirmed in all Fe-based stainless steels. Also there was an almost definite presence of Hagg carbides in samples 2343 and R350.

 

These Hagg carbides were not found in any of the nickel based samples nor the iron samples with the highest nickel content; seeming to confirm previous findings that nickel acts to suppress the formation of iron carbides due to the elements poor affinity to one another.

 

The formation of chromium carbides, often Tongbaite, was thought to be possible in a number of samples, particularly in the nickel based samples. But XPS analysis [YC7] [C8] is required of all the samples to further certifyconfirm these findings. The formation of this carbide would significantly decrease a materials ability to resist corrosion.

 

No carbides were seen in Stainless Steel 353, however due to the wide, asymmetrical nature of the 2nd austenite peak which could act to obscure any carbide peaks in this region, this assumption cannot be proved definite.