The carbon composition of steel stock is given as 0.06wt% and the EDX analysis of the ferrite phase gives 12.7wt% of C. How do you relate the two wt%s in terms of relative abundance? This situation is shown as an attachment below.
I would question the results of the EDX analysis. Carbon weight of 12.7% would not be considered steel. If such a material even exists, it would be considered cast iron, and would be extremely brittle.
I was trained on an EDX attachment to a Scanning Electron Microscope (SEM). In my training, I was told two things about EDX:
1. Lighter elements (C and lower atomic number) are difficult to detect reliably.
2. EDX is not accurate for quantitative measurements of elements.
My suggestion would be to use an alternate method to identify the carbon content of the steel. https://verichek.net/carbon-content-in-steel.html shows 4 methods, of which I think XRF is considered standard by SAE for aerospace steels.
In terms of relative abundance, the elements must be scaled by their atomic weights. So the relative abundance of carbon would be : (12.7%C / 12.011g/mol) /((12.7% / 12.011g/mol) + (0.55%O / 15.999g/mol) ... + (.01%Cu /63.546))
Percentage and grams per mol drop out and you are left with a decimal relative abundance.
EDX is not the right technique to measure trace elements and it is really very approximate to measure carbon. The carbon detected here is probably located only on the surface of the sample.