Instead of trying to “rescue” this image I would instead try to rescan it with better quality. The noise you see here is due to insufficient input data; the details seen in the original X-Ray image are too few to provide enough data for a decent reconstruction. I suggest you increase the scan time (longer exposure with less power) or increase the number of projection images by at least a factor 5. How many projections do you use? My guess is 1500 or less. Try to increase to 6000 or more, that makes a big difference. Yes it takes longer time and costs more in instrument rental, but you get that back many times in the time you will save in the post-processing with better data.
What CT instrument are you using? Are the X-Rays filtered through a metal first? Can you upload the metadata files as well? There is probably a file in some text format or XML that describes the instrument settings for this scan.
Yes you are kind of right here. When the X-rays (which are polychromatic when using non-synchrotron radiation) enter the sample, the longest wavelengths are lost first, shortly after entering the sample. The remaining traveling X-rays are thus of a shorter wavelength, which more easily penetrates the remaining material. The reconstruction software then thinks the material is less dense at the inside, which is not the case in reality. This effect is called “beam hardening” and is quite normal. It is also difficult to correct, as it is a physical limitation of the method. The best remedy is to increase the acceleration voltage as much as you can, and then use physical filtering of the X-rays to deliberately remove the longest wavelengths before they hit the sample. My guess is that this is already done here. When you say the instrument is on “max power” I assume this is what you mean. If this is a steel sample (what diameter?) I would say that the beam hardening effect is quite little here; so the chosen power / filtering seems already adequate.
The instrument I am using (Nikon/XTek 225S) has an option to compensate for beam hardening in the software. I have found that to give better results than physical filters, as the latter gives a big increase in the noise. Perhaps your setup already filters too much; you can filter using a lighter metal and instead accept a more uneven background, but with less noise.