David, I have often speculated whether this type of steel can actually have the carbides refined with a strop, no matter what abrasive is used. An effect can certainly been seen, but what of the carbides themselves? If they have considerably more scratch resistance at a given amount of pressure than the surrounding steel, any stropping activity is going to "attack" the steel support material at a much faster rate per pass, even if diamonds are the abrasive. It would seem to me the only reliable way to work those carbides would be with an abrasive affixed to a base that has virtually zero give such as a steel plate or vitreous stone.
It would make an interesting work-up. If one has an optically smooth surface made from this material (largest point to point deviation of .2u or less), they could etch and micrograph the carbides in the steel matrix - certainly this can and is done, with the carbides being approximated at 2-3 microns. They could then strop with diamond or CBN to a lower finish and gauge relative differences in the scratch pattern created, and how it changes as it cuts across the surface of the exposed, polished carbides. Do Vanadium Carbides even take a scratch pattern or do they simply fracture down like silicon carbide, diamonds, and so many other durable minerals?
Edit to add: I have taken micrographs of D2 and s30v after being worked with SiC, on a stone, sandpaper, and with compound over thin paper. I have observed nothing that looks remotely like 2-3u defects along the apex even when worked with relatively fine particle sizes, and the apex and surrounding surface can be worked to pretty uniform finish as well. I have noticed that in use, these steels all seem to perform better (greater longevity primarily) when left at a medium finish, leading me to speculate about the carbides fracturing under use - leaving them larger to start would have a considerable effect on this.
All speculation ^.
On the question of whether vanadium carbides fracture or not, my own impression has always been they don't. Or, if they do, it would take some extreme forces to make it happen. Don't think I've ever read anything mentioning vanadium carbides themselves breaking down or fracturing; always seems to come down them dislodging or tearing out in use, when 'wear' of vanadium carbide-heavy steels does occur. I suspect this is part of the basis of their reputation for extreme wear-resistance, aside from hardness alone. A lot of users here have frequently commented on the difficulty in truly polishing vanadium-rich steels like S30V; based on that, if the carbides do take a 'scratch pattern' per se, I'd assume the scratches would be very shallow anyway. There are examples posted of high mirror-finishes on S30V and other similar steels, and I'd bet most/all that were done
by hand were likely done using sub-micron diamond/CBN on strops, at least in part.
Other 'hard' materials, like SiC, are also known for fracturing pretty readily under use (best example I've seen: how the abrasive quality of SiC wet/dry paper becomes effectively finer with more use, as the abrasive breaks down in size). That's another reason I'd question SiC's ability to really abrade the vanadium carbides. If both are very close in hardness, but the vanadium carbides are less prone to fracturing (maybe much less prone), I think most of the end result would be the SiC breaking down under pressure from the carbides, limiting SiC's ability to keep working at the same aggressiveness.
I agree with the idea that diamond or any other abrasive being used on the carbides, for stropping, would likely have to be very firmly rooted in an unyielding substrate, in order to be fully effective. The ~3X difference in hardness of diamond over vanadium carbide might help it work to some limited degree on a slightly softish backing, but as with using any other abrasive on a strop, the effective grit of the abrasive would be reduced by the yielding/cushioning effect of the substrate.
I do have a lot of questions (speculation also) about a lot of this, myself. But, at least in terms of the well-established hardness differences between the abrasives (diamond vs SiC/AlOx), and the apparent nearly-same hardness values established for SiC/AlOx, versus the vanadium carbides, it seems as though the diamond would ensure a more predictable result in grinding/refining the carbides themselves. I think this is what's alluded to in the remarks from Crucible about 'grinding difficulties' encountered in vanadium-heavy steels, when the grinding media has to work directly on the vanadium carbides.
And, in reading more from Crucible about the intent/goal of their CPM process, it's all about minimizing the size of the carbides from the beginning (and homogenizing their distribution), so their effect on grinding/finishing operations is mitigated anyway. This is why I also feel, like you, that S30V and similar steels can still work excellently at less-refined finishes (like DMT's 25µ 'Fine' and maybe down to the 9µ EF), in which case, the refinement of individual carbides isn't as necessary.
David
