There is no standard for "fine edge holding" we like to apply it to hair shaving because that is the use for it's application but it there are large differences between hair diameters between different people ranging from ~60-200um
There is also a vast difference between how well something is "shaving" from scraping skin and catching and pulling hairs to quietly treetoping the hair without touching the skin. Huge difference.
There is a huge emphasis on the steel, geometry and heat treatment which is nice but most folks need to be reminded that it's the sharpening at the end of the day if we are dissecting fine edge holding.
Also without good measurement of Geometry it's difficult to rule it out and test the Steel and HT.
Higher hardness is nice because we are chasing a high yield strength at the apex but higher hardness doesn't always translate to more yield since hardness measures Tensile Strength in the plastic region more than purely yield Strength in the elastic region.
Don't get tunnel vision on just the hardness, These steels have wildly different structures even the same steel at same hardness can have different structures that make up that hardness for better or for worse.
All things being equal, K390 will out cut Rex 45 even if the Rex 45 is at higher hardness. This is true if we define cutting edge retention as the least amount of work with the same effort with slicing since the higher volume of harder carbide in the matrix will resist the wearing of that apex in controlled slice cut testing.
If we are using a polished edge, I feel we need to rule out sharpening process since we have more of the harder Vanadium Carbides that resist being cut and shaped in ~2-5 Micron diameter ranges and we are seeking a sub micron radius for the ultimate fine edge it makes since that we need diamond and CBN to shape that down with bonded stones to improve the surface finish/roughness to keep the abrasive grains from penetrating deep into the bevel, so it's not purely just grit size that effects the surface roughness but the combination of how that grit interfaces with the bevel and burnishing effects of different bond types.
Todd Simpson's work at Science of Sharp "Carbides in Maxamet" showed clearly that the mechanism for hard, high volume carbide in high hardness tool steels and why they may not be performing as nice with ceramic. It was not tear out, but actually just breaking of the brittle carbides in the matrix exposed at the apex. They were busted to shape not cut ; giving us fatigued steel at the apex. So the carbides can be busted up to a fine radius but the Stability and Retention is dropped.
My point is, it's difficult to apply claims of fine edge holding to these PM tool steels all with ~8 to +20% carbide volumes and 60 to +69rc. Seems the biggest factor was how it was sharpened, after that is ruled out than things can be narrowed down more to the acutal properties.
This isn't an attempt to marginalize anyone's opinions or efforts but just to get folks thinking more critically about this stuff.
Yes, Cruwear is a nice steel, love that edge it takes
Rex 45 too.