You need to know that hyper specialized alloys are designed primarily for applications other than knives and thus the materials properties you want to see for knives are often lacking. REX121 definitely too fragile to be a good knife steel.
People should stop valued an extremely high alloy steel to be a great blade material. I have experience using many steel like 10V, M390, M4, 3V, XHP, ZDP189 etc. some high carbide steel would be specialized at cutting cardboard/robe, but it real life usage, who needs that much wear resistance? especially if you needs to trade off the ease of sharpening and sharpness taken-able.
The most impressed steel for me is just a well heat treated 52100, its take far much better edge than all of steels mentioned and sharpen much easier. You will chip the edge if you accident hit something hard with steel like 10V or 204p while the plain carbon steel will suffer much less damage.
I have a couple of questions here:
1) Why do rope & cardboard cutting not qualify as "real life usage", and what does? Does "real life usage" entail cutting only low-abrasion materials like soft meat/flesh an reciept/phonebook paper? Why do you reject high wear-resistance as a "real life" concern??
2) With ubiquitous SiC and diamond hones, differences in sharpening times between these steels with similar levels of edge degradation become negligible until the edge is
severely damaged and requires removal of a lot of material.
3) Steels like 52100 and stainless varieties like 13C26 are able to take a much finer edge (<1 micron) compared to high-carbide PM steels (<10 micron), but who needs edges <1 micron for any cutting other than opthalmic surgery?? Such fine edges only matter for VERY thin knives for specialized work. THAT is NOT "real life usage". Furthermore, there is insufficient material support to maintain those <1 micron edges in usage beyond the aforementioned delicate (low stress, low applied force) cutting of low-abrasion materials. The edge bends/abrades/fractures away to thickness similar to what the PM steels provide, but then the high-carbide volume of the PM steels takes over and keeps them cutting without further edge-loss due to abrasion for much longer.
4) I am surprised that you regard
toughness as more important than wear-resistance in a hand-held cutting implement.
But to compare toughness, 52100 @ 58 Rc can absorb ~41 J/cm2 prior to fracture by charpy C-notch compared to 35 J/cm2 for CPM-10V as well as 20CV (same composition as CTS-204P and M390). However, at ~62 Rc M390's toughness
matches 52100 @ 58 Rc, giving you a) stronger edge less prone to bending, b) tougher edge less prone to fracture, c) higher wear resistence, d) also higher corrosion resistance.
The advantages offered by 52100 steel are a) higher attainable initial sharpness, b) easier to establish that edge with poor-quality abrasives, c)
cheaper. Because 52100 is prone to corrosion, that fine edge won't last long anyway but i would also argue that it isn't necessary to begin with for "real life usage". I have already mentioned that ease of sharpening only comes into play when establishing a new bevel (when a lot of material-removal is required) or when using lower quality abrasives. As for price, definitely should be a consideration but in these times of cheap energy folks don't mind paying more for what the PMs offer and the price differences are not as high as might otherwise be given the other costs involved in knife-making. Perhaps that will change in the not-so-distant future, but until then... *shrug*
