If you’ve spent more than five minutes on a knife forum, you know people obsess over steel like it’s a religion. CPM Rex 45 is one of those "holy grail" steels that Spyderco users tend to hoard. But there’s a specific detail that separates the casual collectors from the guys who actually use their knives until the blade is half its original width: the HRC. Specifically, the Spyderco Rex 45 HRC (Rockwell Hardness Scale) rating.
Hardness isn't just a number. It's the soul of the knife.
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Spyderco usually hits their Rex 45 in the 65 to 67 HRC range. That’s incredibly hard. For context, your standard kitchen knife is probably sitting at a 56. A high-end pocket knife in S30V might hit 59 or 60. When you jump to 66, you aren't just getting "better" steel; you're entering a realm of physics where the edge behaves differently. It’s stiff. It’s relentless.
What is Rex 45 Anyway?
Rex 45 is a super-high-speed tool steel. Crucible Industries makes it using their Particle Metallurgy process, which basically means they turn molten steel into a fine powder and then "glue" it back together under immense pressure to ensure the carbides are distributed evenly. It’s almost identical to Hitachi’s HAP40, a steel that Spyderco fans have loved for years in the Pakkawood exclusives and various Mula runs.
It’s got a ton of cobalt. It’s got tungsten. It’s got molybdenum. Honestly, it’s a chemistry set designed to stay sharp while cutting through things that would turn a lesser blade into a butter knife.
The Magic of the 66-67 HRC Sweet Spot
Why does Spyderco push the Spyderco Rex 45 HRC so high? Most manufacturers play it safe. They heat treat steel to be a bit softer because soft steel is "tough"—it bends before it breaks. But Rex 45 isn't meant to be a pry bar.
At 66 or 67 HRC, the edge stability is insane.
You can grind a Rex 45 blade down to a very thin angle, maybe 12 degrees per side, and it won't roll. If you tried that with a softer steel, the edge would literally fold over the first time it hit a cardboard staple or a knot in a piece of wood. Because the HRC is so high, the steel supports itself.
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I’ve talked to guys who use the Rex 45 Para 3 for warehouse work. They go weeks without sharpening. Not days. Weeks. They might hit it with a fine ceramic rod once or twice to realign things, but the actual abrasive wear resistance is through the roof.
The Trade-off Nobody Likes to Admit
Everything has a price. In the knife world, that price is usually "toughness" or "corrosion resistance."
Rex 45 is not stainless. Not even close. If you live near the ocean or you have particularly acidic sweat, this steel will turn brown or grey before you can even finish your lunch. It’s a tool steel. It develops a patina. Some people hate it. They think their knife is "rusting." It’s not; it’s just gaining character. But if you leave it wet? Yeah, it’ll pit.
Then there’s the chipping.
High HRC means high brittleness. If you drop a Rex 45 Manix 2 on a concrete floor at just the right angle, that tip might snap. Or the edge might get a tiny micro-chip. It’s the difference between a glass bottle and a plastic one. Glass stays "clearer" and holds its shape better, but it doesn't like impact.
Sharpening This Monster
If you think you're going to sharpen a 67 HRC Rex 45 blade with a cheap pull-through sharpener or a basic Arkansas stone, you’re in for a bad afternoon. You need diamonds. Or at least high-quality Cubic Boron Nitride (CBN).
Because the steel is so hard, it resists abrasion. That's the whole point, right? But sharpening is abrasion.
The trick is not to let it get dull. Seriously. If you wait until the knife is completely "butter-knife dull," you will spend three hours on the stones trying to move enough metal to create a new apex. If you strop it regularly, it stays terrifyingly sharp forever.
Comparing Spyderco’s Heat Treat
Spyderco is famous for their "Mule Team" projects where they release the same knife in dozens of different steels. This is where we got a lot of our data on how Rex 45 performs.
Expert testers like Larrin Thomas from Knife Steel Nerds have looked into these high-speed steels extensively. He’s noted that while Rex 45 is remarkably similar to M4, the cobalt addition allows it to reach these higher hardness levels without losing all its integrity.
- Rex 45: Harder, better edge retention, loves a patina.
- M4: Slightly tougher, usually run around 62-64 HRC.
- Maxamet: The only thing Spyderco runs harder (can hit 68-70 HRC), but it’s even harder to sharpen.
For most people, Rex 45 is the "Goldilocks" of the super-hard steels. It’s more manageable than Maxamet but noticeably outshines M4 in pure cutting longevity.
Real World Use: The Cardboard Test
Let's get practical. Most of us aren't skinning elk every day. We’re opening boxes from Amazon.
Cardboard is surprisingly abrasive. It’s full of recycled materials, dirt, and glues. It eats edges. In side-by-side testing, a Spyderco Rex 45 HRC 66 blade will outcut an S30V blade by a factor of nearly three to one. That means if the S30V knife gets dull after 100 feet of cardboard, the Rex 45 is still slicing cleanly at 300 feet.
That is the "why" behind the hype.
Why the Patina Matters
If you're buying a Rex 45 Spyderco—maybe one of the burnt-orange G10 models or the white FRN lightweights—don't fight the patina.
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When the steel reacts with the environment, it forms a thin layer of oxidation. This actually helps protect the steel from more "active" rust (the orange, flaky stuff). I've seen Rex 45 blades that look like they were pulled out of a shipwreck—dark purples, blues, and greys. It’s beautiful. It shows the knife has been used.
If you want it to stay shiny, keep a thin coat of mineral oil or Aegis Solutions Corrosion Inhibitor on it. But honestly? Just use it. Cut an apple. The acid in the fruit will start the patina process immediately.
Finding the Right Model
Spyderco doesn't keep Rex 45 in constant production for most models. They do "sprints" or "exclusives."
- The Para 3: Probably the most popular Rex 45 platform. It’s small, slicey, and the compression lock is addictive.
- The Manix 2: If you have bigger hands, the Rex 45 Manix 2 is a beast. The ball bearing lock is robust enough to handle the pressures this steel can endure.
- The Chief: A longer, slimmer blade that really showcases how thin you can get a Rex 45 edge.
Final Technical Insights
When looking at the Spyderco Rex 45 HRC, remember that hardness isn't a guarantee of quality—the consistency of the heat treat is. Spyderco uses Golden, Colorado, and Seki City, Japan, for these runs. Both factories have mastered the vacuum furnaces needed to get these high-alloy steels to their peak performance.
You aren't just buying a name. You're buying a specific crystalline structure in the metal that was achieved through precise temperature control and cryogenic quenching.
If you're a "steel snob," Rex 45 is your playground. If you're a worker, it’s your best friend. Just don't forget your diamonds when it's time to sharpen.
Actionable Next Steps
If you’ve just picked up a Spyderco in Rex 45, or you're eyeing one on the secondary market, here is how you handle it like a pro.
First, force a baseline patina. Don't wait for it to get splotchy. Cut some warm meat or a couple of lemons and let the blade sit for ten minutes before washing and drying. This creates an even, protective layer that prevents "pitting" later on.
Second, invest in a 1-micron diamond strop. Because the HRC is so high, a leather strop with green compound won't do much. You need the hardness of diamonds to actually polish that Rex 45 edge. A few strokes after a day of use will keep that "factory edge" feel for months without you ever having to touch a stone.
Lastly, check your pivot. Rex 45 is often paired with phosphor bronze washers. Since the steel isn't stainless, ensure you put a tiny drop of oil on the tang of the blade where it meets the washers. This prevents internal corrosion that can gritty up your deployment.
Rex 45 is a high-maintenance relationship, but the performance you get in return is unmatched in the EDC world. Treat it right, and it will quite literally outlast you.