How Cryogenic Hardened Steel is Helping Your Knife

Knife Makers Have Been Freezing their Knives for a Long Time

You’ve probably seen the term thrown around forums and knife product descriptions. Cryogenic treating, or processing, or refining, or whatever a knife manufacturer decides to call it, sounds suspiciously like marketing nonsense.

Blade Mag covered this topic pretty well by interviewing some high profile knife makers like Paul Bos and Bob Bealy. The process is explained along with the benefits but they kind of rush through whether it’s worth a knife buyer’s money, leaving it at “a cryogenically treated blade could make the difference in performance and easily justify any minimal added expense”.

This still left me with a few questions, though, like “how much of an added expense” and “how do I know a company is doing it well or just slapping the word ‘cryo’ on the steel to sell off the buzzword”.

SOG’s knife buying guide loosely describes how they do their cryo treatment and say the “process relieves the blade material on an atomic level” making it tougher and less likely to chip. That all sounds fine, but anytime I hear someone selling something as effective because it “works on an atomic level” I get suspicious.

So I tried to figure the thing out for myself. Here’s what I landed on after a couple weeks of research just in case you don’t feel like reading through another thousand words of my stumbling through technical steel crafting processes I still only half understand:

• Yes, cryogenic treatment improves some types (high alloy) knife steels
• It mostly makes steels harder and more stable
• Probably every large knife manufacturer is doing it on some knives
• No, we probably aren’t paying extra for knives as a result.

Update: Shout out to Keith Nix Knives for correcting me on the actual properties of austenite and martensite and a few other minor points of misinformation. You should check out his blog on why you should cryo treat your knives, especially if you’re a knife maker looking for more applicable information, but more especially if you want to read cool things about liquid nitrogen.

What is Cryogenic Processing

The purpose, on a simplified technical level, is to convert the left over austenite of the initial heat treatment of the steel to martensite.

For most people that is not a very helpful sentence. We’ll break it down in a moment, but the main take away is that, on the whole, knife makers need the steel to have as much martensite as possible in order to make their knives hard.

The Four Stainless Steel Structures:

• Martensitic: A hard structure built up around the composition of iron, chromium, and carbon. This structure can be heat treated.
• Austenitic: A face-centric cubic structure that can not be hardened by heat treatment. Generally results from the addition of nickel, manganese, and nitrogen. Generally too soft to be desirable for knives.
• Ferritic: A body-centric structure resulting from high-chromium content. This is usually a very soft and corrosive resistant structure often used in cheaper kitchenware.
• Duplex: A form of hybrid between austenitic and ferritic steels (it’s sometimes called austenitic-ferritic) that tends to be more corrosion resistant than the austenitic structure and harder than the ferritic structure.

That is a grossly generalized overview that’s probably making a few intelligent people cringe, but as we’ll see, there are other people who have explained it better.

In order to get martensitic structures in the steel, you need to austenitize it, which basically means heating it up until the carbon and alloys from the ferritic phase dissolve into solution (this process is described in excellent detail in Larrin Thomas’ articles on Austenitizing). Then you quench the steel so carbon atoms are locked into place, creating the martensitic state.

As I understand the very in depth (but surprisingly readable) articles on cryogenic processing by the Knife Steel Nerds, alloy steels that are high in carbon and chromium tend to still have more austenite after being cooled because the more elements you add to a steel the lower the temperature in which martensite forms will be. You get enough carbon and other alloys, and you get a finishing temperature that’s below room temperature. With a lot of stainless steels, that temperature is so cold it requires cryogenic freezing, and that’s when they bring in the liquid nitrogen.

That’s likely a gross oversimplification, but for our purposes as consumers it’s good enough for now.

What’s Actually Happening Inside the Steel

Most steels, when they’re first made, aren’t perfect, and the many machining processes they go through to become a knife or beam or hammer often add to those imperfections. Heat treatment is one of the ways those imperfections are fixed, but even after heat treatment the steel can still be anywhere from 15-20% austenite, leaving it softer to the point that it negatively affects a knife’s ability to retain its edge and take a proper edge in the first place.

Thanks to the science-magic properties of austenite (again, you should really refer to Larrin Thomas) and the surrounding alloys within the steel, the way to get that austenite to dissolve into more uniform martensite is to bring the steel down to cooler temperatures (sometimes below freezing, but it depends on the steel).

The process, as I understand it, is sort of like elements dissolving into each other. Baking always seems to make a healthy analogy to treating steel, so in this case you could think of it like baking cookies hot enough to break the fat from the butter and the sugar down, causing the dough to flatten out, then freezing the cookies to where the sugar crystalizes and makes it sweeter.

So in a situation where you want your steel to become “sweeter” you need to heat it up to get things moving around, then cool it down carefully so those things can fix themselves into a better structure. Basically it heats up the metal until the atoms are mobile enough to realign and become more symmetrical. The methods used to heat and cool the metal, along with the time spent on each stage determine whether they realign properly.

Is Cryogenic Processing Making Your Knife Better

In theory, yes. But like everything with a complex craft, it depends on the materials and the application.

If done right, the process should make the steel harder, but not all steel types react to the process in the same way. Some react “better”, or more dramatically, and others just don’t have the chemical make up for it to make much difference at all.

The simple way to look at it is steels with more chromium benefit more from the process. In particular, chromium’s presence alongside a lot of carbon gets the most mileage out of being frozen and thawed.

Paul Bos told Blade Magazine he cryo treats high alloy steels, and actually went into some surprising depth about his process. Over his career he’s seen companies cryo treating everything from nylon stockings to gun barrels, and figured it could probably do a lot for knife steel but he clarified that “its effects are more profound in high alloy steel”.

D2 steel in particular seems to benefit a lot from the process, and I would go so far as to guess that it holds doubly true for cheaper Chinese steels like 8Cr13MoV (though admittedly I’m far from a heat treat expert, and most of my knowledge is from a decade-old article and a bunch of internet forums).

The truth is, though, that most of us probably won’t notice the difference. This is steel we’re talking about. Even soft steel is pretty damn hard, and most of us will never use our knives in a way that would make us notice the added toughness between regular steel and cryogenically treated steel.

That’s not to say it doesn’t add some life and value to our knives, just that you shouldn’t expect to see the improvement in any immediate sense like you would in a knife that just has a good heat treat and edge geometry in the first place. Those are the kinds of things that will keep a knife cutting paper after a round of ill-advised batoning. Cryogenic treatment is more about longevity than direct performance.

Is It Worth the Money

First off, how much is the cryogenic process even padding the bill?

Bob Beaty told Blade magazine that it doesn’t make knives that much more expensive.

In a perfect world, knife makers cryo treat their steel to get the most out of the steel properties. What’s far more likely is that companies cryo treat large batches of knives to compensate for heat treatments that inevitably suffer from mass production.

In that situation, the treatment probably isn’t affecting the price much if at all. Either the company is sending the blades off to a third party in bulk or doing it themselves in large batches on site. The reason the consumer doesn’t feel the additional labor is the same reason the knife ends up affordable in the first place: the wonders of mass production.

We might feel it more from custom knife makers: the ones who make one to ten knives to order and want to optimize the steel as much as they can. So they either invests in some kind of nitrofreezer to do it themselves or sends it to another company to do it

That knife maker will feel the extra steps in his bottom line a lot more than Benchmade or Cold Steel, but if he actually knows what he’s doing that small bump in price will reflect a knife with steel that has been made about as hard and stable as it’s possible for it to be.

So Who’s Cryo-Treating their Knife Steel

It’s tricky to find out exactly who does cryogenic treating and what they’re doing it to, because most companies don’t go into that much detail about their treatment process. SOG clearly does it on at least some of their knives since they explain the process in their buyer’s guide. Rumor has it Benchmade uses some kind of cryo process, and Spartan Blades touts their “double deep cryo heat treatment” in most of the product descriptions of their Gold Grade knives.

The real answer, though, is probably everyone. It seems to be well enough established as an effective method for improving certain kinds of steel. If you look at cryogenic processing plants you see they have clients across a huge range of industries including aviation and tooling. Most of us are probably commuting or drilling holes with cryo treated steel every day. If cryo treating is nothing more than a marketing ploy, then it has an enormously wide spread placebo effect.