Cryogenics for sharper knives?

golok

golok

Joined
Oct 20, 2000
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I was just reading about this process and was wondering how many knifemakers are using this method to produce better knives.

Cryogenics is the process of tempering parts by exposure to extreme cold, and then timed stages of warming.

I learnt that it works with golf balls and light bulbs. Apparently, it is claimed that knives stay sharper after undergoing the cryogenic process.

Who can elaborate on this method of treatment on knives?
 
There have been numerous materials studies done showing that cryogenics improves wear resistance. This is achieved by (a) greater martensite % formation, (b) finer resulting grain structure, and (c) precipitation of a different form of high wear carbides. The argument (supported by data) is that with deep cryo (liquid N2), while you can get the first effects traditionally, you can't get the last one at all. The resulting difference in wear resistance is many times to one, depending on the steel, simpler steels have lower effects. However the claim is often made by people who don't promote cryo that you can indeed get the excact same effects by more traditional methods, and cryo is just more expensive. However I have never seen any hard data to support this argument.

As for what effect this has on knives, a finer grain size would allow a more polished edge, as for this being "sharper" this depends on what you are cutting. If you are finishing above the grain size (coarser abrasive), then you are not going to notice any significant effects because the scratch pattern will form micro-teeth that are above the grain size and thus the grit at which you finish will determine "sharpness" and swamp out any steel or heat treat effects. Even at a very fine polish I doubt that you would notice and effects because quite simply I have seen very high push cutting ability on the steels with very coarse grain structures like D2 which has a grain size well over an order of magnitude coarser than the finest grained steels like 52100.

In regards to edge retention in general, there is more to it than wear resistance so it isn't reasonable to expect a direct transfer of abilites. You also have to consider strength, impact toughness and ductility. Deep cryo does improve strength slightly (grain refinement), but offers no advantage to impact toughness or ducility that I have ever seen published, it does increase stress/strain toughness slightly. Now you could argue that the higher strength and wear resistance allow a softer RC to be used and get the same level of edge retention and thus get an overall greater combination of strength and toughness promoting a maximal durability. However most people using cryo go with the higher RC's, and I have never seen that line argued in detail and there are some non-trival complications.

In short, deep cryo doesn't have any drawbacks if done correctly, and has definate advantages so it would be worth it for the knifemaker to explore in detail, many are doing so. Most only claim small performance differences like "~25% increase in edge retention", which isn't a well defined statement anyway as blades don't blunt in a linear manner. Of course there are lots of blades in which wear resistance is only a very minor matter (large bush blade for example) and thus cryo wouldn't give any significant advantage.

-Cliff
 
Good explanation Cliff! For knife makers who are interested, we have found that, when it comes to 5160 and 52100, should you get a great increase in performance after cryo. you can usually gain in performance by altering the variables in your heat treat. ie. a different quench oil, thermal cycles, and tempering temperature. When cruy does not make much difference, you are right on track with the other variables.
 
Another great reply. Thanks Cliff.

Considering the rapid transfer of technology these days, I reckon the cryogenic process will get cheaper in future. These will benefit more people, especially knifemakers.
 
Golok, in its most basic sense Cryogenics is very cheap, the raw liquid is probably not as expensive as the oils used in quenchings, in any case its nothing to break the bank. You will need a dewar to hold it, but this is a one time cost, and again nothing too much, about the cost of a decent custom knife depending on how fancy you want to get, how large etc. . The price comes in in how it is applied. There are basically two schools of thought. The first is that you just slam the knife into the liquid Nitrogen, leave it for awhile, and then draw it out and temper as normal and thus it is a cheap process.

However there is an argument that this is a very bad thing to do because it introduces a high thermal shock to the steel and can thus create internal micro-fractures. Thus you must bring the blade down very slowly, hold for a long time, and then bring it up slowly. This isn't trivial to do without special equipment and this isn't cheap, like ~$50 thousand. The thing that is problematic to the knifemaker is that this has to be integrated into the heat treat process, so you can't farm out the cryo and do the rest yourself, you have to do the whole thing together. So experimention with bringing it into your base methods isn't trivial, like trying a different oil for a quenchant.

As for which one is "right". It depends a lot on the knife as thermal shock is highly dependent on geometry, both in raw cross section as well as details like grind transitions. I have used knives from the "slam" approach and they had no problems with excessive brittlness, and could for example flex to 90 without breaking (S90V at 59 RC). However we are talking about very thin blades (1/8"), with full flat grinds, distal taper, rounded spines, heavy radiusing, etc. . In other words no stress risor points and a high thermal conductivity (surface area to volume ratio). However if you are making big thick bowies, with no distal taper, and they are going to be subjected to a lot of banging around then there might be something to the slow cool approach. This is what Busse Combat does, and it seems to work for them.

I would note though that Ed makes a very critical point, cryogenics is just one small part of the heat treating process (which is just one part of knife performance), in essense it just extends the quench. It is not trivial to know which heat treat process, in general is "better". Just because you use cryo does mean you are at optimal levels. Someone who has looked long and hard at soak temps and times, heating rates, quenchings oils, etc., could easily be far ahead of you, not to mention using different steels, geometry etc. .

-Cliff
 
Just wanted to add
Most of the High carbon s/steels were designed
to be nitrogen soaked in the first place.
so the makers of the steel
really should know why and recommend it.
the molecules, to be effective, should be stopped
(not moving any longer) to finish the quench.
but still we can't get that cold yet so at -305
or so is the best we can do.
I believe we get at least 30%+ better edge holding.
I use it and even soak my files in it to help them
last longer. and my double edge razor blades.
  as the saying goes, if you got it use it.
  it cost me $10.00 a wack I'll do four or five blades with it.
 cost average is nominal less that one 2x72"belt.
 
How exactly do the grains become "smaller" through cryo? How is this possible? Furthermore, how does it cause grain refinement of all things? I am curious.
 
First off what does grain size mean? It refers to two different things. One is the size of the carbides, and the other is the size of the bits that make up the main body of the steel. Steel is a crystal, however it isn't uniform along its entire length, but rather on a small scale (~one micron), there are groups of atoms that are lined up in a certain way, homogeneous orientations. These little groups are the grains.

When a steel is heated up past the critical point the ferrite + carbides (usual state) will transform to austenite. This is a transformation from one crystal structure (body centered cubic) to another (face centered cubic). The austenite structure has a more wide open distribution and thus can dissolve much more alloy, and this is what happens when the transformation takes place. The longer you keep the steel in the elevated temperature the more the carbided alloy bits (grains) dissolve into the austenite. Note the austenite grains will grow as time passes which is why you have to be careful about soak times and temperatures.

Anyway, if you look at the steel in this state you will see carbides (grains) disappearing, as well as big hunks of austenite that are clearly in groups (grains) which are expanding (grain growth). It looks like graph paper, with highly irregular spacing and a bunch of big marker dabs. When you cool the steel slowly the process reverses. The austenite transforms back into ferrite and the alloys all carbide out in hunks. Depending on the speed and where you hold it you can normalize, fully anneal, produce Pearlite, Bainite, Troositite, Sorbite etc. .

However if you do the cooling very fast the transformation from austenite to ferrite is so rapid the alloys don't have enough time to carbide out into nice little clumps (normalize) or long strings (pearlite), but instead there is a very explosive transformation to martenite which is yet another crystal structure (body centered tetragonal). This is not a stable state, it will quickly decay. This is why you temper which locks it into ferrite plus carbides, very similar to normalized steel, except the carbides are much smaller (order of magnitudes).

Ok, what does all of that have to do with grain size? Well when you change crystal structure it doesn't happen all at once in a nice uniform manner. It happens very explosively at certain very specific spots (grain boundaries and points of crystal dislocation). These new crystals "grow" until they smack into other ones. Thus the more you start, the smaller the resulting overall grain (average size), when every thing is done. From a basic viewpoint this is one of the aspects of how deep cryo refines grain size (it also starts the tempering process). As well, it also produces finer grained (due to their nature) higher wear resistant carbides.

This by the way is also why forging can cause a finer grain structure. It doesn't actually refine the grain of the steel while its being hammered on. Note during the hammering, the steel is not even in the state that it will be when its ground into a knife. However when you transform the steel by heating and quenching, because it was hammered on, there will be a greater number of crystal transformation points and thus a lower grain size. In technical terms the plastic deformation increased the dislocation density in the steel and thus increased the nucleation sites for the phase transition.

This is also why multiple quenching can produce a finer grain size, when you transform from martensite to austenite, as opposed to ferrite to austenite, there are more transformation points and thus a finer grain structure. When you quench again, the finer austenite grain produces finer grained martensite. This process can be repeated again, as the finer the grain of the initial state, the finer the grain of the produced state, and so it is an iterative process.



-Cliff
 
I am still not getting it Cliff. I understand how microstructure works. What I was asking is, what does "cold" do for steel in the grain refining department? How do they become smaller through the cold of cryo? Perhaps I missed interpreted something you mentioned or maybe I am asking this question the wrong way. For example... grain size is determined by prior austenite size, well, once cooling occurs and the steel has become either martensitic or some form of ferrite/pearlite/cementite whatever composition, in what way can the grains essentially be "made" smaller by dropping the temperature into sub zero range? :) Am I making any sense? Hmm.

-Jason
 
Epsilon :

once cooling occurs and the steel has become either martensitic or some form of ferrite/pearlite/cementite whatever composition, in what way can the grains essentially be "made" smaller by dropping the temperature into sub zero range?
Click to expand...

In some steels you can have ~25% of austenite retained if you don't go to sub-zero temperatures. Since cryo promotes the increase of further martensite transformation, this will directly decrease the growth of the grains already formed (and those forming). This effect of grain refinement by containment, is enhanced by the increase in dispersal of the carbided out alloy segregates which will act to do the same thing, which by their very nature are also finer grained.

When the crysal structure is transformed yet again through tempering, the greater percentage of martensite transforming and increased carbide precipitation and dispersal will insure a lower grain size in the final result. As well, the very low temp of deep cryo (liquid N2) may inhibit grain growth directly. I have only seen this referenced in a vague manner, usually noted as keeping the steel in a stable state before further experimentation has to be done (at a later time).

So in short, it doesn't actually shrink the grains already formed, but prevents them from growing, causes the formation of very fine grained carbides, and causes a finer grain to result when the steel is tempered due to the previous factors and the greater percentage of martensite transforming.

Graymaker, sure.

-Cliff
 
I gave some thought about the percentage idea before, but didn't think to meditate on it any further for whatever reason. Now that all makes perfect sense. Thank you very much for that clear and concise explaination. I get it. :cool:

-Jason
 
All of our blades at www.knifekits.com are cryogenically treated. We found it made the steel more "predictable" for the starting knifemaker. Plus of course, all of the other reasons.

Peace,

GHEN
 
What I'm basically told is that cryo is a waste of time for simple steels, but has some benefits with the higher alloyed materials like A2, D2, ATS-34 - is that correct?
 
to an extent but may not be worth it on
the simpler steels.
but I'm told Harley Davidson has they're cylinders done too.
and I believe them to be cast iron very, low carbon.
and I believe the new plastic top handles on Stihl chain saws
have nitrogen in the center of them, why or how it got there
? must be a reason for it..? they are tuff..
but most the Hi carbon stainless steels were made to
be treated with L/nitrogen as part of the process.
 check Ed Fowlers testing on this subject, you'll find them interesting.
 
TWG, if you think about what cryo does, then it is easy to understand why the simple steels have little to gain. They don't have large segregated carbides, have an overall very fine grain structure anyway, and a very high martensite% transformation just at a normal quench. As well, without the alloy content of the more complex steels, they can't benefit as strongly from the enhanced carbide formation. Plus, in general the simpler steels are used in blades where impact toughness and overall durablity is the major concern, and cryo has little proven advantage for those aspects.

-Cliff
 
Over the years I have formed a theory concerning grain size. After testing many blades to destruction I have found that absolute uniform gran size tends to result in blades that are less tough than blades that manifest an aggregate in grain size. For example blades that have an aggregate of 14 and finer are tougher than an absolute uniform 14 grain size. The concept becomes complex and may not be measurable at this time, but the influence seems to be very real.

Just entering the concept as something to think about.
 
Ed I agree
it's like cement, if you have all the rocks in it the
same size, say like mortar, it's not so strong
the odd sizes of rocks (grain)will cause
a type of interlock.
  but in turn you can't have all the rocks to
big ether, this weakens it..
    just my point of view a opinion..
 
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