Blade durability and flexibility at -196 degrees

C

Cliff Stamp

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I have used knives in the cold before and never saw them take damage any more readily than they did when it was warm. Even down to about -30 or so, I saw no odd behavior during heavy chopping and prying with the edge. I was curious how much temperature did effect the durablity and flexibilty of a blade steel and the easiest way to see this I figured was to use an extreme case, so I used liquid nitrogen, which is around -196 degrees.

I chose a blade called "The Defender", it was marked "440 stainless steel Japan", so I assume 440A with a not so high quality heat treat. The blade stock is just a little under 1/8", at 0.120", and the primary grind is a *deep* sabre hollow making the blade just 0.018" thick behind the edge bevel which is ground at a decently thin angle, about 21 +/- 2 degrees. A very thin edge profile compared to most "tactical" knives.

As I locked the blade in the vice, 1" back from the tip, a small piece near the tip actually broke off, not a good sign. When I torqued on the blade it bent readily and snapped suddenly at about 35 +/- 5 degrees, just above the 1" mark where it was locked in place. I dipped the blade in the liquid nitrogen to a depth of 2" and held it there for about 30 secs until the temperatures had equalized. I then viced again and leaned on it. It passed 10,20,30 and 40 degrees. I kept going and it hit 50,60, and then 70. I pushed on and the blade held to 80 +/- 5 degrees.

I only stopped then as it was difficult to force it any further because I was standing as far away as possible and just using wrist torque to bend the blade, plus the torque was starting to unbalance the table the vice was bolted to.

As I released the blade it went back close to true. It had taken a significant perm. set but had not broken. I decided to see just how far it could go so I wrapped the blade in thick plastic and stood over it and pressed down (by now the blade had been out of the liquid nitrogen for about 1.5 minutes) . This time the blade cracked quickly at 30 +/- 5 degrees, but not at the lock point, actually under it in the jaws of the vice. The blade broke again and came out of the vice at 40 +/- 5 degrees.

About 10 minutes later, using a 600 g mild steel bar as a mallet and swinging hard from the elbow, I hammered on the spine of the blade driving the edge into a concrete block. The edge came apart quickly which I would expect given the really thin geometry, I stopped after 25 hits. I then dipped the blade into the liquid nitrogen until it equalized and repeated the concrete chopping on a different section of the blade. This time the blade broke after 7 hits. However the damage the edge took was not significantly greater than when it was warm.

Here is the blade, the part near the tip was the part that was cooled when it impacted the concrete :


http://www.physics.mun.ca/~sstamp/images/defender_broken.jpg

here is a shot showing the bend in the main body of the blade :

http://www.physics.mun.ca/~sstamp/images/defender_bent.jpg

here is the concrete (the notch on the bottom was cut with the defender, on the bottom was the WB, 50 chops) :

http://www.physics.mun.ca/~sstamp/images/defender_block.jpg

here is the bar :

http://www.physics.mun.ca/~sstamp/images/steel_bar_600g.jpg

Concering the flex, blades will generally snap near the tips at a low angle when viced unless they have a full distal taper, especially hollow grinds due to the high stress points due to the curvature. All the distortion will happen near the lock point as that area is too weak to induce a flex in the main body of the blade. Considering this it would not have surprised me when warm that the blade would have reached a larger angle I moved back. However it did surprise me when it still did this at a very low temperature. Even more so, it broke at an even lower angle than when bent initially when it had lightly warmed up.

What I figured happened here was the blade was in a high stress position when cold. When the blade was released and allowed to return to true it was at a much higher temperature than when bent. This might have caused it to have a reduction in resilience, which is the ability to release the energy stored during an elastic deformation. As well the plastic deformation damage may have also been increased during the release for similar reasons.

In regards to the concrete chopping. There was an obvious decrease in the impact toughness as the blade snapped, however there was no real difference in edge damage. The reason for this may have simply been that the regardless of the toughness, because the cross-section of the blade was so low near the edge because of the deep hollow grind that the steel did not have the necessary strength to resist deformation and thus it was deformed until it tore. The strength would not be decreased at a low temperature, it would actually be increased.

As well, the exposure to the liquid nitrogen during the bending might have induced stress cracks all along the blade, this might have lowered the edge durability when it warmed up. It would have been better to have used two knives so the results of the bending would not have influenced the chopping.

-Cliff
 
I'm surprised it wasn't more brittle than that, and I wonder if 440A might be less brittle at low temperature than carbon steel. How about trying carbon steel next? It doesn't even have to be a knife, any random scrap steel would be interesting.
 
i use 01 and 1095 and 5160 steels if you could find out how they react to your tests i would really look forword to it
 
look,
if your EVER in -196 degree weather,then you got a whole hell of a lot more to worry about then your knife.

just what is the point of this test,anyway?
 
Cliff,

You should consider changing your on-screen name to "Demolition Man" because when it comes to this kind of testing, YOU DA MAN!:D

If we're ever struck with a really severe nuclear winter, I'll now know to steer away from using 440A as blade steel in my survival knife.
I wonder how Infi would stand up...hmmm...

I gotta give ya credit though.
I NEVER thought I would see this kind of a test done and posted on the forums.
Thanks for posting the results! :)
 
ok i know i am new to some of this knife stuff but what the hell was the reason for that!!!!!!!
 
Seems a lot of people are confused about why Cliff did that test, what it proved....

Some people believe when the temperature gets down around zero Fahrenheit a knife gets very brittle and you'd better be very careful how you use it when it's that cold. The experiment proves that's not true even at much lower temperatures -- at least, it's not true for soft-tempered 440A.
 
Cliff,

Your experiment should be repeated and documented. I would have expected the blade to shatter like a piece of glass, and certainly did not expect to see an increase on thoughness. Very interesting stuff. It would be good to get someone to run a series of RC numbers at different temperatures.

Next time you should go with raw blade stock. You might get some of the makers on this site to kick in some of their scrap steel. This would reduce your cost and remove any bias caused by the blade grind.

N2S
 
Not2sharp, the blade bias is a very important aspect indeed. It would have been much more robust if I had multiple copies of the same blade. That way for example I could have viced one blade at different positions and recorded when it snapped (which would have been interesting on its own) and then repeated it on another one when super cooled. Same with the concrete chopping.

If I do get any bar stock, or multiple copies of a knife and repeat this, I'll do it in a more controlled manner. For example I'll use controlled loads on the concrete impacts, probably around 50-100 ft.lbs, as well as have someone else estimate the blade angles so as to get them to be more precise.

Cougar, I would have expected 440A to actually be worse as it would have a lot of retained austenite especially as it usually doesn't have a high quality heat treat. When exposed to low temperatures you could actually see crystal transformations that should cause internal stress. I would assume however that it would take a longer time that what this blade experienced. An interesting question however.

It would be interesting indeed to try this on one of the tougher carbon steels. I'll update this thread if it happens.

-Cliff
 
Update :

Concerning the effect of the temperature on crystal structure, I had believed that this took far greater time than the 30 s soaks that I did in the above, however some recent emails that I have had (with Zut&Zut) indicate that it may indeed be possible that the soaks induced austenite -> martensite transformation.

As well based on some discussions I had with Phil Wilson some time ago, cold working can also induce such a transformation. If you combine the two it may indeed be the case that the combination of the liquid nitrogen soak plus the bending past the onset of plastic deformation caused the formation of untempered martensite and thus highly localized high stress regions in the blade - which is why is fell apart so badly after it heated up.

It would be interesting to see how a blade which has had a full deep cryo treatment would respond to the above as compared to one that had not.

-Cliff
 
One time....a long while ago...there was a question in the California Knifemakers ASSOC newsletter about how much force it would take to break a knife made of 1/4" 154CM that was properly heat treated. The answer was : "Long before you are engaged in any activity that would break your knife in half you would be a pile of smoldering flesh..." :)
-196 degrees.............long before.....etc!! :(
 
Cliff, Thanks for letting us know of another interesting test. I also would have expected the knife to be more shatter-prone at this temperature.

Saaay... I've been wondering, if a thermonuclear device was detonated above me and my knife, do you think I might have problems later with neutron embrittlement of the blade steel? ;)
 
Cliff -- have never felt I truly understood your bent towards destroying knives. Still, I find most of them at least entertaining, and often enlightening.

I am confused somewhat by your recent discovery that chopping concrete blocks will apparently destroy any knife blade. I mean, everyone would guess that. I think if you asked an "average" 8-yr old, she/he would say that would guess a knife would break if used to chop concrete.

What I'm getting at is that I'd be much more interested in something that was at least a distinct possibility for use of a knife. I know you've done a lot of things, like thrusting points into logs and twisting the point free. That seems a potential even some knife users might encounter.

Are there no tests out there that are severe enough to destroy many blades -- but not all -- that are within the potential for knife users to encounter? I guess in my mind proving that rare knives can indeed handle a test that destroys the majority of knives would be truly meaningful.

WHile acknowledging that my memory lacks greatly, I can remember some of hte knives you've destroyed in your testing. But, I can remember what it was that did in only a couple. All I remember is that you destroyed knives.

Maybe what I'm asking is what is the goal, the learning to be gained from breaking knives on concrete, and some of your previous tests?
 
Cliff,

I couldn't tell from the picture, but if the knife is full tang, could you test the tang of the blade (which I presume was not dipped in nitrogen) with a file or something to see if/how it behaves differently than the blade which was dipped. It would be interesting to know what, if any, permanent change in the metal was caused by the low temperature, and an undipped tang, if there is one, would provide a perfect point of comparison.
 
tom mayo :

how much force it would take to break a knife made of 1/4" 154CM that was properly heat treated.
Click to expand...

Not nearly as much as most seem to think. Look for example at the break tests Fallkniven have done on thier blades. Even thier A1 which has a *very* thick primary grind breaks at about 266 ft.lbs of torque. This is far from extreme, on a large knife, if you tried to pry by standing on the blade you could snap it for example. Note as well if the geometry is changed to a full flat or even worse hollow grind, the strength will drop dramatically. And while the strength is high for high alloy, high carbon blades, the impact toughness is relatively low. I have induced gross fractures in such steels with only moderate impacts like ~64 +/- 9 ft.lbs, assuming hard material contacts. Again very dependent on the geometry, convex ground blades are *much* more difficult to fracture.

Howard, I am not sure. There is a conference later on this year in Japan that I may be attending. I will try to see if I get get permission to do some testing off of the coast. It should be ok assuming there are no Godzilla sightings.


Bugs3x :

I'd be much more interested in something that was at least a distinct possibility for use of a knife.
Click to expand...

The purpose of the the harder contacts, is not really to see if a knife can do that so you could list it as a functional use, as there are usually much better ways to accomplish such tasks. In the case of concrete, since it shatters easily you could use the butt to shape blocks easily without significant damage to the knife. The reason these kinds of things are done is to simulate accidental hard contacts off of inclusions in materials being cut and/or contacts off of hidden much harder materials. It is very difficult to avoid such damage on heavy use knives unless you are very lucky and work in a very careful manner.

The way they should be interpreted is not do the blades get damaged or not, as all blades will if they hit a rock. Rather look at the extent of the damage in the light of how it effects the function of the blade as well as how difficult it would be to repair it. Now of course this should be somewhat in the scope of work of the blade. For example the blade I recent recieved from Ed Caffery/Allen blade is an optomized light use knife, there is simply no way that this knife could every see such a contact. The worse it could ever expect to see is a miscut into bone, a light contact off of a staple, etc. . I am curious as to just how much damage it would take, but not enough to destory the knife as its performance is too high and it will make a very nice gift to a friend once I am finished with it.

I often look at such things as how they happen naturally, but usually when I want them to happen they never do. I increase the chance of this by lending blades out to friends who are not as careful with them as I tend to be in heavy use. The problem with this though it that it is hard to do direct comparisions because the accidental impacts are obviously random in nature so telling if one blade held up better than another or if it was just used lighter is very difficult unless very long work sessions are used. This is why I often resort to artifical testing where I can control the work done with each knife. It does not replace actual use, and never should, but just allows a more precise estimate of the performance.

Now of course there are also times I do things just because I am curious as to how difficult they are to do, often stimulated by something on the forums, or questions asked in email or in person about knife performance. For example awhile ago on the Strider forum there was an article mentioned in which one of their knives was used to cut the windshield out of a car. I did this recently as I wanted to know huw much damage it would do to the knife (it basically took the edge bevel off). As mentioned above, there are usually better ways to do such things. In regards to the windshild break the glass with a tire iron or similar and cut through the binding with the knife, it will only excessively dull it with minor impaction/fracture, sub mm in depth.

medusaoblongata, it is my understanding from talking to a few people that all a cryo will induce is around a 1 point change in RC, I don't have the ability to detect a change in hardness that small. I will look at it though, but I doubt that I will be able to see a difference. I might have them hardness tested later on.

-Cliff
 
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