Destructive Testing 80CrV2

tkroenlein

tkroenlein

Well-Known Member
Good evening, folks. I have a bit of a convoluted question concerning the outcome of some destructive testing I did on a knife today. This was my fourth knife I made about this time a year ago. Forge heat treat, 130F canola quench, 400F temper in 2, one hour cycles.

The spine of the the blade at the ricasso is about .080." The blade seemed fairly flexible, and I had previously flexed the blade quite a lot over its full length.

So today, I'm messing around in the shop and decide to plant the tip of that knife in a board and twist it over, and it popped right off. No pressure at all, really. A real small piece. I assumed I must have burnt the tip, so I stuck about 3/8" in the vise and leaned it over, and that snapped right off as well. I did it again and got the same result.

So seeing how easy it snapped, I just kinda said to heck with it, and put it on a piece of 9/16" mild round stock, and proceeded to moderately peck on the spine with a ball pein hammer, checking frequently. I ended beating about a 1/3 of the way through that rod, with no significant edge damage. No perceivable chipping or roll at all. Not still shaving sharp, but sharper than the average guys pocket knife.

The grain looked fair to good. I can do better now that I'm using a kiln and doing accurate grain reduction cycles, but it really wasn't awful.

My question then is this-

Am I expecting too much for flexibility, or does the not quite dialed in heat treat somehow not present as a failure when driving straight into the edge?

I really have no idea what to think.
 
One of the most common mistakes in the entire knifemaking community has been in misunderstandings arising from “flex” testing. Flex testing almost immediately goes off the rails with terminology, that is in flexing vs. bending; if the blade takes a set whatsoever, we are no longer flexing but are bending the blade. This seems like petty semantics, but it really is not, because where the blade bends can tell you a bit about the heat treatment, but if all it does is flexes and returns to true all it really tells you is how thin you have ground the blade.

On the stress-strain curve, there is an initial straight line at a given angle, this straight line in the curve represent the proportional, or elastic, range. The proportional, or elastic range, is governed mostly by a property of steel known as modulus of elasticity, or Young’s Modulus, and is mostly a function of the cross section of the steel rather than heat treatment. Two blades, one dead soft, and the other full hard will flex the same distance under the same load, because their stiffness is a set number that really cannot be affected by heat treatment.

A softer blade will have a shorter proportional range before you enter the plastic deformation that is beyond the yield point and you get bending, but, obviously, you need to take a set of some sort to know you are there. This change occurs when you exceed the numbers representing the tensile, compression and shear strengths of the steel- and here is where cross section is almost more important than heat treatment.

The thicker the blade, the greater the forces will be on the outside and inside curves of the flex. On a thin blade the forces generated may be so low that you never exceed the yield point and thus a filet knife will go to 90° and spring back to true. But a ¼” thick bowie will either take a set or snap, even if the two have the same heat treatment.

It is for this reason that I never bothered too much with flex or bend testing, one really needs some very precise way of measuring the numbers to make any true sense of it. And, in the end, there are other much more important properties I would want for a blade. One such property would be impact toughness, which is a much more accurate measure of the toughness of a blade, and why I like your edge impact into the rod much better. I have always told people that rather than the well-known “brass rod test”, which really doesn’t tell me anything I want to know, I have my own brass rod test. Instead I will use a ½” brass rod in a more telling way by whacking it with the sharpened edge. This is a much more revealing method of testing the toughness of the blade under sudden load, like it may actually see in knife use. I don’t like the idea of prying with a knife blade, but if I did, I would simply make it thicker.

On your blade the mystery is a little more complicated, and it may be a matter of too high of hardness for the steels application, or there could be overheating, or some other inconsistency in the internal condition. Rather than maintaining contact with the rod and tapping on the spine for force, try actually chopping or impacting the rod and see if there is chipping under sudden load. If it is destructive testing, I would push it until there is yield or failure at the point of impact. Plastic deformation, a wrinkle or wave, in the edge indicates a lower yield point and hardness. Chipping indicates a higher hardness than what is desired for that application. Small divots in the edge very often will display signs of plastic deformation under the microscope, so it is often possible to err on the side of higher hardness. But it is still all heavily dependent on the geometry. An edge that wrinkles just a little, can be ground back and reshaped with more support and never wrinkle again. It is all about finding that sweet spot in the widow where the heat treatment is dialed in for that exact geometry on a given alloy.
 
Thanks Kevin!

I did leave out the test I did prior to the mild rod. I hacked the beegees out of a whitetail antler, carved it all up and hammered it through to the extent that I felt as though the antler was not sufficient to induce a failure of any kind.
 
tkroenlein said:
Thanks Kevin!

I did leave out the test I did prior to the mild rod. I hacked the beegees out of a whitetail antler, carved it all up and hammered it through to the extent that I felt as though the antler was not sufficient to induce a failure of any kind.
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You can do as you wish with your knives but if you whacked an antler (which is like rock) repeatedly with NO damage to your cutting edge then you have surpassed what the knife should see in correct use and I would be very happy with those results...No further testing needed IMO; see if you can repeat those same results on subsequent knives of the same design and then you have information worth having. That information is that you have a good process for working that steel. Again, this is just my opinion but I try to keep testing in the realm of one to two steps above what a knife should see in proper use for its design. Fillet knives get whacked against a deer bone several times then must pass the paper test.. Camp or field knives chop through a 2x4 then must pass the paper test. Then they get whacked against an antler several times...you get the picture. Making a long answer short knives must cut and hold an edge. Flexing and bending (as I have learned here) is more related to thickness than your HT.
 
That's why I was miffed. I had assumed that if the edge held up, I was good to go.

I really didn't really know what to expect from (as Kevin pointed out) a prying test. When you phrase it like that, I really don't expect a knife to pry much at all, though a game knife should survive some level of torquing.
 
I really appreciate the comments. I've put a lot of time and effort (and $$$) into getting one single steel (80CrV2) to perform to its max, and while I'm sure there's always a bit more to get, I feel like I'm getting pretty near it.
 
I could see a game knife need to take some abuse at the tip if you are separating a ball and socket joint for example but you could test that by driving the tip into something hard like maybe a piece of brass or batoning the tip into an antler that has been secured so it does not move on you. If it survives that a hip joint will not slow it down. Don't get me wrong, testing is a very good thing I just feel like it should have practical applications for the intended use of the knife.
 
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