Old traditions die hard.

Another way to have a real world understanding of how hardness, flexibility, elasticity, geometry, etc. work hand in hand is to think of a fillet knife.

Being so thin, they are virtually all fully hardened.

Yet, with tapering from full width and thickness at the handle to a very thin point at the tip, they flex almost in a circle, yet don't break.

Think of that being the far end of the spectrum.

Fighters, hunters, Bowies, swords, etc. lie somewhere back in the middle - yet still agree with the very same laws.

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Old traditions die hard.

Another way to have a real world understanding of how hardness, flexibility, elasticity, geometry, etc. work hand in hand is to think of a fillet knife.

Being so thin, they are virtually all fully hardened.

Yet, with tapering from full width and thickness at the handle to a very thin point at the tip, they flex almost in a circle, yet don't break.

Think of that being the far end of the spectrum.

Fighters, hunters, Bowies, swords, etc. lie somewhere back in the middle - yet still agree with the very same laws.

Thank you Mr Anderson, I'm a huge fan of your work by the way. Especially what you do with Hamons. So passing your JS performance test, your cross section geometry was just as important as the heat treatment. Logic would say to just grind the blade thin as possible and you will have the flexibility but would a blade ground that thin stand up to the 2x4 chop even with the hardened edge? I'm still very early in the learning stage but taking in as much as possible. I'm starting to think other than a Hamon I should fully harden the knife. If I'm correct simple, shallow hardening steels like 1084,1095 won't fully harden anyways so a full heat treatment would be to my best advantage and get the most toughness I can out of it. Thank you for your in sights Mr. Anderson I appreciate it.

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Thank you Mr Anderson, I'm a huge fan of your work by the way. Especially what you do with Hamons. So passing your JS performance test, your cross section geometry was just as important as the heat treatment. Logic would say to just grind the blade thin as possible and you will have the flexibility but would a blade ground that thin stand up to the 2x4 chop even with the hardened edge? I'm still very early in the learning stage but taking in as much as possible. I'm starting to think other than a Hamon I should fully harden the knife. If I'm correct simple, shallow hardening steels like 1084,1095 won't fully harden anyways so a full heat treatment would be to my best advantage and get the most toughness I can out of it. Thank you for your in sights Mr. Anderson I appreciate it.

there are several schools of thought on this. first is to take advantage of the rule allowing a 1/3 crack and only harden that portion of the blade, in that case the rest of the cross section is of less importance. second is to full harden the blade and draw back all but that last third to a much softer temper. in this case cross section does matter but only as a part of the larger whole. last is to full harden the blade and ride rely on cross section to give the necessary flex ability. Remember that nothing is timed so the edge just needs to survive all that chopping but you can take as long as you need to. my passing performance knife from last july was full hardened with a drawn spine, it was thin and I took a lot of time going through the 2/4's (to the good natured ribbing of a few bistanders) but I went 90 easily. I did a second knife at the same time that I tested to destruction in the shop, it was with in 0.005 of my test blade and I did the same heat treat. that blade went 90 then 90 the opposite direction 7 times before failing after passing all the other tests, i figured that was enough of a fudge factor.

MP

Now you are fully grasping the challenge of the performance test.

Yes - thin would flex nicely. But you need the mass and speed/weight to cut the free-hanging rope and chop through wood. Twice.

Fully hardening will also require you to fully austenize even the thick portion of the blade WITHOUT over heating the thin cutting edge! Which will then chip while chopping and possibly crack while flexing.

And even shallow hardening blades harden from both sides, which can get fully hard all the way through in the thinner sections.

The variables are far too many to comprehend.

So - don't try to over-think it.

Do the best you can to make a knife you feel will work and then run through the test yourself and see what works - and what doesn't.

Then make the appropriate adjustments to your regimen.

I tested and tested and sliced and cut and chopped and flex and destroyed knives over and over until I got what worked for me in my shop with my tools and materials.

You will need to do the same.

If it was easy - anyone could do it.

Thank you for your input Mr. Parkinson and again for yours Mr. Andersen (my apologies for mispelling your name earlier Mr. Andersen ). I do believe I'm starting to grasp some of these metallurgical ideas. Before now I automatically figured steel hardness was the main factor in flexibility, cross section geometry never even occurred to me to be a factor. I'm going to try really hard to get into one of the Haywood classes this next year. I see they have a class Oct 3rd so maybe I can talk my wife into a mini vacation for her birthday. I do believe now that I have a little better understanding of what needs to happen to make a passing knife,I should just get comfortable forging and testing performance knives,I have plenty of time to dial it in. Thank again gentlemen.

Got a lot here so this will be a three parter:

The ABS performance test does a good job at balancing out the factors involved so that the knife needs to perform in many categories and not just be skewed towards one aspect. On the 2X4 cut whittling is not allowed it needs to be a chopping type cut, so a filet knife just isn’t going to do it in a realistic manner. While there is no time limit to cut through the 2x4 there is reality and the laws of probability also working against you. The 2x4 is to test the edge strength to toughness relationship as well as overall edge holding. Which edge will be sharpest- one that is honed and sets untouched until you shave hair, one that contacts wood two or three times after honing or one that contacts wood fifty times? The more times the edge contacts the cutting medium the greater the chances for edge degradation. If you try it with a filet knife that edge had better be very darned impressive in its material properties because it is going to have a rather long termed ongoing relationship with that 2x4. On the other hand, a heavy bladed bowie with a serious convex edge can be made from mild steal and still probably survive the mere dozen or so momentary encounters it will have with the wood.

You want to cut the rope- make the edge geometry sharp, thin and streamlined. Heavy secondary bevels from grinding a high angle sharpening on an overly thick edge may work to axe your way through the 2x4 but it will work against you with the rope. The spine can be as thick as you like but the edge is best rolled onto a reasonably thin edge with a slack belt so that your sharpening angle on the stone can be 22 degrees or less with no extreme micro-bevel to resist in the cut. The rope is free hanging so un-streamlined cutting resistance will tend to push it out of the way instead of cut it.

You want to have an un-degraded edge after cutting two 2x4’s- find the balance between the edge reinforcement in the cross section and the previously mentioned streamlining effect, and the strength to toughness ratio required to maintain that particular geometry. The better your heat treatment is to bring out the desired properties, the thinner you can go with the streamlining of the edge geometry.

The mechanical advantage in cutting the 2x4 is not in blade thickness as is almost universally believed, but rather in mass distribution. Mas can be more effectively distributed in the width of the blade rather than the thickness, which is only needed for edge support. Thickness beyond that which is needed for edge support is wasted and can actually be counter-productive if it is used in place of proper blade width. Consider two blades- one is 1 inch wide and 3/8” at the spine, the other is 2” wide and 3/16” at the spine. In cross-sectional area they will have roughly the same volume of steel, but one will not only have greater resistance as a wedge, it will also have less rigidity in the direction of force and thus most of it energy will be wasted in bounce as it deflects in the chop. The thinner and wider blade has the same mass in the given area but it is distributed in such a way that the wedge is more streamlined for deeper penetration on every cut, and with much greater mechanical advantage from rigidity due to the width of the steel in the direction of force.

Now it is time to pull all this together with the heat treatment when dealing with that bend test. There are two things happening when you take a 10” blade to 90 degrees- bending and flexing, and they are NOT the same. Unless your blade snaps back to perfectly straight when you release it (like a filet knife) you are doing a bend test, not a “flex” test. Bending is permanent deformation of the metal via complex systems of strain movement (slip) of planes atoms in the steel. Flexing is temporary deformation, involving much less strain movement that allows the atomic arrangement to return to its relatively original condition. The permanent change via bending is “plastic” deformation, the temporary deformation via flexing is “elastic” deformation.

The dividing line between the two is known as the yield point. The yield point can be dozens of pounds of for soft steel, or it can be hundreds of pounds for hardened steel. Think of it a sliding scale where we can move the pointer with heat treatment. But always remember this- below the yield point, deformation is always elastic, and above it deformation is plastic and permanent.

The elastic deformation below the yield point is said to be “proportional” in that the strain/deformation is proportional to the load applied. If 40 pounds makes the blade elastically deflect 15 degrees that is all it will deflect unless more pounds are applied, and the deflection will return to zero when the load is removed. This will be the case until the yield point is reached, where the proportional deal is null and void and the steel will continue to deform, and in a permanent fashion, even though no more pounds are applied. What hardening heat treatments do is move the yield point higher so that you have more proportional range.

The catch, and there is always a catch, is that there is another point beyond the yield point that is the ultimate strength of the steel. At this point the metallic bonds let go and you have two pieces instead of one blade. <img src=' http://www.americanbladesmith.com/ipboard/public/style_emoticons//sad.gi f' class='bbc_emoticon' alt=':(' /> Since heat treatment merely moves the yield point on the sliding scale, you are trading plastic for elastic deformation. A soft blade will bend with few pounds and continue to bend a lot before snapping, while a hard blade will take many, many more pounds to bend but will not continue to bend much before breaking.

But here is the odd part mentioned in the initial post of this thread- heat treatment has no effect at all on the amount of load required to deform (flex) the blade within the proportional range. That number is the same for an annealed piece of steel as it is for one at 65HRC. If it takes 25 pounds of load to flex a blade ten degrees, be it fully hardened or soft spined, the numbers will not change within the proportional range of flexing. This is called Young’s Modulus, or Modulus of Elasticity. The elastic modulus does not change with heat treatment but it does change with geometry. The more area in the cross section, the more load will be required to flex it. To clarify this even better think about this- if you need a stiffer suspension on your pickup do you have your springs re-heat treated, or do you slip more springs into the stack? Re-heat treating would have no effect since springs are elastic (and it is worth noting that springs break when they are overloaded, they don’t bend <img src=' http://www.americanbladesmith.com/ipboard/public/style_emoticons//wink.gi f' class='bbc_emoticon' alt=';)' /> ).

With all of this new information let’s re-examine the ABS test. A thin blade is great for the rope, a blade with mass but with a strong and streamlined profile is good for the 2x4, but for the bend we are given a whole lot of leeway with options. There are no prohibitions on bending versus flexing, and the hardened edge is even allowed to crack.

If the ultimate strength (snap!) of our blade is “A” and we wish to make a blade that will not break, all we have to do is make a blade thin enough that it cannot elicit the forces necessary to reach “A”. It may exceed the yield point but it will not reach “A”, the ultimate strength. A taller order is to make a blade that will not reach the yield point “B”, we can approach this by pushing the yield point a little higher with greater hardness to give us more proportional range to play with and by making it thinner to reduce the forces involved in the flex.

So there are two ways to approach the 90 degree test, one with an emphasis on flexing and one with an emphasis on bending. The zone hardening with only the edge hardened for bending, or the full quench with differential tempering for more of a flex test, but due to practical needs for mass there will almost invariably be some plastic deformation involved at the spine. For these I always recommend something like 1084 for the bend approach and perhaps a simple alloy steel like 5160 for the flex heavy approach. Either way, the thinner you can make the blade and still effectively chop the 2x4, the better your chances of surviving the 90 degree deformation.

For many years now I have put such an emphasis on heat treatment, on order to help folks understand our materials better, that I have admittedly neglected to stress the importance of geometry on the blades efficacy as tool. Proper geometries, both at the edge and in the overall design of the blade are equally as important as the heat treatment, and the two are so interconnected that one cannot even be effectively defined without considering the other.

Wow Mr. Cashen! Thank you so very much for taking the time to break things down even more for me. This is awsome! Only in the ABS do such talented people share so freely. You have definitely given alot to think about it. Before I received your video on Heat treatment I was at a bit of a cross roads with my HT, being that popular belief is that a hard rdge and soft spine make a superior knife, my problem is/was this. Being when we temper knives in a oven we are holding a specific temperature for a specific amount of time in the hopes that our temper is uniform throughout. So how would taking a torch or any other heating method to just the spine of the steel give it a true temper based on the almost instantly changing color of that steel. We get temperature but no real time at temp. Your video and the other great folks here on the forum are definitely increasing my knowledge and understanding which makes things even more exciting. Thanks again Mr. Cashen, your knowledge seems to know no bounds.