How much does "toughness" matter?

Hmm, this lead me to a few questions:

If the both steel of the same hardness and take the same amount of energy to break, so theoretically speaking, they have the same toughness.

However, one flex more before breaking, and the other flex way less, which one is "stronger"? or "tougher"?
 
Deino said:
Hmm, this lead me to a few questions:

If the both steel of the same hardness and take the same amount of energy to break, so theoretically speaking, they have the same toughness.

However, one flex more before breaking, and the other flex way less, which one is "stronger"? or "tougher"?
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Under ideal conditions those 2 situations won't happen. If they have the same hardness, they will flex the same amount before breaking. If you are talking about breaking under flexing conditions, then toughness doesn't normally enter into it. However under less than ideal conditions you might have a hidden tang knife with a sharp corner where the tang meets the blade, and another hidden tang knife with a better transition where the tang meets the blade. The blade with the more abrupt notch could break prematurely, even if the 2 knives are made out of the same steel with the same heat treatment.
 
afishhunter said:
Agree.
You don't  need a "modern" "high end" steel for a tough thin edge that won't chip, either. 👍
I keep all my folders (slipjoints and friction folders) and non-Puukko sheath knives sharpened to 10° to 12~12.5° per side (20 to 25 degrees inclusive).
In over 60 years of using 1095, 10xx, 440A, 440C (it took a bit of time and elbow grease) to get my old Buck 110 with 440C thinned to 12~12.5DPS.) 420HC, 5160 and other "spring steels", generally at under 60 Rockwell C. I've never chipped a blade. I've had an edge roll, once or twice, (not a biggie, stropping on your boot or belt, or using a Butcher's Steel (contrary to popular belief, an old fashioned grooved Butcher's steel does  not remove any steel from the blade.) gets rid of the rolled edge right quick.)
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What's need got to do, got to do with it? What's need, but a secondhand emotion... :)
 
Deino said:
Hmm, this lead me to a few questions:

If the both steel of the same hardness and take the same amount of energy to break, so theoretically speaking, they have the same toughness.

However, one flex more before breaking, and the other flex way less, which one is "stronger"? or "tougher"?
Click to expand...
Less bending-stronger, more bending-tougher. Extreme generalization and hardly possible in real life.
 
bdmicarta said:
Under ideal conditions those 2 situations won't happen. If they have the same hardness, they will flex the same amount before breaking. If you are talking about breaking under flexing conditions, then toughness doesn't normally enter into it. However under less than ideal conditions you might have a hidden tang knife with a sharp corner where the tang meets the blade, and another hidden tang knife with a better transition where the tang meets the blade. The blade with the more abrupt notch could break prematurely, even if the 2 knives are made out of the same steel with the same heat treatment.
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Maybe my mistake to not say that they are not the same steels? (forgot to put on an s somewhere)
Same hardness won't give you the same amount of flex before breaking. This is why there are steel for spring and steel for nut at about the same hardness.
 
Another common misconception is that the stiffness of steel varies with its alloy or hardness. Within the elastic limits, the modulus of the elasticity is practically identical regardless of alloy or hardness. The only thing hardness does is increase the deflection before plastic yielding begins. In a nutshell, if the workpiece is not taking a set, there is no difference.

People have a hard time accepting this because it seems so counterintuitive.

The only thing that significantly affects the stiffness is the thickness. Or specifically, the area moment of inertia (your sectional modulus).

If you double the width of a beam it will be twice as stiff. If you double the thickness of a beam it will be eight times stiffer. This is why fullers are employed in a sword or knife design. Cutting that fuller does not make something stiffer and stronger, but the redistribution of material, making a thicker piece of the same weight because the material was set farther away from the neutral plane, does make it stiffer and stronger. But that's a discussion for another time.

There is not a steel alloy or heat treat that is more flexible than another. Just soft steel that takes a bend easier or hard steel that springs back (at a higher force than was obtained with the softer steel because the softer steel was yielding in its plastic deformation range). The inherent stiffness of all steel is pretty much the same.
 
Some very high carbide steel are a little bit stiffer than low alloy steel because the carbide content of the steel becomes significant and carbide is stiffer than iron. But even then, you would be very hard-pressed to see the difference without specialized tools, I don't think you could feel it with your hands. And steel with that much carbide in it is pretty unusual. And even then, its flex mod is pretty much the same soft or hard.
 
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Nathan the Machinist said:
Some very high carbide steel are a little bit stiffer than low alloy steel because the carbide content of the steel becomes significant and carbide is stiffer than iron. But even then, you would be very hard-pressed to see the difference without specialized tools, I don't think you could feel it with your hands. And steel with that much carbide in it is pretty unusual. And even then, its flex mod is pretty much the same soft or hard.
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It just shatters sooner. Compression side is stronger and stretches the tension side more, which with that much crack propagation sites (every single carbide) gives up with a bang thinking "what's wrong with that human!".
 
Bending has a lot of factors so it is hard to learn about with forum posts. As mentioned the thickness is the most important factor; thicker is more difficult to bend. At the same time, the stress at the surface is higher with greater thickness, so bending to the same angle is more likely to result in failure with a thicker piece of steel. Stiffness is not affected by heat treatment so when bending the force necessary is the same regardless of heat treatment, until you reach the point of yielding. The stronger/harder the steel the more it has to bend before reaching the yield point. So a soft steel does not have to bend very far until you reach yield at which point it will continue to bend without much increase in force. The stronger the steel is the farther you have to bend to yield so that the force greatly increases to keep bending because you are fighting the elastic modulus of the steel rather than plastic deformation. Once the hardness is increased enough you are more likely to reach a point where the steel will reach its ultimate strength and fracture. Again this is tied to steel thickness; thin knives like fillet knives will never reach the yield stress because they are so thin, and bending these 90 degrees or greater is mostly a parlor trick, not a sign of superior heat treatment, apart from ensuring it is hard enough not to yield. When the knife is a bit thicker and we are in the regular knife hardness range, bending will eventually lead to fracture. When we are talking those situations toughness matters. A steel with large carbides and/or lots of carbide will fail sooner due to the lower toughness. For every steel/thickness combination you can increase hardness and see the evolution. As the hardness goes up the stress required to bend or fracture increases until it peaks and begins to drop from insufficient toughness. Tough steels that are capable of high hardness will peak at the highest levels. Hitachi has some bending test results for their range of steels that are discussed in my book and compared with charpy impact testing. You can look at them here: https://www.hitachi-metals.co.jp/e/products/auto/ml/pdf/cwts_b.pdf
 
Nathan the Machinist said:
I honestly am not on this thread to promote myself. But, this is a sword that we made for a cutting competition TV show that demonstrates the insane durability capable with modern high-end metallurgy and heat treat. Forged historical swords would have been destroyed many times over attempting these things

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But will it keel??

Seriously though, I want one.
 
Hashishiin said:
Yeah, be careful what you say about BRK. All of the people in the video came to unfortunate ends in "accidents" after they complained or spoke up about BRK.
And it isn't just them, or their house.... BRK will take your whole block out just to get to you.
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I wasn't joking. That looked like an unsafe work environment to me. Shit, I even wear a mask when sharpening knives with my Wicked Edge.
 
Larrin said:
As mentioned the thickness is the most important factor; thicker is more difficult to bend.
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To add for simplicity. Bendability is a ratio of thickness. If one 1mm bar is bent with a 20cm radius, 10mm will have a radius of 2m with the same stress. So it is easy to figure out if one wants, how much can a blade flex before collapsing if there is initial data for a steel.
 
Larrin said:
Hitachi has some bending test results for their range of steels that are discussed in my book and compared with charpy impact testing. You can look at them here: https://www.hitachi-metals.co.jp/e/products/auto/ml/pdf/cwts_b.pdf
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Some of the charts in that material make sense but one does not. The charts show tension strength and compression strength increasing with hardness which is what I would expect, but the chart of bending strength vs hardness doesn't show the same thing. There is also a chart in there of stiffness (youngs modulus) for the various steels. That chart shows about 10% variation which is pretty much insignificant for knife discussions. I didn't think there would be that much variation among high strength steels, I thought it might be that much different among a very wide range of alloys.
 
bdmicarta said:
Some of the charts in that material make sense but one does not. The charts show tension strength and compression strength increasing with hardness which is what I would expect, but the chart of bending strength vs hardness doesn't show the same thing.
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It breaks in high hardness. When looking just at HAPs the peaks are close enough to see correlation. It's just the steels where not measured thru the hardness spectrum, but just in their working hardness.
 
Rooster81 said:
Here you go, skip to 1:20 to see the sharpening. Take it for what it is, the video is 10 years old so not sure if the processes have changed? I make no judgements or statements on it, as I'm not a knife maker. Except for the lack of masks, holy moly, I started coughing just from watching that dude sanding the handle!

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Holy shit. That's a workman's comp. claim waiting to happen.
 
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