How much does "toughness" matter?

[bdmicarta]

Forgive me if someone's already answered this, but "hardness" is actually a pretty complicated and fuzzy term. This Knife Steel Nerds article gets into some of the details. Extremely roughly speaking, you could say hardness is the ability to resist plastic deformation. Obsidian, for example, is extremely hard. No matter how much you hammer on a chunk of obsidian you're not going to squish it into a different shape. However, it is also brittle, meaning it's fairly easy to chip/shatter it, which is different from deforming it.

So you want a hard edge so it doesn't "squish," but you also want a tough edge so it doesn't shatter/chip like obsidian.

Hardness is really the simplest term involved in this thread.
I've made some knife scales but I've never made a blade so I won't attempt to say I'm a knifemaker. But what I am is a structural engineer and I've done some study and research into fatigue and fracture.

So back to hardness- the primary property of steel is its yield strength. All of us have taken a piece of wire and bent it. You bend it a little bit and let go, it straightens back to its original position. You bend it more and it keeps a permanent deformation. The point at which it starts to permanently deform is the yield stress limit. You bend it a lot more and it deforms further, or if it's a high strength wire maybe it eventually breaks. How far it deforms before it breaks is its ductility. The point at which it breaks is its ultimate strength, or fracture strength. Very high strength steels can never have an actual yield point, they just get to a point where they can fracture. If instead of a wire we had a bar and we pushed on it with a pointed object- at the point where the pointed object leaves a mark is related to the yield strength. Take a steel with higher yield strength and it will be harder to leave a mark on it so we think it is harder steel. So hardness is nothing more than a measure of the yield strength. I should say if you throw a lot of carbides in there it can make the steel seem harder without increasing its yield strength.

Now remember what we said about ductility- toughness is mostly a measure of ductility. Also, despite the old fashioned test used to measure toughness, toughness has nothing to do with impact. Somebody 100 years ago could have just as well come up with a different way of measuring toughness without using impact.

How to relate this to edge stability- whatever you are putting the edge against, at whatever speed, puts various types of stresses in the steel. There will likely be tension stress- if that point of the steel reaches the yield stress limit then the edge rolls. (You roll it far enough it can eventually fracture.) If it's a steel that is higher strength (harder) but doesn't have a yield point, you have to put more force on the edge to get the stress higher but eventually you reach a point where the tension stress reaches the fracture point and the edge chips. So the rolling point for a less hard steel occurs before the fracture point for a harder steel. This is why hardness gives higher edge stability.

If you want to discuss the entire knife and not just the edge then toughness can come into play in other ways. Blades can have saw teeth, choils, holes, corners where the tang necks down, notches for locking mechanisms, lots of places that cause stress concentrations. If a blade has low ductility (toughness) then when the stress at one of the stress concentrations reaches the fracture stress then the blade fractures. If a blade has lower strength but more toughness, when this point in the blade reaches its yield limit it just yields a little bit and doesn't initiate a fracture. This is the advantage of toughness in a blade as a whole.

 

[Dr Rez]

And a thin edge is key to performance.

Not necessarily. It depends what you are defining as performance. For instance it I am looking to baton through wood thinner is not always better. If I am looking to use the knife to make tables/chairs/etc in the bush I want a thicker edge as I will be smacking it into wood.

 

[000Robert]

You know the knife community and knife world are a lot bigger than just bladeforums.com, right? This place, as great as it is, isn't the be all, end all of the knife world either.

It is for me. I just don't have the time to be at too many places.

And two years isn't very long. I've been a member going on 15 years and I came in during the latter years of greatness of this place when there were literally the best knifemakers and pioneers of the craft actively participating and sharing here daily.

Yeah, I was amazed at how many of the knifemakers that I have never heard of that were on the list of knifemakers at Blade/Texas. It is unreal!

There did, in fact used to be pictures and posts of these things. Where they are now. ....i have no idea. Just because you haven't seen them in your two years here means nothing.

It means everything to me.

Ever been to a custom knife seminar? Forging demo? Hammer in? Stuff like this happens on the regular at such events. I've personally witnessed two different makers at different hammer-ins using 52100 and 1084 to cut through a 3/4" bolt and then immediately slice newspaper cleanly. I have pictures and videos somewhere.....trapped in an old virus-locked laptop.

No, I have not. I'm disabled and don't get out much. I started getting more heavily into knives and BFC because of Covid. My goal at retirement was to make custom knives and swords. But becoming disabled changed that.

 

[000Robert]

Hardness is really the simplest term involved in this thread.
I've made some knife scales but I've never made a blade so I won't attempt to say I'm a knifemaker. But what I am is a structural engineer and I've done some study and research into fatigue and fracture.

So back to hardness- the primary property of steel is its yield strength. All of us have taken a piece of wire and bent it. You bend it a little bit and let go, it straightens back to its original position. You bend it more and it keeps a permanent deformation. The point at which it starts to permanently deform is the yield stress limit. You bend it a lot more and it deforms further, or if it's a high strength wire maybe it eventually breaks. How far it deforms before it breaks is its ductility. The point at which it breaks is its ultimate strength, or fracture strength. Very high strength steels can never have an actual yield point, they just get to a point where they can fracture. If instead of a wire we had a bar and we pushed on it with a pointed object- at the point where the pointed object leaves a mark is related to the yield strength. Take a steel with higher yield strength and it will be harder to leave a mark on it so we think it is harder steel. So hardness is nothing more than a measure of the yield strength. I should say if you throw a lot of carbides in there it can make the steel seem harder without increasing its yield strength.

Now remember what we said about ductility- toughness is mostly a measure of ductility. Also, despite the old fashioned test used to measure toughness, toughness has nothing to do with impact. Somebody 100 years ago could have just as well come up with a different way of measuring toughness without using impact.

How to relate this to edge stability- whatever you are putting the edge against, at whatever speed, puts various types of stresses in the steel. There will likely be tension stress- if that point of the steel reaches the yield stress limit then the edge rolls. (You roll it far enough it can eventually fracture.) If it's a steel that is higher strength (harder) but doesn't have a yield point, you have to put more force on the edge to get the stress higher but eventually you reach a point where the tension stress reaches the fracture point and the edge chips. So the rolling point for a less hard steel occurs before the fracture point for a harder steel. This is why hardness gives higher edge stability.

If you want to discuss the entire knife and not just the edge then toughness can come into play in other ways. Blades can have saw teeth, choils, holes, corners where the tang necks down, notches for locking mechanisms, lots of places that cause stress concentrations. If a blade has low ductility (toughness) then when the stress at one of the stress concentrations reaches the fracture stress then the blade fractures. If a blade has lower strength but more toughness, when this point in the blade reaches its yield limit it just yields a little bit and doesn't initiate a fracture. This is the advantage of toughness in a blade as a whole.

I had a brother-in-law that was a structural engineer. I wish he was still alive. He passed away from the operation they gave him for emphysema. I warned him not to have the operation, but he wouldn't listen to me.

 

[K.O.D.]

Strength is the ability to take a load without permanently deforming.

Toughness is the ability to take an impact without damage, i.e., chipping, cracking, etc.

This is exactly my feeling. High toughness is good for chopping or other impact type activities.

I've never tried to fell a sequoia with a single blow with a folder. I like good edge retention, high hardness, and good edge stability in a folder.

 

[000Robert]

This is exactly my feeling. High toughness is good for chopping or other impact type activities.

I've never tried to fell a sequoia with a single blow with a folder. I like good edge retention, high hardness, and good edge stability in a folder.

My number one requirement is toughness and also having some edge retention and strength.

 

[DangerZone98]

Forgive me if someone's already answered this, but "hardness" is actually a pretty complicated and fuzzy term. This Knife Steel Nerds article gets into some of the details. Extremely roughly speaking, you could say hardness is the ability to resist plastic deformation. Obsidian, for example, is extremely hard. No matter how much you hammer on a chunk of obsidian you're not going to squish it into a different shape. However, it is also brittle, meaning it's fairly easy to chip/shatter it, which is different from deforming it.

So you want a hard edge so it doesn't "squish," but you also want a tough edge so it doesn't shatter/chip like obsidian.

No worries, thank you for adding to the discussion.

 

[scdub]

Or maybe a better question is at what point is additional toughness a moot point, at least in a small to medium sized knife?

If whatever you are cutting doesn’t cause edge damage then there’s no need for additional toughness. Think of a sushi knife for instance. You could make it tougher, but that would detract from the edge holding and probably ultimate sharpness.

I think for most people, most EDC type knives would benefit from higher toughness simply because the folder edges that I sharpen from 99% of my friends are quite damaged. A “knife person” might give up a little toughness for increased edge retention if they are careful about what/how they cut.

On a woods knife I personally value toughness over other attributes because I’m carrying it with the possibility of having to survive with it, so breakage is a bigger issue than edge holding.

So it's there a sort of minimum toughness where going any higher is overkill?

Yes - if your knife is so soft it stops cutting, that was overkill.

I believe that some steels are at maximum toughness at knife edge hardnesses (which is perhaps why they’re knife steels in the first place).

 

[K.O.D.]

It's one of those things where there will always be a compromise. Choosing the right tool for the right job is as/more important than the steel. I'm not going to use a Busse to slice apples, nor would I use a puuko the chop logs.

I think with folding knives edge retention is the most important for the tasks most are designed for. Everyone wants that "perfectly balanced" steel, and it has yet to exist.

With camp fixed blades I'll sacrifice some edge retention for strength and toughness. I like a blade that is thin enough that if necessary I can cut food with but stout enough to handle heavier tasks.

Corrosion resistance isn't a big deal to me, I don't sweat much and it's only humid here for 3-4 months. I take care of my knives, dry them off, keep them dry in a case with desiccant or in the sheath.

 

[patrickguignot]

Science would require more samples, or the knife maker to agree with Larrin, which didn’t happen.

More samples would be useful for testing the properties of the steel. But one is enough for the composition.
And there is no need for the knife maker to agree, especially if this knife maker have an financial interest to disagree. A fact is a fact.

A8mod has large chemical tolerances, Infi has evolved over time

INFI pre-2002 contained Cobalt and Nitrogen. INFI post-2002 don't have Cobalt or Nitrogen and have same composition than A8mod.
How is it acceptable to change the composition of your steel and keep the same name?
The simpler explanation is that Jerry had a proprietary steel pre-2002 but decided, for whatever reason, to switch to A8mod while keeping the same name.

So we don’t know.

It cost me a Busse Park Ranger but now we know the current composition of INFI.

 

[hugofeynman]

This is the kind of "science" that I love! D3V steel knife cut a steel chain link with very minimal damage to the edge. I can't believe that I have to show this photo again so, I'll make it a thumbnail this time.

View attachment 1962942

You can do this with a properly heat treated Vanadis 4 extra blade, and V4e is not a tough steel. It’s tough when compared to other pm steels. Whatch Dulo knives on YouTube cutting steel with one of theirs V4e knife. Steel chain is soft, the knife steel is hard, there’s no reason for the much harder steel doesn’t go through the much softer one. It’s like the edge vs edge impact. The harder knife wins the battle.

 

[hugofeynman]

.

 

[kafolarbear]

One thing of great matter is thickness. HC steel knives usually comes thick. Strength rises exponentially with thickness. And the sharpening angle is high. That's why Nerd site is like a Bible to me. If more strength is needed, just make it thicker. I'm just about to forge weld 14C28N onto M398 to get it thicker. Going for 6mm (1/4"). It will pry anything just as any HC steel.

This is something the knife community always seems to ignore, I can't think of anyone making any real thin slicy knives in 1095, almost every knife in 1095 is a goddamn quarter inch thick crowbar with a 25 DPS angle hardened to 56 HRC that dulls if you look at it wrong so yeah of course it hopefully won't chip doing most stuff, but that geometry doesn't make for a particularly good knife either. Don't get me wrong, I like my high carbon steels, I for one swear by winkler's 80CrV2 but why people like 1095 over other high carbon steels has always been a real head scratcher for me, is it just the nostalgia of it or what?

 

[kafolarbear]

ESEE offers knives in 1095 and S35VN. They have a Youtube video showing that their S35VN blades can break, but 1095 doesn't. I vaguely recall they were batoning a log or some such thing; I might be wrong.

My two ESEEs are S35VN. I like stainless. I will be astounded if I ever break either knife, or if I ever need to baton a log.

The DBK boys have shown a couple times that they can baton with a Benchmade Bugout in S30V or M390.

Sure but also consider that their 1095 is 56-58 HRC while their S35VN is 60 HRC

 

[J. Doyle]

This is something the knife community always seems to ignore, I can't think of anyone making any real thin slicy knives in 1095, almost every knife in 1095 is a goddamn quarter inch thick crowbar with a 25 DPS angle hardened to 56 HRC that dulls if you look at it wrong so yeah of course it hopefully won't chip doing most stuff, but that geometry doesn't make for a particularly good knife either. Don't get me wrong, I like my high carbon steels, I for one swear by winkler's 80CrV2 but why people like 1095 over other high carbon steels has always been a real head scratcher for me, is it just the nostalgia of it or what?

Agree totally. If one were to "dumb down" 1095 to achieve those results for a chopper, they'd be far better off to use 1075. You could have then have the same type of knife with higher hardness and better edge geometry.

 

[ferider]

More samples would be useful for testing the properties of the steel. But one is enough for the composition.
And there is no need for the knife maker to agree, especially if this knife maker have an financial interest to disagree. A fact is a fact.

A Gedankenexperiment: Let's assume Busse had found a steel maker that could make a powdered or droplet version of A8mod and call it Infi. That would strongly affect toughness (see Cruwear vs. CPM Cruwear, D2 vs PSF27, etc.). We don't know how to compare Infi toughness to A8mod. If Larrin had several Infi samples of the same geometry as his A8mod samples for comparison, that would be a different story. A fact is a fact

 

[Dergyll]

000Robert
Hey sorry friend, I just noticed some of your comments were directed at me. No, I don't own a belt and this knife is fairly new. I used it to cut fish and food for awhile, then touched it up on the white sharpmaker stone. It came to hair shaving REALLY QUICKLY, which now I understand why.

And no, it wasn't petrified wood lol...

 

[patrickguignot]

A Gedankenexperiment: Let's assume Busse had found a steel maker that could make a powdered or droplet version of A8mod and call it Infi.

Mmm...OK I grant you this point.
But it's far fetched, I don't think there is a secret powdered version of A8mod.
And still it's outrageous to change the steel and keep the name as is. Imagine Bark River doing this and it would be a nuclear war on BladeForums

If Larrin had several Infi samples of the same geometry as his A8mod samples for comparison, that would be a different story. A fact is a fact

I will NOT sacrifice one of my beloved Busse knives for another experiment

 

[ferider]

On a side note, I applaud you for donating your knife to the cause, Patrick. Not sure why Larrin had to destroy it though ....

 

[patrickguignot]

On a side note, I applaud you for donating your knife to the cause, Patrick. Not sure why Larrin had to destroy it though ....

I used the word "sacrifice" in the figurative sense. In fact I just donated the knife to Larrin (the handle was simply not comfortable for me) and suggested he could test the composition. I don't know if he destroyed it by testing it.

According to his Patreon post he used Optical Emission Spectroscopy (OES) to find the composition of the steel.
I found this link describing the process and after reading the following paragraph I think the knife is no more LNIB

"An excitation generator like an electric spark or arc source. A high voltage pulse ionizes the atmosphere between the tip of the counter-electrode and the sample surface, causing it to become conductive. The gap becomes low impedance, and a stable current is generated. A plasma is formed which heats the material to several thousand degrees. The material is then vaporized, atomized and ionized."