Steel fracture toughness and edge stability, carbon content, HRC and how this translates to knife edges

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Londinium Armoury

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Jun 2, 2020
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Okay forgive the late period Pratchett-esgue title name. It's long winded but sums it up well.
Help me (and everyone interested in this) understand and really get to the bottom of this series of questions. When a lot of people talk about steel toughness for say competition choppers and big blades they are really meaning edge stability and the capability of an apex to withstand abuse without deformation, be it chips or rolls etc.
Fracture toughness in official cicles isn't really specifically talking about knife edge apex deformation, it's more to do with the steel developing fractures, and if im not mistaken this is all tested and gauged by using flat bars and other large engineering shapes, to test structural beams, maybe car parts axel systems and other things that have nothing to do with taking a piece of steel down to micron levels of accuteness, then bashing it into lumps of wood.
So when people are looking for a "tough" knife, for heavy duty chopping and such tasks, they are looking for a steel and heat treatment process that is optimal for that task. Some argue steels with medium to low carbon content (around S7) say .45-.50 carbon in the chemical composition. So this is basically a high impact shock series of steel. This then can be taken in a set range of HRC points for optimal result. INFI comes to mind when people say toughest steel. When it comes to a flat bar I think this has been proven by leaf spring car industry technology and advancements. They have obviously done lots of testing and found the perfect carbon content for a spring, which is a thick flat bar.
That doesn't hold an edge well or hold up to high demands that steels with better edge stability at increased HRC can manage. In fact the edge of an S7 blade will likely roll if it hits a really hard knot in a tree trunk, unless it has a very thick BTE geometry, which will make up for the softer "tough" steel.
The steels with higher HRC are seen as "less tough" when many of them will remain stable long after a shock steel has rolled because of it's comparably low ability to resist forces when ground thin.
So what steels are really the toughest and which HRC and carbon content is actually proven by industry standards to be superior in this specific situation? Which steel has the highest resistance to fracture and also deformations. If you were to make the worlds toughest knife, that is designed to be used in competition what would the formula for success be here?
Also additional note, does having only .45% carbon really make a steel tougher than a steel with .65% carbon? I personally think it doesn't but i'm not a trained metallurgist. But from what I'm thinking, if you heat treat the .65% steel at the same HRC as the .45% steel, the .65% carbon will produce a knife with better edge stability and apex "toughness" This scaling effect would only work up until .84% carbon then the line will be crossed into extra carbon that doesn't need to be there correct? So I'm thinking anything under .84% carbon should be just as tough in a knife as .45%.
If i'm wrong please correct me.

A Carbon content?

B HRC of steel after deciding what is the best steel choice?

C Alloying elements to help toughness are?

D Compromise of edge stability and fracture toughness, if both of those things were the end goal, what steel would you choose, how hard HRC would you run it?



Thankyou.
 
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weo said:
Have you checked out Larrin's site: https://knifesteelnerds.com/
I think you'll find the answers to most of your questions there.
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I have it bookmarked, I just have trouble finding specific data among the sea sometimes there. I can find data on pure toughness, and edge retention for HRC etc. I'm specifically looking for a duel criteria answer. which type of steel is superior at both toughness and edge stability. Not just the toughest steel, or the steel with highest edge stability. But which specific range of HRC, Carbon content, toughness and edge stability produces the optimal steel choice.
I can't really find much specifically on edge stability comparisons that don't involve abrasive cutting tests. There's lots of data there on CATRA abrasive tests using that silicon card. I'm more interested in impact toughness on the Apex into very hard dense material like hardwood.
 
I think this will help compress my rambling into a single question. If you were making a competition chopper and everything was 100% optimized, what steel would it be made out of, and what HRC would it be. for optimal edge stability and toughness.
 
Londinium Armoury said:
I think this will help compress my rambling into a single question. If you were making a competition chopper and everything was 100% optimized, what steel would it be made out of, and what HRC would it be. for optimal edge stability and toughness.
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I would go with the Carothers Competition Chopper if I needed one or could afford to just have one.
 
Richard338 said:
M4 at 63 HRC
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It's funny you say that, I made a machete with convex edge and the geometry of a glaziers hacking blade, out of M2 at 65 HRC. It was capable of hitting steel hollow pipes full force with no edge rolling or chipping. A man in Texas I believe has it now, it was great, couldn't slice a potato without wedging it, but the apex was sharp enough to shave a gnats ass. It could do way more abusive things than a any soft chopper i've played around with.
 
000Robert said:
I would go with the Carothers Competition Chopper if I needed one or could afford to just have one.
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I will have to go and have a look at that, I've never seen one of the models before. I like that you never gave specs, it gave me more motivation to go google it haha.
 

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It seems that V4E is now preferred over M4, but still around 63 HRC.
 
Londinium Armoury said:
It's funny you say that, I made a machete with convex edge and the geometry of a glaziers hacking blade, out of M2 at 65 HRC. It was capable of hitting steel hollow pipes full force with no edge rolling or chipping. A man in Texas I believe has it now, it was great, couldn't slice a potato without wedging it, but the apex was sharp enough to shave a gnats ass. It could do way more abusive things than a any soft chopper i've played around with.
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I remember of that what you call it machete , I even wrote a comment then. Can you put that video here, so that everyone can see that machete and how it performs ?
What you use to make it , it was power hacksaw as much as I remember , right ? Machete from 2.5mm M2 will break if just fill wind my friend !!
 
Natlek said:
I remember of that what you call it machete , I even wrote a comment then. Can you put that video here, so that everyone can see that machete and how it performs ?
What you use to make it , it was power hacksaw as much as I remember , right ? Machete from 2.5mm M2 will break if just fill wind my friend !!
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I think the old videos are still around, I will use snipping tool this evening and create some images of them and post pics and description. You have a good memory, yes it was made out of an "all hard" Eclipse tool company giant metal hacksaw. I used a belt grinder, and flap discs on an angle grinder while spraying it with a hosepipe to keep it cool it was pre hardened M2.
I made 2 knives out of it, one short hacking blade, and a longer wharncliffe machete.
 
Londinium Armoury said:
What HRC do you think 4V would perform best at? I've seen it as hard as 67 by custom makers if I remember right. Maybe 62-64?
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Since I've broken a Spyderco 4V tip once, I would assume it's run lower than what Spyderco does.

Check out Jo Carothers outstanding performance at the blade competition last Fri, and what she had to cut. I doubt edge retention is the top most priority.

Other than that I'm just a user of typically smaller knives, not the person to ask. Like: I Ski, but far below Olympic levels :)
 
ferider said:
Since I've broken a Spyderco 4V tip once, I would assume it's run lower than what Spyderco does.

Check out Jo Carothers outstanding performance at the blade competition last Fri, and what she had to cut. I doubt edge retention is the top most priority.

Other than that I'm just a user of typically smaller knives, not the person to ask. Like: I Ski, but far below Olympic levels :)
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I don't think edge retention is the top of their list, as far as I can tell it's edge stability, toughness, and geometry for cutting the deepest into wood as possible with each strike. Also lanyards, I remember reading the official competition rules and they were very serious about lanyard hole and quality of the lanyard, I assume so a member of the crown doesn't get a face full of heavy duty chopper.
 
I'm just going to leave this rant about terms being tossed around currently right here and let y'all chew on it. I keep hearing makers, MANY of them, say that "edge retention" isn't the top of the list, when I am 99.99% sure what they actually mean is "wear resistance" isn't the top of the list. Well, I don't know about the rest of the community, but for every single knife I make, I want "edge retention" at the top of my list (I'm not talking about handle ergo and overall profile, etc....just the blade). I'll say that one more time, I don't know about the rest of the community, but I want "edge retention" at the top of my list for every knife I make.

What is "edge retention"? I think the answer is pretty simple....the ability for a knife to hold an edge. And what factors come into play to give me the best "edge retention"? I can think of 5 primary factors, and these are in no particular order. 1. "Wear resistance" or abrasive resistance. 2. "Toughness" (the microstructure of the hardened and tempered blade is part of this variable so I'm not really going to add another factor of "microstructure".
3. Hardness 4. Geometry (I would include not only the blade and edge geometry but how it was sharpened, what angle, what grit, etc. 5. Corrosion resistance. So when all of those factors are optimized for a given application, then they all combine together and give "edge retention". One could add how the blade is used (slice cut, push cut) will affect edge retention.

Just to boil it down, in my book, I want maximum retention of the edge on my knives. That may not be the highest wear resistance out there, tho the two can very much be directly proportional.
 
samuraistuart said:
I'm just going to leave this rant about terms being tossed around currently right here and let y'all chew on it. I keep hearing makers, MANY of them, say that "edge retention" isn't the top of the list, when I am 99.99% sure what they actually mean is "wear resistance" isn't the top of the list. Well, I don't know about the rest of the community, but for every single knife I make, I want "edge retention" at the top of my list (I'm not talking about handle ergo and overall profile, etc....just the blade). I'll say that one more time, I don't know about the rest of the community, but I want "edge retention" at the top of my list for every knife I make.

What is "edge retention"? I think the answer is pretty simple....the ability for a knife to hold an edge. And what factors come into play to give me the best "edge retention"? I can think of 5 primary factors, and these are in no particular order. 1. "Wear resistance" or abrasive resistance. 2. "Toughness" (the microstructure of the hardened and tempered blade is part of this variable so I'm not really going to add another factor of "microstructure".
3. Hardness 4. Geometry (I would include not only the blade and edge geometry but how it was sharpened, what angle, what grit, etc. 5. Corrosion resistance. So when all of those factors are optimized for a given application, then they all combine together and give "edge retention". One could add how the blade is used (slice cut, push cut) will affect edge retention.

Just to boil it down, in my book, I want maximum retention of the edge on my knives. That may not be the highest wear resistance out there, tho the two can very much be directly proportional.
Click to expand...

I 100% meant wear resistance, and not specifically edge retention, you are right haha.
 
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