Ductility, strength, toughness, flexing and bending

[91bravo]

I'm no metallurgist, so who knows? I think they are all relative, just different terminology to describe similar actions when certain steels are exerted to different strengths.

I just know that in my fixed blades, I would rather have an edge that will roll or dent, than to chip out. Most of the time, it's a carbon steel, that can be easily brought back to sharp in the field. On my EDC folders for work, I prefer something that will hold a working edge as long as it can, and most of them are a stainless of some flavor. Hence, when I get chipping, it's usually in my folders, as I take them down pretty thin for enhanced cutting performance.

 

[danbot]

Twin: okay, great.

91bravo: that's deforming without chipping or cracking, right?
If ductility is the ability to deform under tensile strength, what is the ability to deform under compression and shear strength?

Thanks guys,

Bo

The ability to deform under compression is called malleability

 

[JacksonKnives]

Once we actually start applying the terms to edge and defining "edge stability," (and leaving aside contributing variables from wear-resistant carbides for a few minutes) things get a little tricky.

For example, you ask "would you rather have a more ductile=tougher edge or a harder=stronger edge?"

If I'm a butcher who swipes his knife on a steel every twenty minutes, I guess I'd go for ductility. I don't care if the edge rolls; swipe-swipe-swipe, good enough for another pig. Don't want chips, those will slow me down, but 54HRC and a wobbly edge is good enough to cleave tendon from bone.

For nearly every other use for a knife in my experience and from my understanding of the metallurgy, edge geometry has far more to do with results than the hardness/ductility tradeoff.

If I'm using an edge as a razor, it doesn't matter whether the blade chips or cracks, if anything happens to the edge I have to fix it or it'll give a bad shave. If it takes more bad accidental abuse to chip my edge compared to a ductile edge that would have rolled with less abuse, so much the better. Sure, you can shave with a rolled-then-stropped edge... but why? Just make the edge harder so it can take more force and be careful with the razor!

If I'm making a splitting axe, I want it to survive and not crack from stress... but that's not a function of ductility, that's all about how carefully the steel was worked, right? The amount of abuse an axe can survive has more to do with geometry and little to do with hardness-vs-ductility. Design an edge that can support the kind of chopping you want to do, period, and run the steel as hard as you can (safely) to make it stronger. If it chips, you're not chopping with it carefully and you would have rolled the edge badly anyway.

Am I wrong about stress and hardness in a splitting tool? (I never thought to check on hardness of axes growing up; I definitely hated sharpening out rolled edges just as much as chips, but in either case it was my own damn fault, LOL. I'd guess they were made soft so you could sharpen them with a file.)

A "bushcraft" or "camp" knife nowadays is trying to meet a strange mix of these use cases: people want them thin enough to shave/carve with, stiff and heavy enough to split with, and easy to sharpen in the field.

That last quality, sharpenability, is one that doesn't often come up in metallurgy tough-vs-hard discussions, but is it the real tradeoff with hardness? In an era of fancy abrasives and super-carbides I don't think twice about asking for/making the hardest carbon steel blades I can get (corrosion is the bigger boogeyman), but is that a long-ago tradeoff consideration that's still lurking behind these discussions?

 

[danbot]

Once we actually start applying the terms to edge and defining "edge stability," (and leaving aside contributing variables from wear-resistant carbides for a few minutes) things get a little tricky.

For example, you ask "would you rather have a more ductile=tougher edge or a harder=stronger edge?"

If I'm a butcher who swipes his knife on a steel every twenty minutes, I guess I'd go for ductility. I don't care if the edge rolls; swipe-swipe-swipe, good enough for another pig. Don't want chips, those will slow me down, but 54HRC and a wobbly edge is good enough to cleave tendon from bone.

For nearly every other use for a knife in my experience and from my understanding of the metallurgy, edge geometry has far more to do with results than the hardness/ductility tradeoff.

If I'm using an edge as a razor, it doesn't matter whether the blade chips or cracks, if anything happens to the edge I have to fix it or it'll give a bad shave. If it takes more bad accidental abuse to chip my edge compared to a ductile edge that would have rolled with less abuse, so much the better. Sure, you can shave with a rolled-then-stropped edge... but why? Just make the edge harder so it can take more force and be careful with the razor!

If I'm making a splitting axe, I want it to survive and not crack from stress... but that's not a function of ductility, that's all about how carefully the steel was worked, right? The amount of abuse an axe can survive has more to do with geometry and little to do with hardness-vs-ductility. Design an edge that can support the kind of chopping you want to do, period, and run the steel as hard as you can (safely) to make it stronger. If it chips, you're not chopping with it carefully and you would have rolled the edge badly anyway.

Am I wrong about stress and hardness in a splitting tool? (I never thought to check on hardness of axes growing up; I definitely hated sharpening out rolled edges just as much as chips, but in either case it was my own damn fault, LOL. I'd guess they were made soft so you could sharpen them with a file.)

A "bushcraft" or "camp" knife nowadays is trying to meet a strange mix of these use cases: people want them thin enough to shave/carve with, stiff and heavy enough to split with, and easy to sharpen in the field.

That last quality, sharpenability, is one that doesn't often come up in metallurgy tough-vs-hard discussions, but is it the real tradeoff with hardness? In an era of fancy abrasives and super-carbides I don't think twice about asking for/making the hardest carbon steel blades I can get (corrosion is the bigger boogeyman), but is that a long-ago tradeoff consideration that's still lurking behind these discussions?

This is why there is no "best" knife. Every tool needs to be tailored to its intended use. There is always a trade off or sacrifice that will be unexceptable when chasing a "do everything" knife.*shrug*

 

[JacksonKnives]

This is why there is no "best" knife. Every tool needs to be tailored to its intended use. There is always a trade off or sacrifice that will be unexceptable when chasing a "do everything" knife.*shrug*

What do you think about ductility considerations if we assume a to-purpose design with ideal geometry, though? (E.g. your machete won't be able to slice tomatoes very well, but the edge will survive a day in the brush without chipping *or* rolling.)

I'm mostly using kitchen knives, folders and razors these days, but I'm really not sure that I'd ever really take a rolled edge over a chipped edge. (I *don't* use a steel, I use a strop, but that speaks to the nature of my collection and my preference for polished edges.)

 

[danbot]

What do you think about ductility considerations if we assume a to-purpose design with ideal geometry, though? (E.g. your machete won't be able to slice tomatoes very well, but the edge will survive a day in the brush without chipping *or* rolling.)

I'm mostly using kitchen knives, folders and razors these days, but I'm really not sure that I'd ever really take a rolled edge over a chipped edge. (I *don't* use a steel, I use a strop, but that speaks to the nature of my collection and my preference for polished edges.)

I think that very hard and polished edges are excellent in the kitchen and on your face. A rolled edge is going to scrape up your strop. Not nice! In kitchen prep work there is little risk of putting too much lateral pressure on the edge to cause chipping. But for butchering I think a softer steel that can be easily "steeled" and easily resharpened is your friend! When you hit bone, a rolled edge is better than a chipped edge.
Edit: the desired flexibility in the edge of a straight razor comes mostly from its "thinness" so a softer steel really isn't a concern IMO.