Hardness vs Toughness vs Strength

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JuPaul
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Hardness vs Toughness vs Strength

Postby JuPaul » Sat Dec 28, 2019 7:52 am

With k390 becoming more common in the spyderco lineup, I feel like I've seen more discussions about this topic lately, and statements along the lines of "k390 might not be super tough, but it's strong." I'd like to talk more about this, and get some clarification on what this means. Here's my understanding:

Hardness = how wear resistant a steel is. Measured in HRC (aka Rc).

Toughness = how much force can be apllied to a steel before it breaks. Measured in ft-lbs.

Strength = how much force can be applied to a steel before it deforms (basically before you can squish it). Measured in ???

Here's what I'd like to know: How is strength measured and why don't we see that property discussed more in steel descriptions like we do hardness and toughness? It seems equally important to me for an edged tool. Are strength, hardness, and toughness generally correlated (for example, as hardness goes up, strength goes up but toughness goes down??).

Also, in reading about this, I came across the terms tensile strength vs yield strength. It seems like yield strength = strength (as we discuss steel strength here on the forum), and tensile strength = toughness (again, in reference to common steel terminology). Is that accurate?

And finally, when people discuss "edge stability" here, is that really code for a steel's strength AND toughness? I.e. it's ability to resist BOTH rolling and chipping?
- Julia

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VooDooChild
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Re: Hardness vs Toughness vs Strength

Postby VooDooChild » Sat Dec 28, 2019 8:39 am

Id like to know how things stack up for stuff that bends instead of breaks.

Also isnt hardness measured in a specific way for other materials? Im pretty sure for hardwoods it is the pressure required to push a steel ball of a certain diameter into the wood by a certain amount.

Couldnt some materials have high hardness with low wear resistance and low hardness with high wear resistance? Although for knife steels they do seem to be positively linearly correlated.

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Re: Hardness vs Toughness vs Strength

Postby Albatross » Sat Dec 28, 2019 9:19 am

VooDooChild wrote:
Sat Dec 28, 2019 8:39 am
Id like to know how things stack up for stuff that bends instead of breaks.

Also isnt hardness measured in a specific way for other materials? Im pretty sure for hardwoods it is the pressure required to push a steel ball of a certain diameter into the wood by a certain amount.

Couldnt some materials have high hardness with low wear resistance and low hardness with high wear resistance? Although for knife steels they do seem to be positively linearly correlated.
K390 at 64 hrc will out-cut Rex 45 at 67 hrc. The composition is very important, and means we can't assume hrc will mean better wear resistance. VG10 at 58 wont cut as well as at 59, assuming both heat treats were done well.
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Re: Hardness vs Toughness vs Strength

Postby The Meat man » Sat Dec 28, 2019 9:21 am

There are others here who are vastly more qualified to speak on this subject than I, but for what it is worth, here's my understanding of the matter:

HRC measures a steel's hardness. Hardness does affect wear resistance, but is only part of that equation. Wear resistance is determined primarily by the steel's carbide volume and type, which in their turn are determined by the steel's chemistry and heat treatment.

Toughness is the measurement of the amount of force applied before plastic deformation occurs. (E.g. edge rolling.) Toughness is a measure of the yield strength of the steel; in other words, the amount of flex it can take before being permanently bent.

Strength refers to a steel's ability to resist all forms of damage, be it chipping or rolling. For example, H-1 may be a supremely tough steel, but you can't whittle brass with it because it isn't a high strength steel. Conversely, while K390 at 65 HRC isn't a super "tough" steel (it can't take a lot of flex), it has a much higher edge strength than H-1 and so is able to do things that H-1 cannot. Edge strength is determined by the steel's chemistry and heat treatment, as well as the edge geometry.
Last edited by The Meat man on Sat Dec 28, 2019 11:57 am, edited 3 times in total.
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Re: Hardness vs Toughness vs Strength

Postby knivesandbooks » Sat Dec 28, 2019 9:48 am

I'm gonna wait for BBB or another steel aficionado to chime in.

BUT I do think the statement "hardness = how wear resistant a steel is" is clearly false. Hardness = the quality of being hard.
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Re: Hardness vs Toughness vs Strength

Postby acer » Sat Dec 28, 2019 10:12 am

Hardness and Toughness and Strength, well that describes me😁 just putting on my Marigolds to do the washing up 👍😂 out

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Re: Hardness vs Toughness vs Strength

Postby JuPaul » Sat Dec 28, 2019 10:39 am

knivesandbooks wrote:
Sat Dec 28, 2019 9:48 am
I'm gonna wait for BBB or another steel aficionado to chime in.

BUT I do think the statement "hardness = how wear resistant a steel is" is clearly false. Hardness = the quality of being hard.
Well, I would've assumed that, but while reading about it earlier, I found several metallurgy sites that defined hardness as "a material's ability to hold together when friction is applied." In other words, wear resistance.

Edit: when I think of hardness in laymans terms, I think "how much pressure can be applied before something will give to that pressure- bend, distort, squish, etc." So diamonds are harder than wood, which is harder than playdoh. But I think that's actually defined as "strength" in metallurgy. But maybe I have it all wrong, which is why I asked the question here!
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Re: Hardness vs Toughness vs Strength

Postby curlyhairedboy » Sat Dec 28, 2019 11:00 am

As a materials engineer, I'd rather not duplicate work. The wonderful Larrin (metallurgist and the author of Knife Steel Nerds) has several articles that should help clear stuff up. This is a good one to start with:

https://knifesteelnerds.com/2019/08/05/ ... -not-fail/
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Re: Hardness vs Toughness vs Strength

Postby JuPaul » Sat Dec 28, 2019 11:11 am

curlyhairedboy wrote:
Sat Dec 28, 2019 11:00 am
As a materials engineer, I'd rather not duplicate work. The wonderful Larrin (metallurgist and the author of Knife Steel Nerds) has several articles that should help clear stuff up. This is a good one to start with:

https://knifesteelnerds.com/2019/08/05/ ... -not-fail/
I was hoping Larrin would chime in here, but one of his articles is great, too! Thanks for the direction.
- Julia

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Re: Hardness vs Toughness vs Strength

Postby ekastanis » Sat Dec 28, 2019 12:00 pm

Not a simple question and an online forum almost certainly will not give you a complete understanding of the topic, but a simple example of axial loading of a steel rod can help explain some of these concepts.

Some terminology:
  • Stress: force normalized by the cross-sectional area over which the force is applied. Stress has units of force per unit area (a.k.a. pressure, e.g. psi, MPa, etc.)
  • Strain: elongation normalized by the length of the sample. Strain is unitless (e.g. in/in or mm/mm)
A typical stress-strain curve for a tensile test:

Image

Strength of a material typically refers to yield strength, the stress at which plastic deformation begins, or ultimate strength, the maximum stress reached in the plastic region before fracture. Fracture strength is the stress in the material at the point of fracture, but is typically not the quantity of interest in engineering problems. Strength is measured in stress units.

Toughness of a material refers to the area under the stress-strain curve, how much energy it absorbs before fracture. It is measured in units of stress times strain, i.e. energy, usually normalized be the volume of the sample. (i.e. ft-lbf/in3, J/m3, etc.)

Hardness is not represented by the stress-strain curve. It is a measure of surface resistance to plastic deformation by indentation or scratching. It is not a bulk material property like strength or toughness. Hardness is determined empirically and each type of hardness test has its own scale (e.g. Rockwell, Brinell, Mohs, etc.). It is possible to some extent to relate via empirical correlations measurements from different types of hardness tests, and to a lesser extent to the strength of the material.

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Re: Hardness vs Toughness vs Strength

Postby Phil Wilson » Sat Dec 28, 2019 12:08 pm

Here is an article I wrote some time ago and it does address some of Ju Paul topics above. We have many more blade materials now but the general concepts are here. Larrin has also written a lot more about all of this and his articles are more up to date.

http://www.seamountknifeworks.com/js/we ... Blades.pdf

Phil

JuPaul
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Re: Hardness vs Toughness vs Strength

Postby JuPaul » Sat Dec 28, 2019 12:53 pm

Thank you all for the info and the links...seems I have some reading to do. My dad was a polymer chemist by profession, and a hobby mechanic, machinist, and welder, and I desperately miss being able to ask him these questions in person. So your help is greatly appreciated!
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Re: Hardness vs Toughness vs Strength

Postby Xplorer » Sat Dec 28, 2019 12:57 pm

JuPaul wrote:
Sat Dec 28, 2019 10:39 am

..Well, I would've assumed that, but while reading about it earlier, I found several metallurgy sites that defined hardness as "a material's ability to hold together when friction is applied." In other words, wear resistance.

Edit: when I think of hardness in laymans terms, I think "how much pressure can be applied before something will give to that pressure- bend, distort, squish, etc." So diamonds are harder than wood, which is harder than playdoh. But I think that's actually defined as "strength" in metallurgy. But maybe I have it all wrong, which is why I asked the question here!
Hardness and wear resistance are closely related but not entirely tied together :) . Hardness is just hardness and it's measured by pressing a diamond into the surface just as one would press steel into wood. It is true that wear resistance increases with increased hardness, but HRC is not the only factor affecting wear resistance. Two different steels at the same HRC will have different wear resistances based on their differing compositions.

Toughness is how much a steel can be stressed before reaching the fracture point. In knife discussions this could be described in the context of bending/flexing forces or it could be described in the context of shock/force, such as pounding or spine whacking.

Strength (in knife discussions) is typically referring to how much force can be applied before it will take a permanent bend, roll or other deformation.

For example, a steel that takes a permanent bend easily would be very tough and not very strong. A steel that is very hard to bend and then suddenly snaps in half would be strong and not very tough. A steel that is very hard to flex but then takes a set instead of breaking would be both strong and tough. The second and third examples would likely both be hard as well. :) I hope this helps

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Re: Hardness vs Toughness vs Strength

Postby JD Spydo » Sat Dec 28, 2019 2:49 pm

knivesandbooks wrote:
Sat Dec 28, 2019 9:48 am
I'm gonna wait for BBB or another steel aficionado to chime in.

BUT I do think the statement "hardness = how wear resistant a steel is" is clearly false. Hardness = the quality of being hard.
Yeah I want to see what Larrin might have to say about these properties of blade steels. The OP does make some good observations and there are so many factors that make up a good blade steel IMO.

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Re: Hardness vs Toughness vs Strength

Postby bdblue » Sat Dec 28, 2019 9:59 pm

ekastanis wrote:
Sat Dec 28, 2019 12:00 pm
Not a simple question and an online forum almost certainly will not give you a complete understanding of the topic, but a simple example of axial loading of a steel rod can help explain some of these concepts.

Some terminology:
  • Stress: force normalized by the cross-sectional area over which the force is applied. Stress has units of force per unit area (a.k.a. pressure, e.g. psi, MPa, etc.)
  • Strain: elongation normalized by the length of the sample. Strain is unitless (e.g. in/in or mm/mm)
A typical stress-strain curve for a tensile test:

Image

Strength of a material typically refers to yield strength, the stress at which plastic deformation begins, or ultimate strength, the maximum stress reached in the plastic region before fracture. Fracture strength is the stress in the material at the point of fracture, but is typically not the quantity of interest in engineering problems. Strength is measured in stress units.

Toughness of a material refers to the area under the stress-strain curve, how much energy it absorbs before fracture. It is measured in units of stress times strain, i.e. energy, usually normalized be the volume of the sample. (i.e. ft-lbf/in3, J/m3, etc.)

Hardness is not represented by the stress-strain curve. It is a measure of surface resistance to plastic deformation by indentation or scratching. It is not a bulk material property like strength or toughness. Hardness is determined empirically and each type of hardness test has its own scale (e.g. Rockwell, Brinell, Mohs, etc.). It is possible to some extent to relate via empirical correlations measurements from different types of hardness tests, and to a lesser extent to the strength of the material.
Since hardness is a measure of resistance to plastic deformation I suggest that hardness is related to the yield point on the stress strain curve, IOW hardness is somewhat related to yield strength.

Now toughness is the area under the stress strain curve so it is related to ductility of the steel- how much the steel can deform beyond initial yield before it reaches fracture point.

Another steel quality that is hard to measure is notch sensitivity. Some steels will fracture easier than others when stress concentrations are introduced such as notches, holes, etc. Fatigue strength is somewhat related to this as well- how a material reacts to stress fluctuations that are lower than its strength. Knife design frequently involves shapes that compromise strength with notches, sharp corners, changes in profile.

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Re: Hardness vs Toughness vs Strength

Postby Larrin » Sat Dec 28, 2019 10:17 pm

Hardness is a measure of strength. In a knife this relates to edge rolling or a knife bending and taking a “set.” Hardness correlates most strongly with ultimate strength such as in a compression or tensile test. Tool steels often have lower tensile strength than compression strength because they are relatively brittle. Yield strength usually correlates with ultimate strength though there can be times when the yield strength can be low relative to the ultimate strength. https://knifesteelnerds.com/2018/11/12/ ... -hardness/

Toughness has many definitions but more generically is the resistance to fracture. Such as an edge chipping or a blade breaking. An impact test is in ft-lbs or joules which is not force but energy. There are many types of toughness tests and there’s not enough space here to talk about what they all mean. https://knifesteelnerds.com/2018/05/28/ ... -of-edges/

Edge stability is a bit harder to nail down a definition for, it has been used almost as a catch-all term. I prefer to define it as resistance of an edge to rolling and chipping. That is controlled by edge geometry, strength, and toughness. https://knifesteelnerds.com/2018/08/27/ ... stability/

Hardness alone does not control wear resistance. Carbides are extremely important. 67 Rc 1095 is much less wear resistant than 58 Rc K390. https://knifesteelnerds.com/2018/11/19/ ... retention/
http://www.KnifeSteelNerds.com - Steel Metallurgy topics related to knives

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Re: Hardness vs Toughness vs Strength

Postby JuPaul » Sat Dec 28, 2019 11:36 pm

Larrin wrote:
Sat Dec 28, 2019 10:17 pm
Hardness is a measure of strength. In a knife this relates to edge rolling or a knife bending and taking a “set.” Hardness correlates most strongly with ultimate strength such as in a compression or tensile test. Tool steels often have lower tensile strength than compression strength because they are relatively brittle. Yield strength usually correlates with ultimate strength though there can be times when the yield strength can be low relative to the ultimate strength. https://knifesteelnerds.com/2018/11/12/ ... -hardness/

Toughness has many definitions but more generically is the resistance to fracture. Such as an edge chipping or a blade breaking. An impact test is in ft-lbs or joules which is not force but energy. There are many types of toughness tests and there’s not enough space here to talk about what they all mean. https://knifesteelnerds.com/2018/05/28/ ... -of-edges/

Edge stability is a bit harder to nail down a definition for, it has been used almost as a catch-all term. I prefer to define it as resistance of an edge to rolling and chipping. That is controlled by edge geometry, strength, and toughness. https://knifesteelnerds.com/2018/08/27/ ... stability/

Hardness alone does not control wear resistance. Carbides are extremely important. 67 Rc 1095 is much less wear resistant than 58 Rc K390. https://knifesteelnerds.com/2018/11/19/ ... retention/
Thank you, Larrin. I spent a good chunk of time on this lazy Saturday reading your article that was linked above, then reading your other articles embedded in that one - truly a wealth of information. I'm learning that overall edge stability, as you define it, is probably more important to me than toughness alone as I previously assumed. The awesome thing about spyderco is that they allow us to try so many different steels in numerous blade shapes, so a person can really find her sweet spot in terms of edge stability vs ease in sharpening vs corrosion resistance.

As an aside, I very much appreciate that you sometimes use feminine pronouns in your articles - thanks for not assuming that all end users and readers will be male!
- Julia

"Be excellent to each other." - Bill S. Preston, Esq.


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