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.VooDooChild wrote: ↑Sat Dec 28, 2019 8:39 amId 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.
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.
I was hoping Larrin would chime in here, but one of his articles is great, too! Thanks for the direction.curlyhairedboy wrote: ↑Sat Dec 28, 2019 11:00 amAs 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/
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.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!
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.
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.ekastanis wrote: ↑Sat Dec 28, 2019 12:00 pmNot 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.
- 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.)
A typical stress-strain curve for a tensile test:
- Strain: elongation normalized by the length of the sample. Strain is unitless (e.g. in/in or mm/mm)
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.
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.Larrin wrote: ↑Sat Dec 28, 2019 10:17 pmHardness 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/