Visualizing the Tradeoff of Higher Hardness

Synov

Synov

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Many thanks to Larrin Larrin for doing all the hard work of testing steels and providing this data to the public. I just wanted to compile some of it in a different way.

This graph shows the effect of hardness on the toughness and edge retention of various steels:

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(The numbers next to each steel are hardness values.)

Increasing hardness decreases toughness and increases edge retention as we all know. Larrin has shown that edge retention increases linearly with hardness, and at the same rate for most steels. This allowed me to predict edge retention where data was missing to produce this graph.

Toughness, on the other hand, does not show the same simple relationship with hardness. Tougher steels lose toughness very rapidly as you increase hardness without much increase in edge retention, while less tough steels benefit more from increased hardness. So making high-toughness steels harder seems to be a bad idea.

Some interesting things to note: Magnacut gets a good tradeoff at 62 RC and then falls off as you go harder. CPM-154 shows a better tradeoff than other steels near it. These may simply be the result of uncertainty in the data.

Again, thanks Larrin for giving us this data. I hope some of you find this useful in making decisions about steel and hardness.
 
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Fun chart! I agree changing the steel (or more specifically the carbide type and volume) matters more than hardness for edge wear. I would also like to see the chart with toughness on a log scale instead. I think it is a better representation of the behavior in some ways than a linear scale.
 
Larrin said:
Fun chart! I agree changing the steel (or more specifically the carbide type and volume) matters more than hardness for edge wear. I would also like to see the chart with toughness on a log scale instead. I think it is a better representation of the behavior in some ways than a linear scale.
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I tried it with log scale, it just gave everything more similar slopes, showing that toughness decreases logarithmically rather than linearly.
 
Thank you for the chart.

I didn't think the drop-off in toughness was that steep when hardness was increased, especially when the increase in abrasive wear is so small.

I've been wanting to try MagnaCut steel in an Inkosi, but Chris Reeve runs MagnaCut at 63-64 Rc. This chart makes me want it at 60-61 Rc.

The TCC cut numbers account only for abrasive wear. But when you lose toughness (resistance to chipping, breaking and cracking), you also lose edge stability, although you gain a bit of strength (resistance to denting, rolling and bending) with hardness.

Actual edge wear in the real world considers toughness, strength and abrasive resistance. Thanks to Larrin, we have good abrasive wear numbers, but we still don't have a good sense of edge wear.
 
Twindog said:
Actual edge wear in the real world considers toughness, strength and abrasive resistance. Thanks to Larrin, we have good abrasive wear numbers, but we still don't have a good sense of edge wear.
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I think you mean that we have good data on edge wear but not necessarily “edge retention” in a broader sense. It gets very complicated with different edge geometries and forces applied. Some cutting is all wear, some leans more toward strength, some toward toughness. For a given edge geometry and cutting task you can observe how the edge lost sharpness and attempt to improve it. So if microchipping is observed then switch to a tougher steel or heat treat to a lower hardness. That is ignoring the possibility of using a more obtuse angle. So in some ways I think it is beneficial to have separate measures for strength (hardness), chipping (toughness), and wear (CATRA). You can also throw corrosion in there. Tests with a combination of factors are certainly also useful but sometimes can be misleading for other types of uses and can be difficult to figure out how all the different properties are adding up to the result. Always more tests that can be done. Metallurgists and materials engineers will often isolate various properties for comparison: tensile testing, hardness, wear, corrosion, impact toughness, fracture toughness, etc.
 
It seems like the gains in edge retention over a certain hardness are pretty minimal in most of these steels, except S60V & 10V.

It makes Cruwear, M4, and especially Magnacut seem really impressive since you can get similar performance at 61 as you do at 64/65. Just the toughness is waaaay better.

And everyone says Magnacut needs to be over 62 or it’s no good 😂
 
tony281sc2 said:
It seems like the gains in edge retention over a certain hardness are pretty minimal in most of these steels, except S60V & 10V.

It makes Cruwear, M4, and especially Magnacut seem really impressive since you can get similar performance at 61 as you do at 64/65. Just the toughness is waaaay better.

And everyone says Magnacut needs to be over 62 or it’s no good 😂
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I don't think that "everyone" says that, although there are probably benefits in edge stability. Of course, I don't even play a metallurgist on BF...so what do I know?
 
Larrin said:
MagnaCut was up and to the right from the prior stainless steels but that’s about as far as I could get for now.
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Would it make sense to have a steel that had that extreme toughness & edge retention but non stainless. And make up for the corrosion aspect by coating it in a very high quality DLC coating? Or something similar to that.
 
Blues said:
I don't think that "everyone" says that, although there are probably benefits in edge stability. Of course, I don't even play a metallurgist on BF...so what do I know?
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As I understand it, edge stability is toughness + strength, although those are two different qualities. The higher the toughness, the better the edge is able to resist breaks and chipping. The higher the strength, the better the edge is able to resist dents, rolling, blunting and bends.

So at 60-61 Rc, MagnaCut has about 17 ft-lbs of toughness. But if you jack up the hardness to 63-64 Rc for a Chris Reeve heat treat, toughness drops to maybe 10-11 ft-lbs of toughness.

So one aspect of edge stability goes (strength) up, while the other (toughness) goes down. If you need more toughness than strength in an edge, edge stability goes down. If you need more strength, edge stability goes up.

To complicate things further, I think Larrin is calling "edge retention" the combination of edge stability and abrasive wear.

We tend to use CATRA TCC cuts on abrasive cards as a proxy for how long an edge will hold up. But those CATRA tests measure only edge abrasion. Those tests do not challenge the edge on toughness or strength. In the real world, the edge degrades not just from abrasion, but also from microchipping (lack of toughness) and rolling or denting (lack of strength).

European knife maker Haaksonsen likes to use A8 steel, despite its low abrasion resistance, because in the real world abrasion resistance is less important to edge retention than toughness.

"Toughness is a property often overlooked in knife steels, often on behalf of corrosion and wear resistance. The thing is that a sharp edge with a decent edge angle (<35° total angle) needs toughness to avoid breakage of the edge/chipping, especially when working with hard materials like wood. Because of this, edge retention of this steel will be better than higher alloyed steels and even powder steels for many purposes."

By jacking up the hardness of MagnaCut so far, I think Chris Reeve has sacrificed too much toughness, giving its knives a bit more wear resistance at the much larger expense of edge stability and total edge retention.
 
I think it's appropriate to use abrasive edge wear as a quantifiable proxy for edge retention, under the assumption that you are using the knife properly for what it is designed to do.
 
T Twindog

It should be easy enough for that to be determined. I wonder if any testing was performed by CRK with lower HRC on some test blades. They may have addressed this previously, but I admit I don't keep up with all the data.
 
DrBC said:
Would it make sense to have a steel that had that extreme toughness & edge retention but non stainless. And make up for the corrosion aspect by coating it in a very high quality DLC coating? Or something similar to that.
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MagnaCut essentially uses the same microstructure (and thus has similar properties to) the best non-stainless powder metallurgy steels. The primary advantage to the non-stainless versions is the lack of chromium in solution lets them achieve 2-3 points more in hardness. However even those tool steels are typically used below their absolute achievable hardness such as in the 59-62 Rc range.
 
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