Ideal Rockwell "C" hardness

Ben Dover

Ben Dover

Joined
Aug 2, 2006
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Does anyone have a chart, or link to a chart, giving the ideal HRC number for various top end knife steels?

what would be "Ideal" or "near ideal" for cutlery useage for steels like A2, D2,M2, S90V, etc?

Thanks,
Ben

P.S. I'm interested in knife steel that will be used as knives, for cutting, not as axes or machetes, etc.
 
Most of them peak in strength, toughness and wear resistance at about their maximum hardness, so this is about 65 for A2/D2 which demands cold/oil, and 66 for M2. A2/D2 get there which low tempers and M2 with a high temper to hit the secondary hardening. S90V is a high alloy stainless steel and difficult to harden significantly past 60 HRC due to it requiring high austenization temperatures. Ref :

http://www.physics.mun.ca/~sstamp/knives/blade_materials.html

-Cliff
 
Cliff Stamp said:
Most of them peak in strength, toughness and wear resistance at about their maximum hardness, so this is about 65 for A2/D2 -Cliff
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I don't quite understand, Cliff, according to the links on your own webpage, toughness, ductility and torsional strength all peak at about 60-61 Rc for A2 and not at 65 Rc?
 
Lack of cold treatment, with a 400 F temper it gets about 64/65 HRC with cold treatment and hits one of the torsional peaks, ref. ASM Tool Steels. I have to update that section.

-Cliff
 
Thanks, Cliff.

I've been wondering about that for a long time. I'm pretty well satisfied with my A2,D2 knives (even 1095) at 59-61 HRC, but I've always wondered if putting a little more time, effort, and of course, money, into the heat treat couldn't make a great knife even better.:)

Ben
 
What does torque have to do with toughness?
 
Torsional toughness is what I think Cliff was talking about. Torsional strength could have been it though.
 
me2 said:
Torsional toughness is what I think Cliff was talking about. Torsional strength could have been it though.
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I don't think there's a difference between tosional toughness and strength. I think all of the talk about "torsional toughness" is just a misunderstanding about what this test is actually showing.
 
Torsional impact refers to a piece of metal broken under impact which causes a torsional strain, it is high strain rate loading, similar to a charpy/izod machine. See for example Culver's paper in experimental mechanics, V12,#9, 1972. Allen also has a description of the machine in his metallurgy text.

-Cliff
 
But unfortunately I think (or fear) most torsional toughness numbers cited are not torsional impact toughness, but static torsional toughness since often the technique is not cited (correct me if I am wrong, Cliff). As long as you know what you are talking about all of the above is meaningful though: torsional strength, toughness and impact toughness.

Larrin: What test are you talking about specifically? A correctly conducted static test will always give all three (main) parameters: Strength, resiliance and toughness, no matter whether you stretch, bend, or torque. An impact test will usually give only a fracture toughness.
 
HoB said:
But unfortunately I think (or fear) most torsional toughness numbers cited are not torsional impact toughness, but static torsional toughness since often the technique is not cited
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Toughness is usually used to describe high strain rate failure as opposed to strength which is low strain rate failure. However some metallurgists will use toughness to refer to strength , from "Tool Steels" by Roberts and Cary :

"Toughness can be defined as a combination of two factors:
1. Ductility: the ability to deform before breaking, which represents the
amount of plastic plus elastic deformation up to the point of failure.
2. Strength: the ability to resist permanent deformation or the elastic
strength of the material.
If only one component is to be used to descibe toughness, the second
(strength) appears more practical for tool steels, since large amounts of
flow or deformation are rarely permissible with fine tools. Strength is
largly determined by hardness. Ductility, although somewhat effected by
hardness, is largely a function of the state of stress."
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It should be obvious from the data if it is a direct impact failure or a bend until break failure because the latter talks about yield/ultimate point and ductility as HoB noted. The units are also usually different, though they are the same in the standard system for torsional data as both energy and torque are in ft.lbs.

-Cliff
 
HoB said:
Larrin: What test are you talking about specifically?
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I don't know, I just know all of the links that Cliff has given to graphs of "torsional toughness" have all looked more like a graph of strength than of toughness. Why are they so different than what is shown in Izod and Charpy testing? Supposing these graphs are a test of toughness, how do we know that torsional toughness affects the toughness of knives more than Charpy? After all, the peak toughness of torsional testing is usually one of the lowest points for charpy.
 
Larrin said:
After all, the peak toughness of torsional testing is usually one of the lowest points for charpy.
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??? Not according to the Crucible data and the graph that Cliff has on its webpage. Both peaks occure at about the same tempering temperature (yes, they might be shifted by 100 deg. but they are definitely pretty close).

Well, I don't think anyone can answer the: "what is more important" question. It's a decision that you or the knifemaker has to make. Clearly torsional toughness has an application in knife uses: All you have to do is turn the point of a knife embedded in a piece of wood, not to mention what will happen to the edge. It is obviously of extreme importance for point first batoning with subsequent splitting.

I see impact toughness (whether it is torsional or charpy) and static toughness of completely different animals testing two different things (well, maybe not completely, they are probably somewhat related). If you open a metallugy testbook they are not even treated in the same chapter. Fracture tests are decribed together with fault and crack propagation, shocks and rapid loads. While static toughness is simply the integrated area under the stress-strain curve in the ductile region. As they pertain to knives. One applies when you bend a blade (or place static loads on the edge), the other when you chop or baton etc.

How did you expect a graph of toughness with respect to tempering temperature to look like? I wouldn't really know what to expect? There is so much going on the material that I would think it is far from easy to say ab initio what properties develop with tempering temperature aside from some basic statements such as a secondary hardening and an embrittlement region that you would expect to develop.
 
HoB said:
??? Not according to the Crucible data and the graph that Cliff has on its webpage. Both peaks occure at about the same tempering temperature (yes, they might be shifted by 100 deg. but they are definitely pretty close).
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I admit my knowledge of torsional toughness is limited. The highest temper for peak torsional toughness I've seen was 325F, which is not the peak for charpy. Which graph are you referring to? Also, the peak for torsional toughness seems to have a very small tempering range, with a steep drop off if you miss it.

I'm just trying to make some sense out of this torsional stuff.

Edit: With carbon steels, impact toughness usually peaks at 400-500F, where torsional toughness is at its lowest point with 1095 and W1 (graphs on Cliff Stamp's site).
 
Larrin said:
Why are they so different than what is shown in Izod and Charpy testing?
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They are not always, the charpy/izod can show the 500F embrittlement if done carefully, see Bain's book for example.

...how do we know that torsional toughness affects the toughness of knives more than Charpy?
Click to expand...

That minimum on the torsional toughness graph is actually a known effect called 500F or temper or single step embrittlement. It is a known physical problem and caused by cementite lathing during tempering. This was discussed in detail on SwordForums some time ago where I posted up comments on this issue as I discussed it with Krauss.

-Cliff
 
If you compare the peaks directly, I see you point but if you look at the values the drop off is really not that steep. Looking at the charpy numbers from Crucible you would definitely want to aim a little higher than lower I would think. I don't know what the cold treatment will do, but at 400 F you have sacrificed 1/4 of your charpy toughness.

Nice page, Cliff!
 
One issue, as pointed out by Alvin and ASM, is that charpy values are unreliable for higher hardness levels. There is too much scatter, the machines themselves can deform, and at such high hardnesses, very small flaws can have a dramatic effect. They are probably ok for same day, same heat, machined on the same milling machine, etc, but should not be compared across the board. Straight tensile tests are unreliable for the same reasons for very high strength levels. This led to the torsional impact and torsional strength tests, as they provide a repeatable way to test very high strength/hardness metals.
 
I really hate to show my ignorance by not having the technical knowledge (YET:D ) to participate in this discussion,:eek: but I do have one more question that I would really like to ask.:)

Does anyone, other than a super expensive custom maker produce a good hunting/field/camp knife of A2, D2 or M2 properly hardened to the HRC 64-67 area?

Thanks,

Ben
 
Japanese knives are usually just below the upper hardness level in the white/blue steels. Phil Wilson would be able to make a South Fork in A2 at a reasonable price. If you go on rec.knives and make an impression on Johnston as a serious knife user he will likely just give you one in M2/1095.

-Cliff
 
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