Ranking of Steels in Categories based on Edge Retention cutting 5/8" rope

JNewell said:
Gonna be interesting to see if the M390 Mule comes back at Rc 61. If it does, that's quite a leap in performance for a single point on the Rockwell scale, no? Or owould you expect one point to make that much difference?
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One point will usually make a pretty big difference in performance. :)

Look at S90V at 59 then at 60, S30V at 58.5 and 60.
 
The Rockwell C Hardness scale (HRC) has a theoretical max of 100. An HRC of 100 means the indenter did nothing (permanent) to the surface!

Details:
http://www.gordonengland.co.uk/hardness/rockwell.htm

I _think_ this means that as you get closer and closer to 100, each point of HRC hardness is a bigger and bigger jump. For example, going from HRC 60 to 61 is a bigger jump than going from HRC 40 to 41. I don't know if this effect is real, and/or significant in practice for the usual range of HRC measurements (HRC 20-80).

In practice, HRC is not used much above 80 because it becomes difficult to measure accurately.
http://www.gordonengland.co.uk/hardness/ehe.htm
http://www.nist.gov/manuscript-publication-search.cfm?pub_id=853006

If there are any metalurgists/material-scientists or other experts out there, comments/corrections would be really appreciated.
 
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So in other words, the HRC scale is exponential, instead of linear.

Lets compare it to the moment magnitude scale (used to measure earthquakes, no we do not use the "richter") where each level on the scale is approximately 30 times more powerful than the previous level.

That would make sense to me. It seems like the performance between a knife that is just 1-2 points different than another in the same steel is too great for the HRC to be linear. Sorry if I didn't articulate this well. My morning coffee has my mind racing. :D



Lagrangian said:
The Rockwell C Hardness scale (HRC) has a theoretical max of 100. An HRC of 100 means the indenter did nothing (permanent) to the surface!

Details:
http://www.gordonengland.co.uk/hardness/rockwell.htm

I _think_ this means that as you get closer and closer to 100, each point of HRC hardness is a bigger and bigger jump. For example, going from HRC 60 to 61 is a bigger jump than going from HRC 40 to 41. I don't know if this effect is real, and/or significant in practice for the usual range of HRC measurements (HRC 20-80).

In practice, HRC is not used much above 80 because it becomes difficult to measure accurately.
http://www.gordonengland.co.uk/hardness/ehe.htm
http://www.nist.gov/manuscript-publication-search.cfm?pub_id=853006

If there are any metalurgists/material-scientists or other experts out there, comments/corrections would be really appreciated.
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Yep, Rockwell hardness measurement is roughly logarithmic. Really sucks when a production blade's advertised hardness window is four points or more. Isn't even worth mentioning that your heat treat protocol can spit something out between 58 to 61 and still meet the standard.
 
hardheart said:
Yep, Rockwell hardness measurement is roughly logarithmic. Really sucks when a production blade's advertised hardness window is four points or more. Isn't even worth mentioning that your heat treat protocol can spit something out between 58 to 61 and still meet the standard.
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They CAN narrow that down to + or - 1 HRC if they really wanted too, even in production heat treating if they really want that kind of standard to be met.
 
Ankerson said:
They CAN narrow that down to + or - 1 HRC if they really wanted too, even in production heat treating if they really want that kind of standard to be met.
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I have bought a LOT of knives over these past couple years. The only company I have found to really be that consistent has a bug on the blade.
 
It seems that Rockwell hardness is definitely "nonlinear", but there are lots of types of non-linearity, of which exponential, or logarithmic are some examples. But there are others, such as power laws, etc. I don't know enough engineering/material-science to know which non-linearity best describes Rockwell hardness.

Some thoughts:

(1) HRC is basically how deep an indentation is made by an indenter which is pressed into the metal by a specific force. (See the links for details: http://www.gordonengland.co.uk/hardness/rockwell.htm and http://www.nist.gov/manuscript-publication-search.cfm?pub_id=853006 )

The indenter is sphero-conical in shape; it is a 120 degree (inclusive) cone with a spherical tip. The spherical tip has a radius of 0.2mm. Here is a diagram from the NIST document:
afCFl.png


The indenter is pressed into the metal with a force of 150 kilograms (kgf).

Each point of the HRC scale represents the indenter sinking in an additional 0.002mm (2 microns).

Because they wanted bigger numbers to represent harder materials, they arbitrarily subtract from 100:

HRC = 100 - h/(0.002mm)

where h is the depth of the (permanent) indentation. Here, h is measured in millimeters. (Okay, this is an over-simplification because I'm ignoring the preliminary minor load. See http://www.gordonengland.co.uk/hardness/rockwell.htm for details.)

So an HRC of 100 means that there was no permanent indentation made (ie: h=0mm). (In practice, HRC is only used up to about 80, because beyond that it's hard to measure.)

(2) Eventhough HRC is linear in the depth of the indentation, it is not linear in terms of the volume of metal displaced. This is partly because volume goes as length cubed, and partly because of the shape, namely the spherical tip and the conical base behind it. It's my understanding that for normal ranges of HRC (ie: 20-80 HRC), the indentation has gone deeper than the spherical tip. (Someone correct me if this isn't true.)

So the displaced volume of metal is basically the cube of the depth (ie: basically proportional to h^3 if you ignore the spherical tip).

(3) One might be tempted to say, then, that HRC has a non-linearity of h^3. But that's ignoring other stuff like how far the volume of metal gets displaced, and hundreds of other effects. It also does not account for h=0 means that the diamond indenter did nothing permanent to the surface!

In practice, hardness is sufficiently difficult to define, that it is defined in terms of test procedures (ie: Rockwell Hardness) rather than fundamental physics.

(4) I'm over-simplifying some things to make them easier to explain. Go see the links above for details.

(5) I'm neither an engineer nor a material-scientist. So I don't know what all this means! But I think it's fascinating.

(6) Many thanks to Ankerson for doing these knife tests, and for including the Rockwell Hardness in the results! :)

Sincerely,
--Lagrangian
 
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A lot of factors can affect the testing. surface condition, temp. de carb on the surface and small differences in the steel make up. The most important is the test block. All machines come with a test block that is supposed to be calibrated to the standard. You would think the standard is somewhere in the National Bureau of stds in Washington DC. But in fact it is with the original Rockwell, Wilson, Instrom company. If you want the true std. test block you really have to get it from them. Some of the off shore machines have test blocks of dubious pedigree. I checked one against my std Rockwell test block recently and differed by 2 pts. My machine reads dead on when the temp is 65 to 80 degrees ambient. Other than that I have to make a test and then compare to the test block after each cycle. Not a big difference but all of these things come into play. Phil
 
A little more on "non-linearity" of Rockwell C Hardness:

I don't know if we should consider Vicker's Hardness linear or not, but I very much like this graph of hardness scales from http://www.grantadesign.com/resources/materials/hardnesscharts.htm. It's a graph of Rockwell Hardness versus Vicker's Hardness.

HR...= Rockwell Hardness (many types)
HRC = Rockwell C Hardness
HSc = Shore Hardness
wbGAu.gif


So if nothing else, Rockwell Hardness is not linear in Vicker's Hardness, and for higher and higher hardness, each point of the HRC scale is a bigger and bigger jump in Vicker's Hardness.

If you are curious about Vicker's Hardness:
http://www.gordonengland.co.uk/hardness/vickers.htm
http://en.wikipedia.org/wiki/Vickers_hardness_test
 
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Ankerson,
You state in your original post that you sharpened to "30 degrees inclusive". Please forgive my ignorance but what does this mean?

Many Thanks,
Loki
 
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