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mainaman
01-25-2012, 01:45 PM
Guys,
I am trying to remember how HRC scale scales? I remember it was not linear, but can't seem to able to dig any threads that mention what it was.
Thanks

ajhuff
01-25-2012, 03:48 PM
Not sure what you mean . Rockwell is a depth on penetration hardness test. On the C scale it is related to the depth of penetration of a hardened cone with a 150 kg load. The less penetration, the greater the hardness. As far as I remember it is linear within the scale.

-AJ

Eamon Burke
01-25-2012, 03:50 PM
Like how the test is performed, or what the typical numbers for steel are?

WildBoar
01-25-2012, 04:08 PM
Found this description:

http://www.indentec.com/Resources/Reference_Rockwell_Test.pdf (http://www.indentec.com/Resources/Reference_Rockwell_Test.pdf)

mainaman
01-25-2012, 04:32 PM
What I wanted to find out is what is the difference between two HRC values, for example 55 and 58? If I remember correctly it is not linear, I remember people posting it was either exponential increase or log increase, I am not sure if this was even correct. I was not able to find info on that online.

Thanks

Larrin
01-25-2012, 04:35 PM
It can be difficult to compare different measures of strength. You can get approximations though.

Here's Rockwell vs Vickers hardness: http://www.gordonengland.co.uk/hardness/rockwell_c_conv.htm

Vickers varies fairly linearly with tensile strength: http://www.gordonengland.co.uk/hardness/brinell_conversion_chart.htm

Also here's a conversion table: http://www.gordonengland.co.uk/hardness/hardness_conversion_2c.htm

mainaman
01-25-2012, 04:38 PM
Thanks guys

ajhuff
01-25-2012, 04:39 PM
55 to 58 hrc is a linear increase.

-AJ

Larrin
01-25-2012, 04:40 PM
Here's a graph I made based on the conversion chart: http://img96.imageshack.us/img96/3527/hrcvstensile.th.jpg (http://imageshack.us/photo/my-images/96/hrcvstensile.jpg/)

Uploaded with ImageShack.us (http://imageshack.us)

tk59
01-25-2012, 04:49 PM
What I wanted to find out is what is the difference between two HRC values, for example 55 and 58? If I remember correctly it is not linear, I remember people posting it was either exponential increase or log increase, I am not sure if this was even correct. I was not able to find info on that online.

Thanks
If I'm understanding correctly, to get a very hard rating, a material has to resist a large amount of force spread over a small area. As the depth of the penetration increases, the area over which the force is spread increases dramatically. In this sense, the rating is not linear with respect to pressure (force per unit area). It would seem that a 55-56 difference is much smaller than a 61-62 difference. I'm sure it's more complex than that but this reduction seems pretty intuitive.

mainaman
01-25-2012, 04:49 PM
Here's a graph I made based on the conversion chart: http://img96.imageshack.us/img96/3527/hrcvstensile.th.jpg (http://imageshack.us/photo/my-images/96/hrcvstensile.jpg/)

Uploaded with ImageShack.us (http://imageshack.us)Ok so not a linear behavior, I guess that is what I have seen in posts before but not explained in terms of tensile strength to HRC relation.

ajhuff
01-25-2012, 08:05 PM
No, it is linear. You just need to draw a best fit line.

-AJ

mainaman
01-25-2012, 08:15 PM
No, it is linear. You just need to draw a best fit line.

-AJ
I am looking at Larrin's plot and the data suggest that the tensile strength change is not linear with HRC increase.

WildBoar
01-25-2012, 09:03 PM
^^ x2. It's parabolic (or maybe even a higher order). But in the 55-60 range, you could interpret linearly and be pretty close.

Andrew H
01-25-2012, 09:27 PM
No, it is linear. You just need to draw a best fit line.

-AJ

Wouldn't a best fit line (if you are actually saying it is a line not a curve) make any function "look" linear?


^^ x2. It's parabolic (or maybe even a higher order). But in the 55-60 range, you could interpret linearly and be pretty close.

Yeah, it looks parabolic to me.

mainaman
01-25-2012, 09:37 PM
^^ x2. It's parabolic (or maybe even a higher order). But in the 55-60 range, you could interpret linearly and be pretty close.
true my interest was for HRC above 60 actually, and in relation to razors. Usually German Carbons are tempered ~61 HRC, Japanese straights are harder some going up to 65-66. I wanted to find out how the tensile strength changes with hardness.

ajhuff
01-25-2012, 10:11 PM
Wouldn't a best fit line (if you are actually saying it is a line not a curve) make any function "look" linear?



Yeah, it looks parabolic to me.

That graph is based on averages and conversions. Hardness has a linear relationship with strength. H = 3x Yield Strength.

I'm not a big fan of Rockwell C as it is more of a micro-hardness test than a macro-hardness test because of using a brale tip. But it is the industry standard of sorts and for measuring the harness of thin strip steel it is a better application than say Brinell.

-AJ

Larrin
01-25-2012, 10:29 PM
That graph is based on averages and conversions. Hardness has a linear relationship with strength. H = 3x Yield Strength.
.

-AJ
But that equation isn't for Rockwell hardness. Rockwell testing isn't perfect, especially in its upper and lower range.

ajhuff
01-25-2012, 11:39 PM
True, but still, the general relationship between hardness and strength is linear.

Also, HRc = 100-(depth of penetration/0.002mm) which means the scale itself is linear.

I'm wondering if I don't understand the question?

-AJ

David Metzger
01-26-2012, 07:09 AM
I couldn't find the reference, so I forgot if it was reliable and if it applied to a particular test steel, but I read once that a 2 pt RC increase relates to about 20% longer edge retention. But don't quote me, I don't know if this was in Verhoeven (reliable) or just internet crap.

banjo1071
01-26-2012, 07:52 AM
Hi everyone

The HRC ist not a measurment for the edgeretention, that is a common misconception. It is a measurment for the hardness of the steel. The edgeretention is a function of the resistance against abraision. For instance: if you harden a VG10 to HRC67, would you think the blade would last for long? No, because the edge would crumble away like old bread. Another example: if you compare a c60 (german standard steel) against lets say a D2 (both hardend at HRC60) you will find the second with a much much better edgeretention. Even tough they have the same HRC...

To answer the initial question: The equasion is linear (the deeper the cone, the lower the HRC). But, since a cone is used, the force needed to sink the cone deeper in the steel, will not be linear. Meaning HRC61 is much harder than HRC60. Not only 1.6%, as a linear function would imply..But then, that ist not really important, because the HRC does not stand in a direct relation to the edgeretention....

Greets
Benjamin

P.S. please excuse my bad english..

banjo1071
01-26-2012, 08:18 AM
only my personal 50cent, of course....

ajhuff
01-26-2012, 10:22 AM
OK, but in hardness testing the load applied is constant, not variable.

-AJ

banjo1071
01-26-2012, 10:46 AM
Yeah, absolutly. But, to my meager understandig of physics, the diameter of the cone is not constant, but increases. And with increasing diameter, more "force" is needed to sink the cone deeper into the steel. Or the other way round: with a constant load, the cone with small diameter will sink deeper into to material, than a cone with an bigger diameter. That means, that with increasing diameter you get less "sinking" per load. And according to the HRC-formular less sinking means more hardness. Thus, to my understanding, while the scale is linear, the actual hardness is not..

Greets
B

mainaman
01-26-2012, 10:51 AM
OK, but in hardness testing the load applied is constant, not variable.

-AJ
yes but as tk mentioned earlier as the cone gets deeper in the metal the cross section of the area on which the force is applied does not increase linearly it increases as r^2, so to get same pressure fro larger diameter one has to apply larger force.

DanB
01-26-2012, 11:59 AM
Thanks for this. Could you say more about this, namely the relationship between hardness and edge retention?
I ask because I've been stalking this board for a few months as I contemplate my own knife upgrades, and I see a pattern of ppl recommending knives with high HRC grades, as if this automatically translated into better edge retention. Some studies I've seen confound this equation.
So how is resistance against abrasion achieved with a knife? By the heat treatment? And if so, you just have to know the reputation of the maker?
Sorry if these questions seem dumb. Still a little new to all this.

tk59
01-26-2012, 12:14 PM
Thanks for this. Could you say more about this, namely the relationship between hardness and edge retention?
I ask because I've been stalking this board for a few months as I contemplate my own knife upgrades, and I see a pattern of ppl recommending knives with high HRC grades, as if this automatically translated into better edge retention. Some studies I've seen confound this equation.
So how is resistance against abrasion achieved with a knife? By the heat treatment? And if so, you just have to know the reputation of the maker?
Sorry if these questions seem dumb. Still a little new to all this.

Edge retention is a function of several factors mostly relating to the size and quantity of carbides in the steel. Higher hardness mainly prevents the edge from deforming (rolling, etc) so it would be a factor in allowing you to cut in steeper bevels (sharper edge). To much hardness leads to brittleness. In the end you want a nice balance. Japanese knives lean toward higher hardness.

Larrin
01-26-2012, 12:58 PM
The percentage increase in edge retention idea came from Wayne Goddard using rope cutting with hunting knives. The stopping point for his testing was force (he cut on a scale), so hardness is fairly important in the test.

ajhuff
01-26-2012, 01:53 PM
yes but as tk mentioned earlier as the cone gets deeper in the metal the cross section of the area on which the force is applied does not increase linearly it increases as r^2, so to get same pressure fro larger diameter one has to apply larger force.

I do understand what you guys are saying that to push the indenter into a harder material to the same depth requires more force because the area of the cone increases with depth. But I don't follow where this increase in force is coming from. The test is not predicated on equal depths of indentation. That would be a different kind of test.

You are almost describing the Brinell test where instead of depth of indentation we measure the area or rather diameter of the indentation but that is based on a constant load being applied at constant time not constant depth.

Sorry, I really am confused with this increasing force aspect that keeps getting referred to since there is no increasing force in the test. I have not seen hardness data that was not linear for a given alloy, other factors, yes, such as hardness vs carbon content in steel, but not hardness vs. strength or within the scale itself.

-AJ

DanB
01-26-2012, 02:02 PM
Edge retention is a function of several factors mostly relating to the size and quantity of carbides in the steel. Higher hardness mainly prevents the edge from deforming (rolling, etc) so it would be a factor in allowing you to cut in steeper bevels (sharper edge). To much hardness leads to brittleness. In the end you want a nice balance. Japanese knives lean toward higher hardness.

So if this is true, then the technique some Japanese producers use of cladding a rather hard steel (like VG-10) with softer steel would indeed be the best of both worlds: hard steel with good edge retention protected from crumbling with softer exterior?? Or does cladding present other problems? Am contemplating a cladded knife for my next purchase, so thoughts are appreciated.

tk59
01-26-2012, 02:17 PM
So if this is true, then the technique some Japanese producers use of cladding a rather hard steel (like VG-10) with softer steel would indeed be the best of both worlds: hard steel with good edge retention protected from crumbling with softer exterior?? Or does cladding present other problems? Am contemplating a cladded knife for my next purchase, so thoughts are appreciated.The cladding does nothing for the edge. It might prevent the blade from snapping in half. It is mostly a cost saving measure, reactivity or aesthetic consideration. Cladded knives are fine. Drawbacks are: scratching, limited modification, and a deadening of sensation during cutting (according to some).

WildBoar
01-26-2012, 02:21 PM
Lots of clad knives out there. Hirimoto AS may be one of the most popular; the stainless steel cladding means you can be a little less diligent about wiping the knife down right after use. Some others use iron cladding, like Shigis, and that cladding is pretty reactive. Not only can it impart off colors and flavors into food (until you build or force a patina), but it scratches a little easier.

EdipisReks
01-26-2012, 03:25 PM
knives cladded with soft iron scratch easier, but they are also a lot easier to refinish.

David Metzger
01-26-2012, 05:30 PM
'In summary, the edge stability of a steel, which is the ability to both take and hold a highly polished edge, is increased with greater hardness, decreased with a higher carbide volume and increased with a reduction in retained austenite. Steels with lower edge stability require higher edge angles for the edge to take and hold a high polish. Higher alloy carbide steels also require harder abrasives to obtain maximum sharpness." Cliff Stamp

I believe the above conclusions are rather accurate about edges, steel, and also relate to hardness. Steels are heated and quenched to develop full hardness and liquid nitrogen increases this slightly more. The blade is then tempered reducing the hardness to a workable level. It is important to note that some steels used in blades will not harden to the same extent based on carbon content and other factors. You just can't get RC 63 out of some steels as I understand it.

When we talk about hardness in a blade it is measured on a flat portion of the knife but relates to how the edge will deform under pressure. You would love for your edge not to deform, right? But as hardness increases, brittleness also increases and toughness of the edge decreases. Some steels will perform well at high hardness, others will suck - (they will be too chippy) And as mentioned before, you just can't make some steels that hard even if you wanted to and the heat treatment becomes more detailed and expensive to get everything out of the steel.

So if you are hard on a knife because of your technique, the cutting boards you use, or the products you cut you will probably find you edge chipping and you would want RC hardness at the lower end in the acceptable range. If you already have a knife that is chippy, you can change the edge angle to be more obtuse. A high hardness edge will chip under high pressure and twisting, a low RC hardness will dent. Most kitchen knife enthusiasts like RC hardness 60+ in most styles of kitchen knives.
D

One great reason to sharpen with waterstones is that you aren't changing the tempering of the edge "Lowering the RC Hardness" and it should make your edge last longer. (See reports by Roman Landes)

SpikeC
01-26-2012, 08:27 PM
Hoss posted that an increase of 2 points on the C scale yields 20% more edge holding, I think. I need to get someone to test a sample of my O1!