Question about hardness

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

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

Like how the test is performed, or what the typical numbers for steel are?

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

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

Thanks guys

55 to 58 hrc is a linear increase.

-AJ

Here's a graph I made based on the conversion chart:

Uploaded with ImageShack.us

mainaman said:

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

Click to expand...

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.

Larrin said:

Here's a graph I made based on the conversion chart:

Uploaded with ImageShack.us

Click to expand...

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.

No, it is linear. You just need to draw a best fit line.

-AJ

ajhuff said:

No, it is linear. You just need to draw a best fit line.

-AJ

Click to expand...

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

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

ajhuff said:

No, it is linear. You just need to draw a best fit line.

-AJ

Click to expand...

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

WildBoar said:

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

Click to expand...

Yeah, it looks parabolic to me.

WildBoar said:

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

Click to expand...

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.

Andrew H said:

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.

Click to expand...

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

ajhuff said:

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

-AJ

Click to expand...

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

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

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.

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..

only my personal 50cent, of course....

OK, but in hardness testing the load applied is constant, not variable.

-AJ

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

ajhuff said:

OK, but in hardness testing the load applied is constant, not variable.

-AJ

Click to expand...

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.

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.

DanB said:

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.

Click to expand...

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.

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.

mainaman said:

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.

Click to expand...

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

tk59 said:

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.

Click to expand...

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.