averageguy
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What is the difference between 420 at 55rc and 440C at 59rc? A 7% harder blade? A 200% harder blade? A blade that is 6x harder?
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Steel types / hardness?
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averageguy
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btt
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Here is a great article on blade steel written by Joe Talmadge. It answers you question and a whole lot more.
http://www.bladeforums.com/features/faqsteel.shtml
http://www.bladeforums.com/features/faqsteel.shtml
averageguy
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Yes, It's a nice FAQ but it doesn't help me understand the difference in hardness between say 55rc and 59rc. One is harder - How much harder?
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I'd have to look up the answer, but I'm too lazy. Let me answer a different way.
The Rockwell hardness test is conducted by pressing a diamond point under a prescribed load into the blade, the measuring the size of the dimple that results. You're looking for a precise answer to your question, and I'll tell you that the answer is going to be something like: "59 Rc means that the dimple is 'x' microns deeper than 55 Rc". Will that really help you answer your question?
I have a very rough-and-sloppy answer that will probably be more useable for you: chances are, you won't notice a real difference in hardness for most uses when there's just 1 Rc separating the blades. You definitely start to notice at 2 Rc, for uses where hardness is an important cause of edge degradation. 3 Rc is definitely and obviously noticeable.
The edge on a 55 Rc knife will indent and flatten and roll much much easier than an edge at 59, pretty much regardless of steel types. That doesn't say anything about toughness or wear resistance, though. It's possible that a steel at 55 Rc can be more wear resistant than a steel at 59 Rc, and so hold its edge longer for jobs wear abrasion is the mean reason for edge degradation. In fact, you'll find exactly this situation with Spyderco's S60V (55-56 Rc) versus most 440C (~58 Rc) or even ATS-34 (~59-61 Rc).
Anyway, agian, I know it's not as scientific as you'd like, but my rule of thumb: if hardness and strength play a non-trivial role in edge degradation, 1 Rc isn't incredibly important, 2 Rc becomes interesting, 3 RC a major advantage.
Joe
averageguy
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Thank you for trying to answer that Joe. I did some research and the shallower / deeper dimple theory is all I could find. It appears to be a fairly complicated matter. Yes, 59rc is noticeably harder but identifying exactly how much harder it is may not be possible.
Cliff Stamp
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The forumla for HRC :
HRC = 100 - (Major-Minor)/0.002 mm
Major = major load (150 kg) distance
Minor = minor load (10 kg) distance
The forumla was written in this way so it would scale a complicated measurement to a simple easy to understand scale. The downside is that it is very misleading because people think of it as a linear quantity which it isn't. A hardness of even 1-2 points, which is often described as insignificant, once plugged into the above formula can show a significant increase in compressability.
An easier to understand formula is for Vickers hardness :
V=1.854*Load/d^2
Load = is the load
d=is the length of the side of the indendation produced.
This number, while a bit messier than HRC, is more straightforward to interpret as it is simply the load divided by the induced compressed area so it can be directly thought of as indicating the resistance pressure of the material.
For example, the difference of 59 to 55 HRC, is 715 to 615 V. So you are looking at an increase in hardness of about 20%, a fairly large difference indeed. If you look at the limit of most knives (65 to 45 HRC), this is a Vickers change of ~100%. Showing that there is quite a performance difference indeed available.
The only thing I would like to add to Joe's excellent answer is that besides edge retention on materials which primarily act to roll the edge, you can see the effects of hardness change on large chopping blades as softer blades will take edge ripples much easier. I am not talking about the edge rolling and going blunt, but large sections of the edge being bent to the side.
This effect, is the primary reason why you can make much better cutting knives out of harder steels becuase the edge can be ground much thinner. If you take a traditional machete (~45 RC), and grind a thin bevel on it, you will find that it rolls very quickly on hard woods. Take a tool steel blade (~60 RC), and at the same angle it will hold up fine, and in fact it can go much finer.
So as a general rule of thumb, for the ultimate in cutting performance, you want the hardest blade possible which has the required level of durability. As a bonus you get the highest edge retention as well.
-Cliff
HRC = 100 - (Major-Minor)/0.002 mm
Major = major load (150 kg) distance
Minor = minor load (10 kg) distance
The forumla was written in this way so it would scale a complicated measurement to a simple easy to understand scale. The downside is that it is very misleading because people think of it as a linear quantity which it isn't. A hardness of even 1-2 points, which is often described as insignificant, once plugged into the above formula can show a significant increase in compressability.
An easier to understand formula is for Vickers hardness :
V=1.854*Load/d^2
Load = is the load
d=is the length of the side of the indendation produced.
This number, while a bit messier than HRC, is more straightforward to interpret as it is simply the load divided by the induced compressed area so it can be directly thought of as indicating the resistance pressure of the material.
For example, the difference of 59 to 55 HRC, is 715 to 615 V. So you are looking at an increase in hardness of about 20%, a fairly large difference indeed. If you look at the limit of most knives (65 to 45 HRC), this is a Vickers change of ~100%. Showing that there is quite a performance difference indeed available.
The only thing I would like to add to Joe's excellent answer is that besides edge retention on materials which primarily act to roll the edge, you can see the effects of hardness change on large chopping blades as softer blades will take edge ripples much easier. I am not talking about the edge rolling and going blunt, but large sections of the edge being bent to the side.
This effect, is the primary reason why you can make much better cutting knives out of harder steels becuase the edge can be ground much thinner. If you take a traditional machete (~45 RC), and grind a thin bevel on it, you will find that it rolls very quickly on hard woods. Take a tool steel blade (~60 RC), and at the same angle it will hold up fine, and in fact it can go much finer.
So as a general rule of thumb, for the ultimate in cutting performance, you want the hardest blade possible which has the required level of durability. As a bonus you get the highest edge retention as well.
-Cliff
averageguy
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Thank you Joe. Thank you Cliff. You have given me some stuff to think about.
Being sort of new to knife collecting, I'll try to share what little I've learned so far. My CRKT with ATS34 blade, holds an edge noticably better, than the CRKT I have with an AUS6 blade. 440C of a 1970's vintage Parker falls just below the ATS34, and the 420 stainless chineese folders fall just below the AUS6. If the cheap 420 blades need sharpening, I throw them away and take a new one out of the box. Hopfully some day I'll run out of them! I have to touch-up the AUS6 blade every half dozen times I use it, to cut anything more abrasive than paper. I have to sharpen the 440C blade three or four times a year, and have not had to touch up the ATS34 blade again since I got the knife, though I've only owned it a few months now. Several years ago, I got a nice notch in my 440C blade cutting some wire, took a couple hours with a course Lansky hone to get it out, and took 1/32" off the width of the blade. Hope this helps.
Interesting thread!
Joe, Cliff and JD, some excellent replies there!
Cliff,
I am not too familar with the Vickers tester, about all I know is that it uses a diamond pyramid penetrator. My reference gives the Vickers hardness number formula as:
H = P/A
where H is the hardness number, P is the load in kgf, and A is the area of the indentation in square mm.
I think you are on the right track with the relative hardness relationship of the Rockwell C scale. As I am sure you know, the Rockwell C scale uses a sphero-conical diamond penetrator and the test material resisting penetration increases as the depth of penetration increases. I think the relative relationship of these types of hardness tests is linked to the amount of material compressed or displaced (volume of the indent) and the applied load.
The Brinell Hardness tester is similar to the Vickers except that a 10 mm tungsten carbide ball is used for a penetrator. The Brinell hardness number equiv. to Rc56 is 578 and 653 for Rc60; while there is a 7 % in Rc, there is a 13% increase in the Brinell. I would expect the the Brinell to be closer to the increase in hardness in a relative sense.
-Frank
Joe, Cliff and JD, some excellent replies there!
Cliff,
I am not too familar with the Vickers tester, about all I know is that it uses a diamond pyramid penetrator. My reference gives the Vickers hardness number formula as:
H = P/A
where H is the hardness number, P is the load in kgf, and A is the area of the indentation in square mm.
I think you are on the right track with the relative hardness relationship of the Rockwell C scale. As I am sure you know, the Rockwell C scale uses a sphero-conical diamond penetrator and the test material resisting penetration increases as the depth of penetration increases. I think the relative relationship of these types of hardness tests is linked to the amount of material compressed or displaced (volume of the indent) and the applied load.
The Brinell Hardness tester is similar to the Vickers except that a 10 mm tungsten carbide ball is used for a penetrator. The Brinell hardness number equiv. to Rc56 is 578 and 653 for Rc60; while there is a 7 % in Rc, there is a 13% increase in the Brinell. I would expect the the Brinell to be closer to the increase in hardness in a relative sense.
-Frank
Cliff Stamp
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frank k :
Yes exactly, it is supposed to represent how hard the material is to squash.
[vs RC]
Yes, it is just because the RC scale is a difference, it is scaled to make the numbers "nicer", like the mohs scale. Of course you tend to lose information when you compress it.
-Cliff
Yes exactly, it is supposed to represent how hard the material is to squash.
[vs RC]
Yes, it is just because the RC scale is a difference, it is scaled to make the numbers "nicer", like the mohs scale. Of course you tend to lose information when you compress it.
-Cliff
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Folks,
I agree that this is an excellent thread. Averageguy, thanks for bringing it up... and guys who have responded, thank-you for providing the knowledge! I sincerely appreciate your educatin'.
Stay Safe,
I agree that this is an excellent thread. Averageguy, thanks for bringing it up... and guys who have responded, thank-you for providing the knowledge! I sincerely appreciate your educatin'.
Stay Safe,
Cliff,
The Rc scale was not intended to make "nicer" numbers, it was intended as a way to use a depth gage to give a direct reading, instead of a indent diameter that had to be measured manually before putting it in a formula and calculating. The Rc may not be as pure theoretically, it can be converted into Brinell numbers for estimating tensile and compressive strengths. Eliminating "eye ball" measurements and calculations make the Rockwell tester much easier to use and more accurate.
BTW, Rockwell, Brinell, and Vickers don't really measure hardness at all, they are essentially quick and dirty compression testers. If you want to see a real hardness tester, check out the Shore scleroscope, which works by measuring the rebound of a diamond tipped hammer after it is dropped on the sample (a measure of elastic limit instead of compressive strength).
-Frank
The Rc scale was not intended to make "nicer" numbers, it was intended as a way to use a depth gage to give a direct reading, instead of a indent diameter that had to be measured manually before putting it in a formula and calculating. The Rc may not be as pure theoretically, it can be converted into Brinell numbers for estimating tensile and compressive strengths. Eliminating "eye ball" measurements and calculations make the Rockwell tester much easier to use and more accurate.
BTW, Rockwell, Brinell, and Vickers don't really measure hardness at all, they are essentially quick and dirty compression testers. If you want to see a real hardness tester, check out the Shore scleroscope, which works by measuring the rebound of a diamond tipped hammer after it is dropped on the sample (a measure of elastic limit instead of compressive strength).
-Frank
Cliff Stamp
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frank k :
I should have clarified that statement. I was not speaking of the method, just the way the measurements of depth were scaled to compress them into a 0-100 scale. This is the same for example as the absolute temperature scale transformed to C, it just allows for a "nicer", set of numbers as otherwise you would go around saying things like "Its a bit chilly today, a brisk 273 degrees."
But measuring the indent area is the same as the depth if the shape is constant. They are in a fixed ratio to each other. The area, usually just width, is just a lot easier to measure than the depth.
Well yes, but isn't that what hardness is generally used to mean? In laymans terms it also covers aspects like resistance to scratching and bending, both of which of course are highly correlated to compression resistance.
-Cliff
I should have clarified that statement. I was not speaking of the method, just the way the measurements of depth were scaled to compress them into a 0-100 scale. This is the same for example as the absolute temperature scale transformed to C, it just allows for a "nicer", set of numbers as otherwise you would go around saying things like "Its a bit chilly today, a brisk 273 degrees."
But measuring the indent area is the same as the depth if the shape is constant. They are in a fixed ratio to each other. The area, usually just width, is just a lot easier to measure than the depth.
Well yes, but isn't that what hardness is generally used to mean? In laymans terms it also covers aspects like resistance to scratching and bending, both of which of course are highly correlated to compression resistance.
-Cliff
Not exactly, you need to do some calculations to find areas that a simple depth gage can not do.
Hardness can be defined as the ability of a material to resist being permanently deformed. There are three general ways that hardness is measured: resistance to penetration, resistance to wear/abrasion and elastic hardness (this is what the Shore scleroscope measures).
Most hardness testers measure penetration resistance (compressive strength). As you know if you if you have 420HC and 440V both at Rc 55, they are not equal in terms of wear reistance, even though that is what could be inferred from the Rc numbers. For knives, wear resistance would be generally more useful than Rc, but as you know, you also want to know the Rc, since resistance to indentation tends to influence things like edge roll and blunting under impact. Neither wear or compression resistance tell the whole story and neither is right or wrong, yet they both could be defined as hardness.
I always felt that the elastic hardness was the purest from a theoretical view point, which I suppose it is not, it just fit my own notion of how hardness is best defined. I thought you might be interested it since you have a background in Physics.
Bending is complicated, since there are tension, compression and shear forces at work at the same time. Bending should not be confused with simple tension or compression.
-Frank