Rockwell C hardness scale question

A

Allen Rumme

Guest
Do any of you know if the Rockwell C hardness scale is a linear scale, logarithmic, exponential, or ??. In other words, is a hardness of RC59 20% harder than RC58, or twice as hard or what? I’ve looked online and have not found any clear explanation of the progression.

Why do I ask? I have a blade that was tempered to RC60 and it is significantly harder to work than a blade tempered to RC59. I would like to understand the relative hardness I’m getting if a blade is tempered to RC60 vs. to RC58 or RC59.

Thanks!

Allen
 
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Hardness and toughness are not the same thing. A CPM S30V blade tempered at 60 will be much harder to grind than a CPM 154 blade tempered at 60. You may be trying to compare apples to oranges.
-John
 
Well, maybe you should put that in your post to start with. Toughness and hardness are different properties. There is not enough of a difference in the Rockwell measurement to affect the toughness. Call Crucible and ask to speak to someone about it. Maybe they can give you the answer you are looking for.
-John
 
Toughness is the ability of a material to resist the start of permanent distortion plus the ability to resist shock or absorb energy

Wear resistance is defined as both material loss and deformation at contact surfaces.

Hardness is the property of a metal, which gives it the ability to resist being permanently, deformed (bent, broken, or have its shape changed), when a load is applied. The greater the hardness of the metal, the greater resistance it has to deformation.


Each of these properties depend on the compostion of the steel. But, in many cases a decrease in hardness results in an increase in toughness. Wear resistance may not have that same relationship. I would tend to think that the wear resistance would be related to the structure of the carbide formed for that particular steel. I know there are some other guys here that can give a better explaination of this. Hopefully they will post.
-John
 
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From this website

http://steel.keytometals.com/Articles/Art140.htm





Rockwell Hardness Test

The most widely used hardness test is the Rockwell hardness test. Its general acceptance is due to its speed, freedom from personal error, ability to distinguish small hardness differences in hardened steel, and the small size of the indentation, so that finished heat-treated parts can be tested without damage.
This test utilizes the depth of indentation, under constant load, as a measure of hardness. A minor load of 10 kg is first applied to seat the specimen. This minimizes the amount of surface preparation needed and reduces the tendency for ridging or sinking in by the indenter. The major load is then applied, and the depth of indentation is automatically recorded on a dial gage in terms of arbitrary hardness numbers.

The dial contains 100 divisions, each division representing a penetration of 0.00008 in (0.002 mm). The dial is reversed so that a high hardness, which corresponds to a small penetration, results in a high hardness number. This is in agreement with the other hardness numbers described previously, but unlike the Brinell and Vickers hardness designations, which have units of MPa, the Rockwell hardness numbers are purely arbitrary.

Major loads of 60, 100, and 150 kg are used. Since the Rockwell hardness is dependent on the load and indenter, it is necessary to specify the combination which is used. This is done by prefixing the hardness, number with a letter indicating the particular combination of load and indenter for the hardness scale employed. A Rockwell hardness number without the letter prefix is meaningless.

Hardened steel is tested on the C scale with the diamond indenter and a 150-kg major load. The useful range for this scale is from about RC 20 to RC 70. Softer materials are usually tested on the B scale with a 1/16-in-diameter steel ball and a 100-kg major load. The range of this scale is from RB 0 to RB 100. The A scale (diamond penetrator, 60-kg major load) provides the most extended Rockwell hardness scale, which is usable for materials from annealed brass to cemented carbides. Many other scales are available for special purposes.

The Rockwell hardness test is a very useful and reproducible one provided that a number of simple precautions are observed. Most of the points filled below apply equally well to the other hardness tests:

The indenter and anvil should be clean and well seated.
The surface to be tested should be clean and dry, smooth, and free from oxide. A rough-ground surface is usually adequate for the Rockwell test.
The surface should be flat and perpendicular to the indenter.
Tests on cylindrical surfaces will give low readings, the error depending on the curvature, load, indenter, and hardness of the material. Theoretical and empirical corrections for this effect have been published.
The thickness of the specimen should be such that a mark or bulge is not produced on the reverse side of the piece. It is recommended that the thickness be at least 10 times the depth of the indentation. The spacing between indentations should be three to five times the diameter of the indentation.
The speed of application of the load should be standardized. This is done by adjusting the dashpot on the Rockwell tester. Variations in hardness can be appreciable in very soft materials unless the rate of load application is carefully controlled.
 
Delbert,

Thanks for posting that. I was aware of the general testing procedure, but not the details.

I have done a bit more searching for information on my original question, and everything that I've read says that hardness is a poorly defined term which has different meanings for different materials and in different applications. For steels, the only quantitative piece of information that I've found says that hardness can be empirically related to tensile strength, but they also go on to say that there are a whole raft of other characteristics that can contribute to hardness. So, best I can tell, there is no way to quantitatively define the difference between RC58 & RC60. RC numbers are just an arbitrary way to specify relative hardness based on a specific testing procedure. Not the answer I was looking for, but an answer none the less.

Allen
 
Rockwell Rc scale hardness is determined by the depth of a diamond penetrator with a 150 kg load, the depth of the penetration gives a number indicated on the dial of the machine. The lower the number the deeper the penetration. Rockwell Rc gives what is refered to as macrohardness, which is an average of the carbides present and the matrix. Hardness does affect wear resistance in carbides, especially in chromium and molybdenum carbides.
In this case even a single point of hardness can significantly increase the wear or abrasion resistance of the carbides, especially as you get closer to the maximium attainabe hardness of the steel.
 
Delbert Ealy said:
In this case even a single point of hardness can significantly increase the wear or abrasion resistance of the carbides, especially as you get closer to the maximium attainabe hardness of the steel.
Click to expand...

Yes, I am aware that this is the case. I wanted to be able to put a number on it, RC60 is 20% harder than RC59, for example. But that just isn't possible, from what I can tell, because there are just too many variables and no way to quantify the changes in all of the characteristics.

Allen
 
Allen Rumme said:
Delbert,

Thanks for posting that. I was aware of the general testing procedure, but not the details.

I have done a bit more searching for information on my original question, and everything that I've read says that hardness is a poorly defined term which has different meanings for different materials and in different applications. For steels, the only quantitative piece of information that I've found says that hardness can be empirically related to tensile strength, but they also go on to say that there are a whole raft of other characteristics that can contribute to hardness. So, best I can tell, there is no way to quantitatively define the difference between RC58 & RC60. RC numbers are just an arbitrary way to specify relative hardness based on a specific testing procedure. Not the answer I was looking for, but an answer none the less.

Allen
Click to expand...


Allen,
The reason for "the whole raft of characteristics" is that steel as we know it is comprised of carbides and the matrix, and the carbides behave in certaian ways and the matrix behave in certian ways and for the most part we measure these characteristics as a whole, and with such a complex system, small variables become extremely important. Small changes in composition can have huge observable results. To complicate things further, most specs for a given alloy of steel are ranges, not exact quantities. So each specific melt may have variablity that can affect the finished product. The heat treatment is likewise given as a range and this too can have an effect on the finished product.
 
Allen Rumme said:
Yes, I am aware that this is the case. I wanted to be able to put a number on it, RC60 is 20% harder than RC59, for example. But that just isn't possible, from what I can tell, because there are just too many variables and no way to quantify the changes in all of the characteristics.

Allen
Click to expand...

Allen,
you actually can do this, but not with rockwell Rc. You can do it with a microhardness tester which allows you to test the hardness of just the carbides, but that is a whole different ball of fish
 
Delbert,

As I am finding, this is a much more complex topic than I had first thought. Given the complex nature of steel and the number of variables than can effect the characteristics of a heat treated & tempered blade, I now have a better appreciation for why one can't really assign a number to the differences between various RC ratings.

Thanks for the information, its never too late to learn new things.

Allen
 
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