How does the Rockwell scale work?

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Evolute

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Mar 19, 2001
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I understand that the Rockwell hardness scale refers to how hard material is--I think this means resistance to deformation from applied pressure. (Please correct me if I am wrong.) But that's about all I know about the rockwell hardness scale.

Could someone explain the Rockwell scale to me? Is it logarithmic? Could you give me the equation to translate it into something else I may be familiar with? I'd like to understand what Rockwell hardness means without referring to a closed system of other examples of the scale (i.e., without saying "tempered steel is about 55-61, which means it's much softer than glass, around 80").

What specifically is a given Rockwell hardness number a measurement of?

I'd also be interested in any other pertinent information you care to add.

Thanks in advance.
 
Here's some info from Materials Evaluation and Engineering, Inc:

DESCRIPTION OF TECHNIQUE

Rockwell hardness testing is a general method for measuring the bulk hardness of metallic and polymer materials. Although hardness testing does not give a direct measurement of any performance properties, hardness correlates with strength, wear resistance, and other properties. Hardness testing is widely used for material evaluation due to its simplicity and low cost relative to direct measurement of many properties. Conversion charts from Rockwell hardness to tensile strength are available for some structural alloys, including steel and aluminum.

Rockwell hardness testing is an indentation testing method. An indenter is impressed into the test sample at a prescribed load to measure the material's resistance to deformation. A Rockwell hardness number is calculated from the depth of permanent deformation of the sample after application and removal of the test load. Various indenter shapes and sizes combined with a range of test loads form a matrix of Rockwell hardness scales that are applicable to a wide variety of materials.

Regular Rockwell hardness testing-Measures the bulk hardness of the material. There are separate scales for ferrous metals, nonferrous metals, and plastics. Common regular Rockwell hardness scales include B and C for metals and M and R for polymers.

Superficial Rockwell hardness testing-A more surface sensitive measurement of hardness than for the regular Rockwell scales. This technique is useful for samples with hardness gradients at the surface, to test small areas, and for thin samples. Superficial Rockwell hardness scales are N and T for metals and W, X, and Y for nonmetallic materials and coatings.

The C scale is the one you will usually see used with blade steels.

Use one of the good search engines, like Google and you'll probably find quite a bit about it.
 
Since you seem to have a technical background, there's an excellent book on the market that gives alot of background on the Rockwell testing methods and other methods as well: Metallurgy Fundamentals, by D.A. Brandt and J.C. Warner. pub. by Goodheart-Willcox Company, Inc., 1999, International Standard Book Number 1-56637-543-6. It's available through Barnes & Noble (www.bn.com)
 
Hardness is simply a measure of how difficult it is to squash something. For steels, the hardness is usually measured on the Rockwell "C" scale and is well correlated in a linear manner to tensile strength, which measures the resistance to being pulled apart.

-Cliff
 
Thank you for your answers. However, that's not exactly what I was looking for.

Let me ask another way. First I'll give some analogies to what I want to know:

1 degree Celsius means that the temperature is 1/100th of the way between where water freezes and where it boils (at 1 atmosphere of air pressure).

1 Degree arc means 1/360th of the circumference of a circle (with Euclidean Geometry).

1 meter means 1/10,000,000 of the distance between the North pole (at the time it was measured) and the equator--approximately 1/40,000,000 of the circumference of the Earth.

So, Cliff, when you say--

"Hardness is simply a measure of how difficult it is to squash something"

--What I'd like to know is, if something measures 1 on the Rockwell "C" hardness scale, how difficult is it to squash?

Also, if something measures 2 on the Rockwell "C" scale, does this mean that it is twice as difficult (takes twice as much pressure) to squash?
 
Evolute,
The Rockwell tester is a press that forces a penetrator into a sample being tested, with a gage that gives a hardness number, based on the depth of the permanent indentation left in the sample, after a prescribed load was applied (essentially a measure of compressive strength). The C scale uses a 150 kgf load and a Brale sphero-concical diamond penetrator.
I'm not sure, so someone please let me know if I'm wrong here, but I understand the RC number to be the depth of penetration caused by the 150 kgf load, divided by 0.00008" (which represents one RC number), subtracted fram 100.
 
Thank you, Frank.

I assume that the "kg" part of "kgf" is stands for kilogram? What does the "f" in kgf" stand for?
 
Evolute,
Yes, kg is for kilogram, the f is for force. A kg is a mass unit, it's weight is dependant on gravity, on earth a kg is about 2.2 lbs.(note that a pound is a force unit), "kgf" is used to denote this is the force or weight of a kg on earth and not a mass which is constant reguardless of gravity. I hope this is not too confusing, but a kgf is the weight of a kg on earth or about 2.2 lbs.
 
I think I now have the information I was looking for to understand Rockwell C measurements in terms I can understand. I still welcome anyone to further elaborate on anything you consider pertinent.

Codeman, knifenerd, Cliff, Frank,

Thank you very much.
 
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