What Is the Rockwell Hardness Test?

Many people don't realize there is more than one Rockwell test

Workers examining metal part in manufacturing plant
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What is the definition of the Rockwell hardness scale? With this review, get the facts you need about this method. This includes why the Rockwell test is used, who originated the scale and how effective it is in measuring steel. 

The Basics About the Rockwell Hardness Test

Simply put, the Rockwell hardness test is a method of measuring the hardness of materials. The Rockwell hardness scale is typically administered to characterize the hardness of metals, such as thin steel, cemented carbides, lead, aluminum, zinc, copper alloys, titanium and iron.

But the scale is also administered to test certain plastics.

In any case, hardness as measured by the Rockwell scale, refers to resistance to penetration. The test gets its name from Stanley P. Rockwell, who devised the test and original machines, later selling the rights. The test was first administered in 1919.

How the Test Works

The test measures the hardness by pressing an indentor into the surface of the steel with a specific load and then measuring how far the indentor was able to penetrate. More specifically, the Rockwell test includes steel balls of various diameters and a diamond cone penetrator with an angle of 120° and a spherical tip with a radius of 0.2 mm.

While there are a number of Rockwell tests, the most common one is Rockwell B. In addition, Rockwell C is used on hard materials. When the material is very thin, lighter loads must be used, resulting in Rockwell 30T, 1ST, Rockwell 15-N, 30-N scales.

As you can see from these measurements, Rockwell hardness numbers feature a prefix that distinguishes the Rockwell scale used in correlation to the combination of load and indenter. 

There are conversion charts that will allow conversion from one method of hardness to another, but it must be remembered that these conversion charts cannot precisely convert from one to another.

Unfortunately, most customers do not recognize that there are different Rockwell tests. They may also fail to realize that conversion charts are not totally accurate. That's because Rockwell measurements have no units. As Sizes.com explains, "The symbol is HR followed by a letter indicating one of a number of possible scales. For example, 'HRC 96' means 96 on the Rockwell C scale."

A Comparable Scale to Rockwell Hardness

The Rockwell scale has been compared to the Brinell scale. The Brinell scale was developed by Swedish engineer Johann A. Brinell in 1910. This scale operates as follows. A load is applied to a hardened steel ball that sits on a flat surface of the metal waiting to be evaluated. After this, the diameter of the dent that forms is assessed.

According to Sizes.com, the hardness is measured with this formula: "The Brinell number indicating the metal's hardness is then the load on the ball in kilograms divided by the spherical surface area of the dent in square millimeters."

The Mohs test is another hardness scale.

An understanding of other hardness scales may help you better grasp how the Rockness scale works in comparison.