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Stanley P. Rockwell invented the Rockwell hardness test. He was a metallurgist for
a large ball bearing company and he wanted a fast non-destructive way to determine
if the heat treatment process they were doing on the bearing races was successful.
The only hardness tests he had available at time were
Vickers, Brinell
and Scleroscope. The Vickers test was too time consuming, Brinell indents were too
big for his parts and the Scleroscope was difficult to use, especially on his small
parts.
To satisfy his needs he invented the Rockwell test method. This simple sequence
of test force application proved to be a major advance in the world of hardness
testing. It enabled the user to perform an accurate hardness test on a variety of
sized parts in just a few seconds.
Rockwell test methods are defined in the following standards:
Types of the Rockwell Test
There are two types of Rockwell tests:
- Rockwell: the minor load is 10 kgf, the major load is 60, 100,
or 150 kgf.
- Superficial Rockwell: the minor load is 3 kgf and major loads are
15, 30, or 45 kgf.
In both tests, the indenter may be either a diamond cone or steel ball, depending
upon the characteristics of the material being tested.
Rockwell Scales
Rockwell hardness values are expressed as a combination of a hardness number and
a scale symbol representing the indenter and the minor and major loads. The hardness
number is expressed by the symbol HR and the scale designation.
There are 30 different scales. The majority of applications are covered by the Rockwell
C and B scales for testing steel, brass, and other metals. However, the increasing
use of materials other than steel and brass as well as thin materials necessitates
a basic knowledge of the factors that must be considered in choosing the correct
scale to ensure an accurate Rockwell test. The choice is not only between the regular
hardness test and superficial hardness test, with three different major loads for
each, but also between the diamond indenter and the 1/16, 1/8, 1/4 and 1/2 in. diameter
steel ball indenters.
If no specification exists or there is doubt about the suitability of the specified
scale, an analysis should be made of the following factors that control scale selection:
- Type of material
- Specimen thickness
- Test location
- Scale limitations
» Rockwell Scale Application Guide
Principal of the Rockwell Test
-
 Select image to enlarge The indenter moves down into
position on the part surface
- A minor load is applied and a zero reference position is established
- The major load is applied for a specified time period (dwell time) beyond zero
- The major load is released leaving the minor load applied
The resulting Rockwell number represents the difference in depth from the zero reference
position as a result of the application of the major load.
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