Rockwell Hardness Test

Rockwell Hardness Test

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:

  • ASTM E18 Metals, ISO 6508 Metals , ASTM D785 Plastics

Types of the Rockwell Tests

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. Most 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.

For soft materials such as copper alloys, soft steel, and aluminum alloys a 1/16″ diameter steel ball is used with a 100-kilogram load and the hardness is read on the “B” scale.

In testing harder materials, hard cast iron and many steel alloys, a 120 degrees diamond cone is used with up to a 150 kg load and the hardness is read on the “C” scale.

There are several Rockwell scales other than the “B” & “C” scales, (which are called the common scales). A properly reported Rockwell value will have the hardness number followed by “HR” (Hardness Rockwell) and the scale letter. For example, 50 HRB indicates that the material has a hardness reading of 50 on the B scale.

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

The principle of the Rockwell Test

  • 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)
  • The major load is released leaving the minor load applied