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 Test

There are two types of Rockwell Tests:

  1. Rockwell: the minor load is 10 kgf, the major load is 60, 100 or 150 kgf.
  2. 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 on 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

Principal of the Rockwell Test

  1. The indenter moves down into position on the part surface
  2. A minor load is applied and a zero reference position is established
  3. The major load is applied for a specified time period (dwell time) beyond zero
  4. The major load is released leaving the minor load applied
Rockwell

The resulting Rockwell number represents the difference in depth from the zero reference position as a result of the application of the major load

Applications

With the two test ranges available, the Rockwell test can be used on almost any metal sample as well as some hard plastics. The test can normally be performed in less than 10 seconds and the indent is usually small enough to allow the part to be used. Some parts with a critical hardness specification are tested 100%.

Strengths

  1. Rapid test, usually less than 10 seconds
  2. Direct readout, no questionable optical measurements required
  3. Non-destructive, part normally can be reused

Weaknesses

  1. Multiple test scales (30) needed to cover the full range of metal hardness
  2. Conversions between scales can be material dependent
  3. Samples must be clean and have a smooth test point to get good results