- THE MAGAZINE
- WEB EXCLUSIVES
Stanley P. Rockwell invented the Rockwell hardness test. As a metallurgist for a large ball bearing company, he wanted a fast, nondestructive 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 the time were Vickers, Brinell and Scleroscope. The Vickers test was 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 advancement in the world of hardness testing. It enabled the user to perform an accurate hardness test on a variety of part sizes 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 Rockwell Tests
There are two types of Rockwell tests:
• Regular Rockwell: the minor load is 10 kilograms of force (kgf) and the major load is 60, 100 or 150 kgf.
• Superficial Rockwell: the minor load is 3 kgf and major load is 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 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-inch 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
Principle of the Rockwell Test
The Rockwell principle is described as follows: An accurately shaped indenter is used to make an indent in the sample to be tested. This indenter is either a spheroconical diamond, a 120-degree cone with a 0.2 millimeter radius tip, or hard metal ball with a 1⁄16, 1⁄8, 1⁄4 or 1⁄2-inch diameter. The instrument is designed to position the centerline of the indenter perpendicular to the test surface and has a depth-indicating device that can measure the vertical movement of the indenter into the sample.
Following are the steps included in Rockwell testing:
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.
The resulting Rockwell number represents the difference in depth from the zero reference position as a result of the application of the major load.
The indenter is first pressed into the sample a small amount by a relatively low preliminary force of 10 kg (HRC scale). After the preliminary force has been fully applied, the depth-indicating device is set to a zero indication. Then, a much larger additional test force of 140 kg is applied to the indenter for a total force on the sample of 150 kg. This large total force causes the indenter to penetrate the sample much farther. After the total force is fully applied, it is maintained for a short time to make sure that all penetration has stopped. Then, the additional force (140 kg) is removed while still maintaining the preliminary 10 kg force. After the preliminary force has been removed, the depth-
indicating device indicates the difference of penetration of the indenter as a result of the additional force. This depth is converted into the Rockwell number by subtracting the value from 100 (HRC scale). This will result in a harder material giving a higher number than a softer material. In the HRC scale, the formula is set up to have one Rockwell point be equal to 0.002 millimeter, 2 microns or 0.00008 inch.
The drawback of the Rockwell test is that the indenter travel is limited to 100 Rockwell points or 0.2 millimeter. This limitation requires different combinations of test force and indenter shapes to accommodate the hardness of all the possible materials to be tested.
Initially, 15 scales were created using a 10 kg preliminary force; 60, 100 or 150 kg total test forces combined with diamond; 1⁄16, 1⁄8, 1⁄4 and 1⁄2-inch diameter ball indenters; and 0.2 millimeter per Rockwell point. These forces are called the Regular Rockwell scales. The HRC, for hardened steels, and the HRB, for softer steels and brass, are the most common scales.
While the HRC scale uses a maximum of 0.2 millimeter (100 Rockwell points) penetration, the HRB and other regular scales use a ball indenter because they test softer materials and have a maximum depth of 0.26 millimeter (130 Rockwell points).
As the Rockwell test gained popularity, the need to test thin and case-
hardened parts became apparent. To accommodate these samples, the Superficial Rockwell scales were created. The same indenters are used, but the preliminary force is reduced to 3 kg and the total forces are 15, 30 and 45 kg. In addition, because of the shallower indents the depth value of one Rockwell point was reduced to 0.001 millimeter. The same maximum depth of 0.1 millimeter is used for both diamond and ball indenters.
Testers can be designed and sold as either Regular or Superficial units. However, the most common units currently produced are called twin testers because they can perform both Regular and Superficial test ranges.
Information courtesy of Wilson Instruments, An Instron Company (Canton, MA). For more information visit www.instron.com or call (800) 564-8378.