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.