Aug 26 2013
Rockwell hardness testing is a method of determining the relative hardness of a material. In this test, hardness is measured by determining the depth of residual penetration by a steel ball or a diamond point under load. Rockwell testing is the most commonly conducted hardness testing, ashe testing procedure is simple and the readings can be directly attained from the testing machine.
Hardness may be defined as the ability of a material to resist plastic deformation caused by penetrating forces. The Rockwell hardness scale is based on the indentation hardness of a material, which is simpy the resistance offered by a material to indentation.
The Rockwell testing apparatus was first invented by Hugh M. Rockwell and Stanley P. Rockwell. The first Rockwell testing machine was a differential-depth machine. This machine was later named the Wilson tester. The value of hardness measured by this method is represented as a dimensionless number followed by an abbreviation. The abbreviation used to designate Rockwell hardness is HRA, where A is the scale. The most common scales for this hardness testing are ‘B’ and ‘C’.
A variation of Rockwell hardness testing is the Rockwell Superficial Hardness testing. This testing method is used to measure the hardness of thin materials that tend to bend or get crushed under regular testing conditions. In this testing method, the indentation material is the same as in standard Rockwell testing but the applied load is reduced. To designate the superficial hardness test value a ‘T’ is added along with the HR number. This article will elaborate on the technique and applications of Rockwell testing.
Technique of Rockwell Hardness Test
A specific load is applied on the indenter of a Wilson tester and the depth of the penetration is measured. The indenter may be a steel ball or a spherical diamond-tipped cone of 120° angle and 0.2 mm tip radius (called a brale). A minor load of 10 kg is applied first, which causes a minor indentation. This is done in order to seat the indenter and also remove any surface irregularities.
Then the dial is reset and the major load is applied. The depth of penetration is measured after removing the major load but retaining the minor load. The hardness number is read directly from the scales. A brale with a load of 100 kg is used for very hard materials and the hardness is read on the 'B' scale. For most materials, a steel ball indenter is used.
Advantages of Rockwell Hardness Testing
Some of the advantages of Rockwell testing are listed below:
- Simple and easy test procedure
- Readings can be directly read from scale
- Able to test hardness of a variety of materials.
Industrial Applications
Rockwell hardness test is used for measuring the hardness of the following materials:
- Cemented carbides, thin steel and shallow hardened steel
- Copper alloys
- Aluminum alloys
- Cast iron
- Magnesium alloys
- Phosphor bronze
- Beryllium copper
- Soft bearing metals, plastics and other soft materials.