Metallurgists determine the suitability of metals like aluminum, steel, or iron for construction by testing their hardness, a material characteristic, not a physical property. Hardness refers to resistance against indentation, scratching, abrasion, and temperature. The Rockwell Indentation hardness test is widely preferred due to its simplicity and interpretability.
Method Description of Rockwell Hardness Test
Invented by metallurgist Stanley P. Rockwell, this method provides a non-destructive and efficient means to assess the impact of heat treatment on bearing races, unlike other complex or time-consuming methods like Vickers, Brinell, or Scleroscope.
Rockwell testing, besides being quick and straightforward, offers higher accuracy. It applies to all metals and materials, except where variations in structure or surface complexity inhibit accurate readings.
The Rockwell hardness test employs a diamond cone or hardened steel ball indenter, applying two loads to the test subject. A minor load of 10kg-f initially penetrates the surface, followed by a major load of 60, 100, or 150kg-f to deepen the indentation. The difference between the two loads determines the Rockwell hardness value.
Superficial Rockwell testing, a variation, employs lighter loads for thin or smaller surfaces. While the method and process remain similar, superficial testing uses loads of 3 kg-f as the initial force and 15, 30, or 45 kg-f as an additional force.
However, different materials require specific testing methods. The Rockwell hardness test offers 30 hardness scales, denoted by the indenter and loads. A Rockwell hardness value combines a numerical hardness number and Rockwell hardness scale letters, e.g., 70 HRA.
What’s the difference between Rockwell hardness testing and Brinell hardness testing?
The Rockwell hardness test has been compared to the Brinell test which was discovered by a Swedish engineer in 1910. In a Brinell test, a hardened steel ball under a set force is applied on a flat surface of the test subject. Once the indenter is removed, the diameter of the dent is measured.
Unlike the Brinell test, the Rockwell hardness tester uses a smaller indenter and starts by applying a minor load to cut through the surface. This process makes it less destructive and more accurate.
Using a larger indenter in the Brinell test makes the process slower and leaves irreparable imprints on the test object.
Additionally, the Brinell test favors materials that are rough, irregular, or unsuitable to go through any of the other tests, including the Rockwell method. Cast iron is one of the materials perfectly suited for the Brinell test as it requires a larger indenter.