Rockwell hardness test

The result is a dimensionless number noted as HRA, HRB, HRC, etc., where the last letter is the respective Rockwell scale.

[2] The differential depth hardness measurement was conceived in 1908 by Viennese professor Paul Ludwik in his book Die Kegelprobe (crudely, "the cone test").

The Brinell hardness test, invented in Sweden, was developed earlier – in 1900 – but it was slow, not useful on fully hardened steel, and left too large an impression to be considered nondestructive.

[4] The requirement for this tester was to quickly determine the effects of heat treatment on steel bearing races.

At the time of invention, both Hugh and Stanley Rockwell worked for the New Departure Manufacturing Co. of Bristol, CT.[5] New Departure was a major ball bearing manufacturer which in 1916 became part of United Motors and, shortly thereafter, General Motors Corp. After leaving the Connecticut company, Stanley Rockwell, then in Syracuse, NY, applied for an improvement to the original invention on September 11, 1919, which was approved on November 18, 1924.

[7] Stanley collaborated with instrument manufacturer Charles H. Wilson of the Wilson-Mauelen Company in 1920 to commercialize his invention and develop standardized testing machines.

[10] The later-named Wilson Mechanical Instrument Company has changed ownership over the years, and was acquired by Instron Corp. in 1993.

Its commercial popularity arises from its speed, reliability, robustness, resolution and small area of indentation.

Legacy Rockwell hardness testers operation steps: In order to get a reliable reading the thickness of the test-piece should be at least 10 times the depth of the indentation.