Rockwell scale

The Rockwell scale characterises the indentation hardness of materials through the depth of penetration of an indenter, loaded on a material sample and compared to the penetration in some reference material. It is one of several definitions of hardness in materials science.

Related Topics:
Hardness - Materials science

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It is typically used in engineering and metallurgy and is most common in the USA. Its popularity arises from its speed, reliability, robustness, resolution and small area of indentation.

Related Topics:
Engineering - Metallurgy - USA

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There are several alternative scales, the most commonly used being the "B", and "C" scales. Both express hardness as an arbitrary dimensionless number.

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The B-scale is used for softer materials (such as aluminum, brass, and softer steels). It employs a hardened steel ball as the indenter and a 100kg weight to obtain a value expressed as "HRB".

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The C-scale, for harder materials, uses a diamond cone, known as a Brale indenter and a 150kg weight to obtain a value expressed as "HRC".

Related Topics:
Diamond - Cone

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The depth of penetration is converted to a scale in which the harder the material the higher the number.

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Common values

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• Very hard steel (e.g. a good knife blade) HRC 55 to HRC 62 or so
• Axes, chisels, etc. HRC 40 - 45

Readings below HRC 20 are generally considered unreliable, as are readings much above HRB 100.

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Several other scales, including the extensive A-scale, are used for specialised applications. There are special scales for measuring case-hardened specimens.

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Good practice includes:

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• Cleaning indenter and test-piece to be clear of dirt, grease, rust or paint;
• Measuring on a perpendicular, flat surface ("round work correction factors" are invoked to adjust for test-piece curvature);
• Ensuring that the thickness of the test-piece is at least 10 times the depth of the indentation;
• Maintaining an adequate spacing between multiple indentations; and
• Controlling the speed of the indentation.
 
Rockwell scale: Standards ►