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It is typically used in engineering and metallurgy and is most common in the USA. Its commercial popularity arises from its speed, reliability, robustness, resolution and small area of indentation.
There are several alternative scales, the most commonly used being the "B", and "C" scales. Both express hardness as an arbitrary dimensionless number.
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".
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".
The depth of penetration is converted to a scale in which a harder material gives a higher number.