From Wikipedia, the
The Rockwell scale characterizes
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. It involves the application of a minor load
followed by a major load, and then noting the hardness value directly from
a dial. Its chief advantage is its ability to display hardness values
directly, thus obviating tedious calculations involved in other hardness
measurement techniques. Also, the relatively simple and inexpensive set-up
enables its installation in college laboratories.
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.
- 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.
Several other scales, including the extensive A-scale, are used for specialized applications.
There are special scales for measuring case-hardened specimens.
Good practice includes:
- 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
- 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.
- ISO 6508-1 : Metallic materials -- Rockwell
hardness test -- Part 1: Test method (scales A, B, C, D, E, F, G, H,
K, N, T)
- ASTM E18 : Standard methods for rockwell hardness and rockwell superficial
hardness of metallic materials