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Chapter VIII Micro-hardness studies…
1. Brinell Test
It is perhaps the first scientific technique to measure hardness of
materials. In a standard Brinell test [16] a hard steel ball of about 1 cm.
diameter is pressed normally on the surface of metal under examination for a
standard period of 30 sec. After the removal of the load, the indentation
impression is measured. The impression may not be circular due to some
reasons necessitating the measurement of two diagonals at right angles and
using the mean value for estimating the hardness number. To reduce various
errors, usually, the geometrical similarity of indentation marks should be
preserved and hence the diameter of the impression mark should be kept
within the range of 0.25 to 0.50 times the diameter of the ball.
The Brinell hardness number, or simply the Brinell number, can be
obtained by dividing the load used, in kilograms, by the actual surface area of
the indentation, in square millimeters. The result is a pressure measurement,
but the units are rarely stated. The Brinell hardness test uses a desk top
machine to press a 10 mm diameter, hardened steel ball impressed onto the
surface of the test specimen. The machine applies a load of 500 kilogram for
soft metals such as copper and brass. A 1500 kilogram load is used for
aluminum castings and a 3000 kilogram load is used for materials such as
iron and steel. The load is usually applied for 10 to 15 second. After the
impression is made, a measurement of the diameter of the resulting round
impression is taken. The hardness is calculated by dividing the load by the
area of the curved surface of the indention. Among the three hardness tests,
the Brinell ball makes the deepest and the widest indentations, so the test
averages hardness over a wider amount of material, which is more accurate
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