Brinell number

A measure of the hardness of metals, defined in 1910 by Johann A. Brinell, a Swedish engineer. Abbr, Bhn.

A known load is applied to a hardened steel ball resting on a flat surface of the metal to be tested; the diameter of the dent made is measured. The Brinell number indicating the metal's hardness is then the load on the ball in kilograms divided by the spherical surface area of the dent in sq. millimeters. In practice workers have standardized on a certain ball size (10 mm in diameter) and loads. For a 10-mm ball, the Brinell number is:

$A fraction. The numerator is the load in kilograms times the quantity five plus the square root of 25 minus the square of the radius of the dent expressed in millimeters. The denominator is ten pi times the square of the radius of the dent expressed in millimeters.$

A load of 3,000 kilograms is used for hard metals, 1,500 kg for those of intermediate hardness, and 500 kg for soft metals. For very soft metals, 100 kg is sometimes used. The length of time the load is applied also influences the size of the dent; at least 10 seconds is necessary for iron and steel and at least 30 for other metals.

Around 400 Brinell, hardened steel balls become noticeably flattened under 3,000 kg, and workers departed from Brinell's specifications by substituting, for example, tungsten carbide balls.