☙

Brinell hardness number

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

A known load is applied to a hardened steel ball resting on a prepared, 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 square millimeters.

An equation. The numerator is the load in kilograms. The denominator is the diameter in millimeters of the steel ball times pi, divided by 2, times the quantity ball diamtr minus the square root of the square f the ball diamters minus the square of the diameter of the dent expressed in millimeters.

Where,
P is applied load in kilograms,
D is diameter of the steel ball, in millimeters,
d is average diameter of the indentation, in millimeters.

In practice workers have standardized on a certain ball size (10 mm in diameter) and loads. For a 10-mm ball, 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 and thin sections. For very soft metals, 100 kg is sometimes used. If the size of the ball and the load are not included in a report of a Brinell test, 10 mm and 3000 kg are assumed.

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. Standards now specify that steel balls are not to be used above 444 BHN. The ASTM recommends that, even with tungsten carbide balls, Brinell testing is not appropriate for hardnesses that would measure over 627 BHN.

For comparison with the other common measure of hardness, the Rockwell scales, 627 BHN is about 58.7 on the Rockwell C Scale, 100 on the Rockwell B Scale (22.8 on the C scale) is about 241 BHN, and 111 BHN is about 65.7 on the Rockwell B Scale.

standards

ASTM E 10 Method of Test for Brinell Hardness of Metallic Materials.

ASTM A 370-64 (and subsequent editions) Standard Methods and Definitions for Mechanical Testing of Steel Products.

sources

Brinell Hardness Numeral (abbreviated B. h. n.). — Ratio of pressure on a sphere used to indent the material to be tested to the area of the spherical indentation produced. The standard sphere used is a 10mm diameter hardened steel ball. The pressures used are 3000 kg for steel and 500 kg for softer metals, and the time of application of pressure is 30 seconds. Values shown in the tables are based on spherical areas computed in the main from measurements of the diameters of the spherical indentations, by the following formula:

B .h .n . = P + π t D = P + π D (\frac{D}{2} - \sqrt{\frac{D^{2}}{4} - \frac{d^{2}}{4}})

P = pressure in kg, t = depth of indentation, D = diameter of ball, and d = diameter of indentation, — all lengths being expressed in mm. Brinell hardness values have a direct relation to tensile strength, and hardness determinations may be used to define tensile strengths by employing the proper conversion factor for the material under consideration.

Frederick E. Fowle.
Smithsonian Physical Tables, 7th revised edition, reprint.
Smithsonian Miscellaneous collections, vol. 71, no. 1.
Washington, DC: Smithsonian Institution, 1921.
Page 74.