Brinell hardness
/ brih-NELL /
The Brinell test, devised by Swedish engineer Johan August Brinell around 1900, is the ball-bearing method of measuring hardness. A hard ball is pressed into the surface with a heavy load, then removed, and the width of the round dent it leaves is measured with a little microscope. A wide dent means a soft material; a narrow one means a hard material. Because the ball is fairly large, it averages over many grains, making Brinell well suited to coarse, uneven materials like castings and forgings.
In practice a tungsten-carbide ball, usually 10 mm across, is pushed in with a load of up to 3000 kgf for around 10 to 15 seconds. The dent diameter is measured and the Brinell hardness number (HB) is the load divided by the curved surface area of the dent. A soft aluminium might read around 30 HB, a mild steel about 120 HB, and a hardened steel 500 HB or more. Results are written with the conditions, for example 250 HBW 10/3000.
Brinell's strength is exactly why it is loved for structural metals: the big indent samples a large volume and shrugs off local surface bumps. The same big indent is its weakness — it is too coarse for thin sheet, small parts, hard coatings, or very hard materials (where the ball itself would flatten). It also leaves a visibly large mark, so it is unsuitable on finished visible surfaces. For metals the Brinell number ties neatly to strength: tensile strength in MPa is roughly 3.4 x HB.
A foundry checks a large iron casting with a 10 mm Brinell ball because the coarse graphite structure would give scattered readings under a tiny indenter — the big Brinell dent averages it all out.
Brinell's large ball averages over many grains — ideal for castings, too coarse for thin or very hard parts.
The large Brinell indent is both its strength (averages coarse material) and its limit (too big for thin sheet, small parts, or very hard surfaces). Numbers are only comparable when the ball and load are the same, so always quote the test conditions.