BHN stands for Brinell Hardness Number, a standardized way to measure how resistant a material is to being dented. It works by pressing a hard ball into a surface under a known load, then measuring the size of the dent left behind. The harder the material, the smaller the dent and the higher the BHN value. It’s one of the oldest and most widely used hardness scales in engineering, particularly common for testing metals like steel, cast iron, and aluminum.
How the Brinell Test Works
The test uses a desktop machine that presses a hardened ball, usually 10 mm in diameter, into the flat surface of the material being tested. The ball is either hardened steel or tungsten carbide, depending on how hard the material is. Steel balls work for materials up to about 444 BHN, while tungsten carbide balls handle harder materials up to around 627 BHN.
A specified load is applied and held for a set period, typically 10 to 15 seconds. The load varies by material: 3,000 kgf for steel and cast iron, 1,500 kgf for aluminum castings, and 500 kgf for softer metals like copper and brass. After the load is removed, the circular dent left in the surface is measured under a microscope or with a portable measuring device. Two diameter measurements are taken across the impression at right angles and averaged.
The BHN is then calculated by dividing the applied load (in kilograms) by the curved surface area of the indentation (in square millimeters). A small, shallow dent means the material resisted the force well, producing a high BHN. A large, deep dent means the material is softer, giving a low number.
What BHN Values Actually Mean
BHN values for common materials give you a practical sense of the scale. Pure aluminum might fall around 15 BHN, while mild steel sits in the range of 120 to 180 BHN. Hardened tool steels can reach 600 BHN or higher. Armor plate and wear-resistant steels are routinely tested with the Brinell method, pressing a 10 mm tungsten carbide sphere into the surface at 3,000 kg to verify they meet specification.
The number is expressed in units of kg/mm², though in practice most people just refer to the number itself. A steel rated at 200 BHN is roughly twice as resistant to indentation as one rated at 100 BHN, though the relationship between BHN and other mechanical properties like tensile strength isn’t perfectly linear.
Why Brinell Over Other Hardness Tests
Several hardness scales exist, including Rockwell and Vickers, so why use Brinell? The biggest advantage is the size of the indentation. Because the 10 mm ball creates a relatively large dent, the measurement averages out local variations in the material’s grain structure. This makes the Brinell test especially well suited for materials with coarse or uneven grain, like castings and forgings, where a smaller indenter might land on a hard spot or a soft spot and give a misleading reading.
Rockwell tests use smaller indenters and are faster since the machine reads the hardness directly without needing a microscope measurement. Vickers tests use a diamond pyramid and work well on very thin or very hard materials. But for raw castings, rough surfaces, and large parts where you need a reliable average hardness, Brinell remains the standard choice.
Test Requirements and Limitations
For a valid Brinell reading, the test piece needs to be thick enough that the ball doesn’t push through or distort the bottom surface. The general rule from ISO 6506 is that the material should be at least eight times thicker than the depth of the indentation. Thin sheet metal, for instance, won’t give reliable results with a standard 10 mm ball and full load.
The surface also needs to be reasonably flat and clean where the ball contacts it, though the Brinell test is more forgiving of rough surfaces than smaller-indenter methods. Indentations should be spaced far enough apart that one test doesn’t affect the next, and they shouldn’t be placed too close to the edge of the material.
In Europe, Brinell testing extends beyond the standard 10 mm ball setup. Smaller balls, down to 1 mm in diameter, are used with loads as low as 1 kgf for testing small parts. These are sometimes called “baby Brinell” tests. The 5 mm ball is another common option for thinner stock and smaller components, bridging the gap between full-size Brinell testing and methods like Rockwell that use even smaller indenters.
Where You’ll See BHN Used
BHN shows up most often in material specifications and quality control for metals. If you’re buying steel plate, bar stock, or castings, the supplier’s certification will frequently list a Brinell hardness value or range. It’s the go-to test for incoming inspection of raw materials in manufacturing, especially for parts that will be machined, since BHN correlates roughly with how difficult a metal is to cut.
Machinists use BHN to estimate appropriate cutting speeds and feeds. Tooling engineers use it to verify that heat-treated parts reached the target hardness. In mining and construction, wear plates and crusher components are specified by BHN to ensure they’ll hold up under abrasive conditions. If you’re reading a material data sheet or a specification that lists a BHN value, you now know exactly what that number represents: how much force it takes to push a standardized ball a given depth into the surface.