Material hardness measures a substance’s resistance to permanent localized deformation, such as indentation or scratching. Understanding this property is important for engineering and manufacturing to predict how a material will perform under mechanical stress. The Brinell Hardness Number (BHN) is one of the oldest and most widely adopted standardized methods developed to quantify this characteristic. It provides a numerical value that allows engineers to compare the surface resilience of various metals and alloys.
The Brinell Test Mechanism
The Brinell test procedure begins by selecting a hardened, spherical indenter, typically made of hardened steel or tungsten carbide. Tungsten carbide is chosen for testing harder materials because it resists deformation better than steel, ensuring accuracy. The indenter’s diameter is precisely known, often standardized to 10 millimeters (\(\text{mm}\)), though smaller sizes are used for thinner specimens.
The test involves applying a specific, predetermined force, known as the major load, perpendicular to the material’s surface. This load is applied gradually and maintained for a standardized duration, usually 10 to 15 seconds for ferrous metals. This sustained pressure ensures the plastic deformation process is fully completed, resulting in a stable, permanent indentation.
Once the prescribed time has elapsed, the load is removed from the specimen. The resulting impression is a spherical cap, which provides the necessary geometric data for calculating the hardness number. The impression must be distinct, well-formed, and free from excessive material piling up (ridging) or sinking in (barreling) around the edges.
The final step is the precise measurement of the indentation’s diameter. This measurement is taken using a low-powered microscope or an automated optical system. It is standard practice to measure the diameter across two perpendicular axes to account for slight irregularities or non-uniformity. Averaging these two measurements provides the most representative value for the final calculation.
Calculating the Brinell Hardness Number
The Brinell Hardness Number is not a direct measurement but is mathematically derived from the physical parameters recorded during the test. The calculation relates the applied load to the curved surface area of the resulting indentation, not its simple projected area. The governing standard for this procedure in the United States is outlined in the ASTM E10 specification, which ensures consistent and comparable testing.
The formula involves three primary variables: the applied load (\(P\)), the indenter diameter (\(D\)), and the measured indentation diameter (\(d\)). The load (\(P\)) is expressed in kilograms-force (\(\text{kgf}\)), a unit reflecting applied force, while both diameters (\(D\) and \(d\)) are measured in millimeters (\(\text{mm}\)). This calculation quantifies the pressure exerted over the surface area of the impression.
The mathematical relationship accounts for the spherical indenter’s geometry, ensuring the result is independent of the load used, provided the load-to-diameter ratio is maintained. This ratio is chosen based on the material type to ensure the indentation size falls within an acceptable range, typically 25% to 50% of the indenter diameter. Following these guidelines ensures the BHN value is standardized and comparable.
The resulting BHN value is technically expressed as units of pressure: kilograms-force per square millimeter (\(\text{kgf}/\text{mm}^2\)). For reporting convenience, the unit is almost always omitted. The result is stated as a number followed by the designation HBW or HBS. ‘W’ indicates the use of a tungsten carbide indenter, while ‘S’ indicates a steel indenter, providing context about the test conditions.
Applications and Material Suitability
The Brinell test is well-suited for materials exhibiting non-uniformity or a coarse internal structure, such as large-grained castings and forgings. In these materials, a smaller indenter might only test a single grain, yielding a non-representative hardness value. The relatively large diameter of the Brinell indenter addresses this issue effectively.
By creating a large indentation, the test encompasses several grains and phases, providing a more reliable average hardness reading. This averaging effect is a significant advantage when evaluating bulk metal components used in heavy machinery or structural applications. Consequently, it is a preferred method for quality control in foundries and metal mills producing large components.
The method is recommended for testing materials in the softer to medium-hard range, including aluminum alloys, copper alloys, cast iron, and non-heat-treated steel. Materials that are too soft, like lead, may deform excessively, causing inaccurate readings due to material flow. Conversely, extremely hard materials, such as fully hardened tool steels or ceramics, can cause the indenter to deform or shatter, limiting the test’s upper range.
The Brinell test is advantageous because it leaves a relatively large impression that can be easily measured with standard optical equipment, sometimes including portable microscopes. This visibility and the simplicity of the measurement process contribute to its widespread use in industrial environments. The consistency of the resulting indentation makes it a reliable choice for assessing bulk material properties.
BHN vs. Other Common Hardness Scales
While Brinell focuses on measuring the diameter of a large, permanent, spherical indentation, other popular hardness methods utilize different mechanisms. The Rockwell hardness test, for instance, determines hardness by measuring the depth of penetration under minor and major loads. This approach makes the Rockwell test quicker to perform and read, often making it preferred for high-volume manufacturing environments where speed is a priority.
The Vickers hardness test employs a square-based diamond pyramid indenter that produces a geometrically similar indentation regardless of the applied load. This geometry allows the Vickers method to be used across a wider range of materials, including extremely hard substances. It is also suitable for testing very thin materials or small surface areas. Unlike Brinell, which has a practical upper limit before the indenter deforms, the Vickers test provides accurate readings for the hardest known substances.
The primary limitation of the Brinell test is the requirement for a large, flat test surface and the potential for indenter deformation when testing materials above approximately 650 BHN. Ultimately, the choice between Brinell, Rockwell, or Vickers depends on the material’s properties, thickness, required precision, and measurement speed. BHN remains valuable due to its capacity to average the hardness of non-homogeneous materials.