The indenter used in the Brinell test method is made of a hard metal ball of diameter D = 10; 5; 2.5 or 1 mm. It is applied to the specimen with a test force (to standard from 1 kg to 3000 kg) and held according to the holding time.
To calculate the Brinell hardness, two perpendicular diameters of the test indentation are measured on the specimen surface. In addition, the test force N and the diameter of the spherical indenter are required.
In the Brinell hardness test, an optical method, the size of indentation left by the indenter is measured. In contrast to the optical Vickers method, in which a pyramid-shaped indenter is pressed into a specimen, the Brinell method uses a spherical indenter.
The larger the indentation left by the Brinell indenter with a specified ball diameter and under a defined test force on the surface of a workpiece (specimen), the softer the tested material.
According to ISO 6506, the spherical indenter made of hard metal (tungsten carbide) is pressed into a specimen (workpiece) with a defined test load (between 1 kgf and 3000 kgf) to determine the Brinell hardness (HBW).
Brinell hardness is a unit of measurement that indicates the hardness of a material. It is measured using the Brinell hardness method, for which a hardmetal ball is pressed into the material. The size of the resulting indentation is optically measured to determine the hardness of the material.
Brinell hardness is normally used for materials with large grain size, rough test surface or inhomogeneous materials, such as castings, alloys and forged components. It is defined as the ratio of the applied force to the surface area of the indentation. The unit of Brinell hardness is HBW (hardness according to Brinell with a tungsten carbide ball).
The Brinell hardness HBW results from the quotient of the applied test force F (in newtons N) and the surface area of the residual indentation on the specimen (the projection of the indentation) after removal of the test force (see Brinell formula). To calculate the surface area of the residual ball indentation, the arithmetic mean d of the two perpendicular diagonals d1 and d2 (in mm) is used, because the base area of Brinell indentations is often not perfectly round.
In practice, when determining the hardness value, the formula is not calculated for every individual test. The hardness value can be alternatively determined from tables, that is a specially programmed hardness testing software, which displays the hardness value as a function of the average indentation diameter d for all standardized ball diameters and test loads.
The test force is to be selected so that the average indentation diameter d is between 0.24 D and 0.6 D.
To be able to comply with these limits, it is necessary to coordinate the test force with the ball diameter. This results in different force-diameter indexes (also referred to as loading levels or load factors) within the Brinell method, where the quotient of test force and square of the ball diameter is kept constant: B = 0.102*F/D2. The five common force-diameter indexes are 1, 2.5, 5, 10 and 30. Testing of a material with different ball diameters and test forces must be conducted within the same force-diameter index in order to achieve comparable test results (see overview table "Brinell methods and applications”).
The ball diameter must be selected in such a way that the indent covers the largest possible workpiece area – representative for the specimen.
According to the standard (ISO 6506), the test load should be increased to its final value within a minimum of two to a maximum of eight seconds. Generally, the dwell time for the test load is 10 to 15 seconds (s). If the dwell time is any longer, the duration in seconds must also be specified in the hardness value, e.g.: 210 HBW 5/250/30 (dwell time of 30 s).