Brinell hardness test

The Brinell hardness (HBW) results from the ratio between the applied test force and the projected indentation area that a Brinell indenter leaves on the material surface. The indentation is determined by the mean value of two perpendicular diagonals, as the shape of the impression is often irregular. In practice, hardness is usually determined using tables or special hardness testing software. To ensure that the impression remains meaningful, the ball diameter and test load must be correctly matched. The load is applied in accordance with the ISO 6506 standard - including an exposure time of 10 to 15 seconds. Comparability of the measurement results is ensured by the application within a defined load level. The Brinell method is particularly suitable for soft to medium-hard materials and provides reliable values for material hardness.

Brinell hardness is a parameter for the resistance of a material to plastic deformation caused by the penetration of a test specimen. It is determined using the Brinell method, in which a hard metal ball (Brinell indenter) is pressed into the material under a defined test force. The resulting indentation surface is then analysed optically.

This method is particularly suitable for coarse-grained, uneven or rough materials such as castings, forgings or alloys. The Brinell hardness HBW is calculated from the ratio of the test force to the projected surface of the permanent indentation. The abbreviation HBW stands for ‘Brinell hardness’ using a tungsten carbide ball.

The Brinell hardness test provides reliable results for materials with different structures - standardised, practical and tried and tested.

Normally, the surface of the specimen to be tested must be prepared for Brinell hardness testing, because the surface quality must be good enough to allow correct optical evaluation of the indent.

Compared with the Vickers method, which also tests hardness in the micro range, the surface quality requirements for the Brinell method are nevertheless lower, because the latter normally involves testing with higher loads (in the macro range) and larger indenters (balls), so the surface can be rough.

In detail, the specimen to be measured must meet the following requirements for the Brinell method:

• The specimen must be plane-parallel (clean separation of the specimen from the workpiece required during specimen preparation).
• The specimen to be tested must have a level, metallically bare and smooth surface (grinding and polishing of the specimen surface only necessary for tests with small ball diameters).
• The specimen must be fixed in a rigid mounting fixture, so that is cannot move during the testing process (practical tip: measure with the specimen clamped, or embedded or fixed in a specimen holder).

The Brinell method is a versatile hardness testing method and is suitable for materials with very different degrees of hardness - from soft metals such as aluminium, tin or lead to harder materials such as steel or cast iron.

In order to obtain comparable Brinell hardness values, the test force and ball diameter must be matched to each other within the same degree of stress. This means that the Brinell hardness test can be carried out precisely and in accordance with standards, even with different materials and test parameters.

In practice, the hardness value for the Brinell test is not determined by calculation for each individual measurement. Instead, it is usually determined using tables or corresponding testing software. This displays the hardness value as a function of the average indentation diameter d for all standardised ball diameters and test loads.

The test load must be selected so that the average indentation diameter d lies within the range of 0.24 D to 0.6 D.

In order to reliably maintain this range, the test load and ball diameter must be matched. This results in different degrees of stress - also known as load degrees or load factors - within the Brinell method. The load factor results from the quotient of test force and ball diameter squared (B = 0.102 × F / D²) and remains constant within a load factor. Common degrees of stress are 1, 2.5, 5, 10 and 30. If a material is tested with different ball diameters and forces, this must be done within the same degree of stress in order to ensure comparable results.

The ball diameter must be selected so that the test indentation covers as large a representative zone of the material as possible.

According to ISO 6506, the test load must be applied within a minimum of two to a maximum of eight seconds. The duration of application of the test load is generally 10 to 15 seconds. If the load duration is longer, this must be added to the hardness specification, for example: 210 HBW 5/250/30 (exposure time 30 s).

A Brinell hardness value consists of the following components:

1. The numeric hardness value;
2. The three letters "HBW", standing for "Hardness according to Brinell" (with hard metal ball made from tungsten carbide);
3. The ball diameter in mm;
4. The applied test load in kgf;
5. According to ISO 6506: The dwell time of the test load, but only if this is not between 10 and 15 seconds (uncommon in practice)

Example:
210 HBW 5/250/30

• 210 …hardness value
• HBW …as per Brinell with tungsten carbide ball
• 5 …with ball diameter of 5 mm
• 250 …with test load of 250 kgf (or test force of 2.452 kN )
• 30 …dwell time to test force outside recommended duration, in this case: 30 seconds