Brinell hardness test
Brinell hardness test dome impression
The industrially applicable Brinell hardness test goes back to the Swedish engineer Johan August Brinell. He developed the hardness testing method named after him and presented it at the World Exhibition in Paris in 1900. The method is standardized according to DIN EN ISO 6506-1 to 6506-4. The Brinell method is particularly suitable for the following materials: soft and medium-hard materials as well as materials with an inhomogeneous or porous structure (shrinkage cavities in cast iron). The large sphere surface "bridges" defects and thus forms a "mechanical average". In contrast, the small tip of a Vickers diamond pyramid could penetrate a pore - the result would be grossly distorted.
- unalloyed cast iron, low-alloy steel / structural steel
- Aluminium alloys, non-ferrous metals: brass, copper…
- inhomogeneous structures (here almost all other metal hardness testing methods fail)
- sintered metal (not sintered)
- wood (ISO 3350)
Video Animation Functional Principle
Brief description of the Brinell hardness test
In this hardness testing method, a hard metal ball is pressed onto the surface of a test piece with a defined test force (F) and penetrates to different depths (depending on the hardness). The resulting ball impression cap is optically measured in terms of its diameter. The hardness value HBW Hardness Brinell tungsten carbide (ball) can be calculated using the formula below.
After a loading time of 10 to 15 seconds (steel and iron) or 10 to 180 seconds (non-ferrous metals and their alloys), the test force is reduced. Adhering to the loading time is important in order to take the flow behavior of the material into account. Every material has a flow behavior (soft materials have a higher flow behavior): Over time, the indenter can penetrate deeper into the material until the penetration process comes to a standstill or the material is no longer compacted.
The surface of the impression is determined by optically measuring the resulting impression diameter in the workpiece. The diameter d to be determined is calculated from the average of two measurements rotated by 90 degrees (d1+d2)/2. If the impression is not circular, the diameter required to calculate the hardness is averaged from the largest d1 and smallest diameter d2.
Previously, a steel ball was used as an indenter (HBS – Brinell hardness steel ball). Since 2006, a hard metal ball (tungsten carbide) has been required (HBW hardness Brinell tungsten carbide). Depending on the homogeneity, material thickness and hardness, balls with different diameters are used: Ø 10 | 5 | 2,5 | 2 | 1 | 0,625 mm. Which test method (combination of ball diameter and test force) must be used depends on the different required load levels (see table below). For example, a load level of HB30 (force ratio in relation to the ball surface – formula F/D²) is to be used for steel and a load level of HB10 / HB5 / HB2,5 / HB1 for softer non-ferrous metals. Load level HB10/3000: ball diameter 10 x 10 = 100 x load level 30 = 3000 (kg).
The load level is calculated from the force divided by the square of the ball diameter (F / D²) and enables the correct choice of test load or test method. If too high a load level (related to the hardness of the material) is selected, the Brinell ball would be pressed too deeply into the material - an evaluation is no longer possible or would be incorrect (no hole should be "punched" but a dome impression should be created)
At the time the Brinell hardness test was developed, the test force was still given in kg or kp. In order to ensure that the hardness values, conversion tables and material specifications were valid, the test forces were retained - the test methods retained their names, which are based on the test force in kg (kgf). The load forces given in the table (after the slash) give the test forces in kgf. Since the conversion of the test forces into Newtons results in odd values, the metrologically correct specification in Newtons is omitted in this table.
Special features of the Brinell hardness measurement
The measurement of Brinell hardness indentations is more complex than the optical methods according to Vickers or Knoop, since the relatively large balls cause a large amount of material to be displaced. This effect is greater the softer the material. A crater-shaped edge is created, which, depending on the lighting and the crater bulge, reduces/enlarges the measurable indentation edge to a greater or lesser extent. The light-dark boundary moves into the crater or is visible at the crater edge.
A first significant improvement was achieved by changing the type of lighting. Instead of the usual lighting through the lens, a ring light or a mixture of ring light (dark field lighting) with some lens light is now used as standard.
The manufacturer INNOVATEST went a step further and integrated AI – Artificial Intelligence – into the Brinell hardness testing machines or in the IMPRESSIONS testing software. For this purpose, thousands of Brinell test impressions were collected, recorded by the testing software and compared with cap impressions measured manually by the operator. The integration of human intelligence into the AI module of this software leads to increased accuracy and relieves the material tester of the manual correction of the measuring lines. This development, among others, has made INNVOATEST an innovation leader.
Standard-compliant specification Brinell – hardness test
According to DIN EN ISO 6506-1 from 03/2006, the hardness value, the method, the diameter of the ball and the test force must always be specified.
Example: 246 HBW 10/3000
This means:
246 = hardness value
HBW = Brinell hardness with tungsten carbide hard metal ball
10 = ball diameter D in mm
3000 = test load (force F) in kg or kgf
If the test load is applied for >15 s, the duration of the load must also be stated.
Example: 246HBW10/3000/60
The not or low-alloyed For steels, the Brinell hardness can be converted into tensile strength (Rm in N/mm² or MPa) with a certain tolerance. The factor Rm x3,5 can be used for this purpose.
Brinell standards:
- European & international: EN IS
- American: ASTM E10-08
|
Example |
load level 30 |
HBW10/3000 |
test force 3000 kp – ball diameter 10 mm |
3000 / (10 x 10) |
3000 / 100 = 30 |
|
degree of stress |
30 |
10 |
5 |
2,5 |
1,25 |
|
ball Ø 10 |
HBW 10/3000 |
HBW 10/1000 |
HBW 10/500 |
HBW 10/250 |
HBW 10/125 |
|
ball Ø 5 |
HBW 5/750 |
HBW 5/250 |
HBW 5/125 |
HBW 5/62,5 |
HBW 5/31,25 |
|
ball Ø 2,5 |
HBW 2,5/187,5 |
HBW 2,5/62,5 |
HBW 2,5/31,25 |
HBW 2,5/15,625 |
HBW 2,5/7,8125 |
|
ball Ø 1,25 |
HBW 1,25/46,875 |
HBW 1,25/15,625 |
HBW 1,25/7,813 |
HBW 1,25/3,906 |
HBW 1,25/1,953 |
|
ball Ø 1 |
HBW 1/30 |
HBW 1/10 |
HBW 1/5 |
HBW 1/2,5 |
HBW 1/1,25 |
|
ball Ø 0,625 |
HBW 0,625/11,72 |
HBW 0,625/3,906 |
HBW 0,625/1,953 |
HBW 0,625/0,977 |
HBW 0,625/0,488 |
|
Materials |
Steel |
Al Aluminum > 55 HB |
Al Aluminum |
Al Aluminum |
Pb lead |
The table only lists the most common methods. Other methods are possible but are not common due to the poor comparability of the results.
Optical measurement: The measurement of the ball impression diameter (spherical cap) used to be carried out using a microscope with low magnification (14 to 100 times - depending on the ball diameter and test force). In Europe, in the middle/end of the 20th century, it became common practice to Hardness Tester built-in optics projected the image of the spherical cap from behind onto a ground glass screen. The enlarged impression could now be easily measured on the ground glass screen (similar to a profile projector).
Initially, a ruler was used to measure the magnification of the lens used. However, this particular ruler did not have a mm graduation: for example, if a lens with a 20x magnification was mounted, the ruler's scale was divided by this value. So, with a screen diagonal of, say, 60 mm, you could read the true size of 3,0 mm directly. To determine the hardness, the average of the true diagonals was used and the hardness could be read from a table, which was faster than calculating it using the formula shown below.
Later, the diagonals were measured using quite expensive electronics and a measurement calculator then displayed the measured value.
At the beginning of the 90s, our company developed a solution that used a simple digital measuring device similar to a caliper (SHP150). This invention was copied by almost all competitors because it was not patentable. Today, the use of a video camera with automatic image processing (image analysis software) is state of the art.
Originally, the test force application using a lever arm and weights hanging from it. Depending on the required test force, weights could be removed or added.
Less frequently, systems with leaf springs and leaf spring packages (which were also outdated) were used and had to be replaced. In rare cases, the springs aged - the accuracy of the test force application was no longer guaranteed.
Machines were also built (particularly in the USA) where the test forces were applied hydraulically. However, these machines usually did not have an integrated optical measuring system, but were used exclusively for force application. The measurement had to be carried out externally with a portable measuring magnifying glass respectively.
For test pieces that are too large or too heavy to be placed on the test table (anvil) of the hardness testing machine, portable test clamps similar to a screw clamp are still used today. After the test force is applied via a spring and eccentric (small test forces) or via a hand crank with gear (up to 3000 kgf test force). In this case, the operator must observe the force on a pointer instrument and stop at the desired test force. For portable test clamps, there is also a system in which the test force is applied hydraulically via a hand pump and is also observed via a pointer instrument.
Portal systems or radial stands are also used in heavy industry. Here, the test object, which can be huge and weighs several tons, is first placed down using a crane. A portal then moves or a radial arm with the flanged hardness tester swings over it.
In today's stationary hardness testing machines (also portal and radial hardness testing machines), it is state of the art that a force measuring cell with motorized delivery is integrated into the machine. The reasons for using this technology are
- Flexibility – hardness tester from 1 gram to 3000 kg in one machine (multiple force transducers)
- Lower manufacturing costs compared to extreme manufacturing costs of e.g. 50 weights
- Error minimization - Closed-loop electronics detect force deviations and regulate the forces
- And last but not least, the ease of use with turret head (tool changer), several indenters and lenses – up to hardness tester – "all-rounder": Universal hardness testing machines for Brinell, Vickers, Knoop and Rockwell and motorized XY cross table for unmanned series testing.
A little mathematics to calculate the Brinell hardness: The Brinell hardness is calculated from the ratio of test force to indentation surface. The test force in Newtons is divided by the value 0,102 (reciprocal of 9,81 = factor for converting Newtons to kgf). In this way, measured values from current metrology (for which a force specification in Newtons is generally required) correspond to the measured values determined using the currently invalid metrological unit for force (kg or kgf). A change to even values in Newtons (factor 9,81) does not make sense, as otherwise all existing hardness testing machines, evaluation algorithms, measured values, etc. would have to be converted.
To calculate the Brinell hardness, the force F in N, the ball diameter D in mm and the average indentation diameter d in mm must be entered in the formula below. The value in the denominator is the result of the formula for calculating the surface area of the ball indentation produced (spherical cap).
NEMESIS 9600 Heavy Duty Brinell
