The hardness of a material is its ability to resist localized permanent deformation, penetration, scratching, or indentation. So, it is an important parameter in engineering. Hardness Testing provides a means to quantify the hardness of a material and it is a key element in many quality control procedures and R&D work. Several methods are available for hardness testing. However, Brinell, Rockwell, Vickers, Knoop, Mohs, Scleroscope, and the files test are the most widely used hardness tests. In this article, we will learn about Brinell Hardness Test, its procedure, related formula, and standards.
What is the Brinell Hardness Test?
The Brinell Hardness Test method is the most commonly used hardness measurement technique in the industry. In the Brinell Hardness Testing, the hardness of a metal is determined by measuring the permanent indentation size produced by an indenter. Harder materials will generate shallow indentations while the softer materials will produce deeper indentations. This test method was first proposed by Swedish engineer Johan August Brinell in 1900 and according to his name, the test is popular as Brinell Hardness Test.
Brinell Hardness Test Procedure
The Brinell Hardness test is performed in a Brinell hardness test unit. In this test method, a predetermined force (F) is applied to a tungsten carbide ball of fixed diameter (D) and held for a predetermined time period, and then removed. The spherical indenter creates an impression (permanent deformation) on the test metal piece. This indentation is measured across two or more diameters and then averaged to get the indentation diameter (d). Using this indentation size (d) Brinell Hardness Number (BHN) is found using a chart or calculated using the Brinell hardness test formula. The equipment used for Brinell Hardness Testing are:
- Brinell Hardness Testing Machine
- Indenter Sphere, and
- Brinell microscope to measure the generated impression.
Brinell Hardness Testing Machine:
The Brinell Hardness Testing Machine (Fig. 1) consists of a loading system that includes leavers, weights, hydraulic dashpot, and plunger enclosed in the body of the machine. The test material is kept on the adjustable anvil. Using the lever, the spherical ball indenter descends on the material with a pre-decided force that can be read on the screen.
For softer metals the force used is less than harder metals. The force value varies from 1 kgf to 3000 kgf. Common test forces range from 500 kgf often used for non-ferrous materials to 3000 kgf for steels and cast irons.
There are four sizes of the indenter used for the Brinell hardness test. They are 1 mm, 2.5 mm, 5 mm, and 10 mm in size.
To obtain the same BHN with different ball diameters, geometrically similar indentations must be produced. It is possible if F/D2 is maintained constant.
Brinell Hardness Test Formula
Once the average indentation diameter is measured the Brinell Hardness Number (BHN or HBW) can be calculated using the following Brinell hardness test formula:
Note that, the term HBW stands for Hardness Brinell Wolfram carbide.
Wolfram carbide (= tungsten carbide) underlines the use of tungsten carbide balls, as opposed to the (softer) steel balls previously used (HBS).
The minimum Test Specimen thickness is at least 10 times the indentation depth as per ASTM standard and the same is at least 8 times the indentation depth as per ISO standard.
Specifying Brinell Hardness Number
While specifying a Brinell hardness number (BHN or HB), the test conditions used to obtain the number must be specified. The standard format for specifying is “HBW 10/3000”. “HBW” refers to a tungsten carbide ball used as an indenter, as opposed to “HBS”, which means a hardened steel ball. The “10” is the ball diameter in millimeters. The “3000” is the force in kilograms force.
Sometimes, the Brinell hardness is also specified as “XXX HB YYD2” where
- XXX is the force to apply (in kgf)
- YY specifies the material type (5 for aluminum alloys, 10 for copper alloys, 30 for steels).
Thus the Brinell hardness of a typical steel could be written: 250 HB 30D2. The following image from wikipedia provides some typical Brinell Hardness Values of common materials.
Requirements for Brinell Hardness Testing
- Before the test, the sample must be cleaned thoroughly. Preferable if the test surface is machined, ground, and polished to get better indentation measurement.
- Proper indenter (Steel ball or carbide Ball) as per requirement need to be selected.
- The applicable force needs to determined beforehand.
- The load on the specimen to be maintained for the exact period of time.
- The indents must be positioned to keep sufficient clearance from the specimen edge and between the individual indents.
Standards for Brinell Hardness Test
The widely used standards for Brinell Hardness Test in industries are:
- ASTM E10
- ISO 6506
- JIS Z 2243
Advantages of Brinell Hardness Test
Brinell Test method has many advantages:
- The hardness of rough samples can be measured which is difficult in other methods.
- Application of high test load (up to 3,000 Kg) is possible.
- Wide measuring range due to availability of a range of indenter sizes and loads
- A Brinell hardness tester can determine the hardness of all types of metals.
- Provides reliable results.
However, Brinell test method has some disadvantages as well:
- There could be measuring errors due to using of optical instruments.
- Surface imperfections can interfere with the test result if the surface not prepared thoroughly.
- The requirement of a flat surface makes this test redundant for cylindrical surfaces.
Brinell vs Rockwell hardness Test
The main difference between Brinell and Rockwell Hardness Test is provided in the table below:
|Brinell Hardness Test||Rockwell Hardness Test|
|In Brinell Hardness Test the indenter is a spherical Tungsten Carbide Ball||For Rockwell Hardness Test the Indenter is a Small Steel Ball (HRB) or a diamond cone (HRC)|
|Hardness greater than 650 HB can not be measured with this set up.||No such limitation.|
|It measures the diameter of the indentation to calculate hardness value.||Depth of indentation is measured for calculating Rockwell hardness.|
|A comparatively slow method.||Quicker process.|
|Surface preparation required.||No surface preparation.|
|Comparatively costly||Cheaper option|
Brinell, Rockwell, and Vickers Hardness Conversion Table
The approximate conversion of Brinell, Rockwell B & C, and Vickers hardness is provided below for sample only. One more column indicating approximate equivalent tensile strength is also added.
|Brinell Hardness (HB)-3000Kg-10 mm Ball||Rockwell Hardness (HRC)-150Kg Brale||Rockwell (HRB)- 100Kg 1/16″ Ball||Vickers Hardness (HV)- Diamond Pyramid 120 Kg||Approx. Tensile strength (N/mm²)|