The hardness of a material is its quality to withstand localized deformation. This mechanical property is important when suitable material is needed for a specific environment where material wear can occur.
In practice, the wrong material in your project can lead to a disastrous effect, especially for larger projects. This is because hard materials have a higher chance of resisting indentations while soft materials suffer indentations. Hence, it is important to evaluate the hardness of the material you intend to use for your project before embarking on production. To understand material hardness, its measurement, and the comparison of different units, we have curated the most comprehensive article on material hardness.
Read on to learn more about material hardness.
What is Material Hardness?
Material’s hardness is the measure of its resistance to localized permanent deformation. In other words, permanent hardness is known as plastic deformation. This means that the material can withstand external force without indentation, penetration, and deformation.
For the benefit of understanding, an elastic deformation means that a material can only change its shape when a force is applied. On the other hand, a resulting plastic deformation means the material fails to return to its original form.
Types of Material Hardness
Since all materials used in CNC machining behaves differently under different condition, machinist categorizes hardness into three types. The value of the hardness of all materials falls under these three types. They include:
- Scratch Hardness
- Indentation Hardness
- Dynamic or Rebound Hardness
√ Scratch Hardness
This involves the ability of a material to withstand scratches on the surface when exposed to external forces. Scratches are continuous narrow indentations on the material’s surface due to contact with harder or sensitive materials.
A Scratch test is used for brittle material to confirm if the material in question possesses scratch hardness. Your material choice depends on its use. If its usage is in an environment where more rigid materials will brush through the surface on one another, then a harder material should be considered. The use of soft scratch hardness material may be disastrous on a long-term basis. The long-term effect of this is that the material loses its smooth surface, which may affect the overall functionality of the machine. To avoid this, a scratch hardness is performed to test if the selected materials can withstand frictions in the environment of use.
√ Indentation Hardness
Indentation hardness is a type of hardness that refers to the resistance of materials to permanent deformation when you subject them to a continuous load. It involves indenting the test material with a continuous load until it forms an impression. To perform this type of test, you can use both the microscopic and macroscopic scales based on the use and nature of the material.
In practice, when machinist refers to hardness, they talk about indentation hardness. The measurement of a material’s indentation value helps understand the amount of load it can carry before it deforms.
√ Dynamic or Rebound Hardness
Dynamic or rebound hardness is more elastic hardness other than plastic hardness. In the contest of elastic hardness, a material does not deform permanently. What happens is that they lose shape when you apply external force. Once you remove the external pressure you applied, the material gains its original form. This is different from the case of plastic deformation, in which material cant regain its original shape.
Here, machinists make use of an indenter to test material for rebound hardness. This is because it is the material to absorb the energy on impact while returning it to the indenter. So, the material’s hardness is then compared with that of the hardness of the indenter. A way to do this is by dropping a diamond-tipped hammer on the material to measure your material’s rebound or dynamic hardness. Note that the material’s rebound hardness is high if the hammer returns closer to the original dropping height. On the other hand, if the hammer does not return closer to the dropping height, the material’s rebound hardness is low.
Different Forms of Hardness Units
This section of this article will help you eliminate the confusion in the measurement of hardness. It involves the use of different measurements with their SI units. Other types of hardness explained in the previous section have a different scale of measuring hardness. While the SI units are derived from different measurement techniques, they are not suitable for direct comparison.
Hence, the method of measurement of scratch, rebound, and indentation hardness varies. For example, Brinell, Knoop, Meyer, Rockwell, and Leeb’s hardness values are different for material. While these units are derived from other measurement techniques, they are not suitable for direct comparison. However, there is a conversion table that you can use for comparison between different means of measurements.
There are varieties of units for the measurement of hardness; they include:
- Rockwell Hardness
- Brinell Hardness Test
- Vickers Hardness Test
- Scleroscope Test
- Mohs Hardness Test
- Knoop Hardness Test
- Case Depth Hardness Test
Measuring Material Hardness
√ Rockwell Hardness Test
Rockwell Hardness test features over 30 different scales to choose from: hence you can select any suitable one for your material. The most commonly used scale for this type of test is “B” and “C.”
The test involves the application of a minor load before the application of the testing load. The essence of the nominal load is to fix the indenter into test samples to eliminate all forms of irregularities on the material’s surface. This will help to achieve a more accurate result during the test process.
Next is the application of the major load to create an impression on the material. The impression is then measured to determine the hardness. This is done by calculating the hardness value via the increased indent dimension using the formula below:
N is the scale factor based on the scale used
d is permanent indentation depth in comparison to minor load (mm)
s is the scale factor based on the scale used
√ Brinell Hardness Test
This test is one of the most widely accepted hardness tests used by a machinist. Here a ball of 10mm is used as an indenter to make an impression on the material to help calculate its Brinell hardness value. The ball is held in place for a few seconds, usually 30 seconds, while a force is applied. The force you will apply depends on the test material.
In practice, the standard load of 3000kg is used and can be reduced to 500 kilograms for softer materials. For harder materials, tungsten carbide can be used to prevent the distortion of the ball. In such a case, the hardness unit HB will be converted to HBW to notify the use of tungsten.
Once the indenter is removed, the dent is observed with a low-power microscope. Then the size is calculated by measuring at right angles. It is calculated as follows:
F is force, N
D is the diameter of the indenter, (mm)
d is the indentation diameter, mm
√ Vickers Hardness Test
Vickers test is another way of measuring material hardness. It is suitable for softer materials that require fewer loads. It is most accurate for soft materials.
To do this, place the part onto the machine while the microscope is used to find the suitable height. The correct place is determined using the images. The diamond indenter is made in the form of a four-sided pyramid. The machine reaches the predefined force value after touching the part. While it stays at the same load for a certain time, the measuring of the indentation takes place. It is calculated as follows:
F is Force, N
d is indentation diagonal, mm
√ Scleroscope Test
This is a device used to measure the dynamic or rebound hardness of a material. The device features a hollow vertical glass tube connected to a stand. A diamond is dropped onto the material through this tube while the bounce of the hammer is recorded. The bounce for a soft metal is lower as a portion of the impact energy is exhausted in creating a dent in the material. The hardness is measured with the shore unit as the glass tube has a gradient to measure the bounce height.
√ Mohs Hardness Test
This is a test for the scratch hardness of a material. To do this, a numerical value is given to the material based on the test result. This test uses ten reference materials featuring varying hardness in the form of a scale. The softest material used is Talc with a value of 1, while the hardest material is diamond with 10. Please note that the Moh’s scale lacks accuracy, and it gives a rough estimate of hardness.
√ Knoop Hardness Test
This test is otherwise known as the microhardness test method used for thin sections or small parts. This test specifies a range of light loads using a diamond indenter. This indenter is used to make an indentation that can be measured and converted to a hardness value. It is applicable in testing a wide variety of materials as long as they are carefully prepared.
√ Case Depth Hardness Test
Case depth means the thickness of the hardened layer on a specimen. Also, case hardening help to improve both the fatigue strength and wear resistance of material under thermal or dynamic stresses. Hence, a case depth test involves performing a range of hardness impressions from the edge of the test material towards the center. The distance from the surface straight to the hardness limit is calculated from the harness progression plotted on a graph.
Consideration For Hardness Test
For an effective hardness test, the following should be considered:
Hard material may require a test load of 3,000 kg, while soft materials will only need a test load of 500kgf. The higher the load used, the higher the accuracy of the test
Range of Hardness
This factor dictates the test material of the indenter. For example, if you are working on a Hardness material of over 650 HB/30, a diamond indenter is needed. For an object with a value less than this, steel or hard metal indenter is effective.
Level of Accuracy
This depends on the surface to be tested. Consider factors such as texture, cleanliness, roughness, and many more of the surface material.
The adaptability of the device
The size and shape of the test material should be considered when selecting a hardness tester.
Across varieties of industries, including aerospace, auto manufacturing, construction, oil, and gas, to name but a few, testing the hardness of the material is very important. This is to ensure the material to be used will not critically fail down the line over time. Hence, a proper material hardness test is of utmost importance to determine the most suitable material for your project. Based on our experience, we offer varieties of hardness tests to help achieve your long-term goal. Request a quote today – we a just a step away!