What is Hardness Test?
Hardness testing is the method for determining the hardness of any material.
The hardness of a material is defined as its ability to resist deformation or indentation under an applied force. Hardness testing is an important step in engineering industries as it helps to understand the suitability of materials for specific applications.
It helps to produce the best quality products which meet the prescribed safety standards and regulations.
How to Choose a Hardness Test?
Choosing the ideal hardness testing method is important for getting accurate and reliable hardness testing values. Some of the key factors which decide the type of hardness test method are discussed below:
Specimen Hardness Range
The hardness range of the specimen for which hardness is to measured decides the hardness testing method. If the specimen hardness is more than 650 HB/30, then diamond indenter should be used for hardness testing.
Diamond indenter cannot be used under the Brinell method; hence the method is not suitable for measuring the hardness of such specimen.
Rockwell method, on the other hand, allows the use of diamond-tipped indenter cone. Vickers method uses only diamond indenter and can be used to measure the hardness of any type of specimen.
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The test load is also an important factor to decide the hardness testing method.
If higher test loads are to be applied, then microhardness testing methods like Rockwell and Brinell testing should be used. If smaller test loads need to be used, then microhardness test methods like Vickers and Knoop tests should be used.
The smaller the specimen size, the lighter the test load needed for measuring hardness.
Hardness testing for small sized specimens or specimens which are too thin should be measured using microhardness testing methods like the Vickers or Knoop tests.
When testing the hardness of smaller specimens, it becomes even more important to ensure the minimum thickness criteria for getting reliable hardness values.
The specimen should also be clamped using an anvil for getting accurate hardness readings.
Hardness testing method can be selected based on your budget also.
Brinell and Rockwell hardness testing methods are a basic and cost-effective way of hardness testing of specimens. Vickers method requires more time and cost as the specimen surface has to be prepared before testing.
It also requires advanced and modern optical systems for measuring the depth of the indentation made on specimen surface for determining hardness values.
This makes the Vickers method costlier than other methods of hardness testing.
How is Hardness Measurement More Convenient than a Tensile Test?
Tensile testing is used to study the behavior of the metal when it is being pulled under an applied force.
It helps to measure yield strength, proof strength, and ultimate tensile strength. Hardness testing is used to measure the hardness values for different types of materials. The key differences between hardness testing and tensile test are as follows:
It is much easier to measure the hardness value of materials using hardness testing compared to measure the tensile strength.
The procedure for hardness testing is also quicker compared to the tensile test.
Preparing the specimen for conducting tensile strength is more complex and time-consuming than the specimen preparation in hardness testing as the specimen has to be made to proper shape and size.
Not only is conducting hardness testing more convenient, but it is much cheaper also compared to tensile testing. The cost of a calibrated hardness tester is only a fraction of the cost of a tensile tester.
Effect on Specimen
Hardness testing is different from tensile testing as it is a non-destructive testing method. It makes shallow penetrations on the specimen surface using an indenter and the specimen is not destroyed.
The specimen can be used for other purposes after completion of the hardness test.
Tensile testing is a destructive method as it applies force on the specimen to pull it apart till it breaks. The specimen is destroyed after conducting the tensile test and can not be used for other purposes. This non-destructive nature of hardness testing also makes it a more cost-effective test method compared to the tensile test.
How Does the Hardness of Steel Change After Tensile Test?
Tensile strength represents the characteristic of a metal when force is applied for pulling it. As long as the force is lower than the yield point, the steel specimen will undergo elastic deformation, which means it will return back to its original shape. When the force reaches the yield point, plastic deformation takes place which is irreversible.
There is a change in mechanical properties and hardness of steel after conducting a tensile test. When a tensile test is conducted, it can lead to grains fragmentation and lattice reconstruction of the crystal lattice.
The steel specimen becomes elongated due to the result of tensile or compressive stress.
The hardness of steel increases after conducting a tensile test on it. Grains fragmentation and austenite deformation take precedence over martensite transformation during a tensile test of steel.
There are also changes in grains shape, dimensions, orientation, fragmentation, and other parameters.
What Hardness Test is for Stainless Steel?
The hardness of stainless steel can be understood as its ability to resist indentation or deformation on its surface. The higher the hardness, the higher is its resistance to deformation, indentation, scratches, etc. on its surface. There are different methods for testing the hardness of stainless steel.
The most common methods involve testing the hardness by a diamond-tipped indenter. The hardness of steel can be measured by Rockwell, Brinell, or Vickers test.
In the engineering industry, steel hardness is important to measure as the ability of steel to withstand friction, abrasion, stress, etc. increases with the hardness of steel. The higher the hardness of stainless steel, the more resistant its surface will be.
Brinell test was the first method used commonly for measuring the hardness of stainless steel. This method uses a hardened steel or carbide ball with 10mm diameter and test load of up to 3000Kgf.
Due to higher test loads, the Brinell method is used for measuring the hardness of hardened steel. It requires a large specimen and leaves a large indentation on the specimen surface which limits its usefulness.
Rockwell method of hardness testing is also based on measuring the depth of indentation made on the specimen surface.
On the Rockwell scale, the most commonly used scales used to measure the hardness of stainless steel are B and C. Rockwell hardness testers are lighter and more mobile than Brinell hardness testers.
Vickers testing method is also used for hardness testing of stainless steel. It uses a diamond-tipped right pyramid indenter. The Vickers test can be used for measuring the hardness of all types of stainless steel.
Each of the hardness testing methods provides a particular hardness value which needs to be converted using a conversion table to get the hardness of stainless steel.
How to Test Hardness of a Mineral？
The hardness of minerals is measured using the Mohs scale which was developed by the German mineralogist Frederick Mohs. It was invented as back in the 19th century, there was no means of determining the hardness of minerals.
Mohs hardness scale is a comparative scale for minerals where the softest mineral is talc which is placed at a hardness level of 1 and the hardest mineral diamond is placed at the highest hardness level of 10. Since diamond is the hardest mineral in the world, it can scratch every other mineral on Earth.
Due to this property, it is widely used in rock drills, cutting equipment, etc. All other minerals are placed between these two values relative to their hardness. Mohs hardness scale is based on the principle that a harder mineral will scratch a mineral which is softer to it. Talc which is the softest mineral can be scratched by all other minerals.
Similarly, glass which has a Mohs hardness value of 5.25, can scratch minerals placed lower to it on the Mohs scale (talc, gypsum, calcite, fluorite, and apatite) but it can’t be scratch orthoclase, which is placed higher than it.
To measure the hardness of minerals, select a fresh surface of an unknown mineral and scratch it with a mineral of known hardness.
You should press the mineral of the known hardness firmly against the mineral with unknown hardness.
If the test object is harder than the mineral with unknown hardness, then it should leave a clear scratch on its surface. If there is no scratch or line on the surface of an unknown mineral, then it is harder than the tested mineral for which the hardness is already known.
In the current times, the Mohs hardness scale is used by mineralogists and gemologists. It has relatively limited use for metallurgists.
How to Test Steel and Metal Hardness？
Hardness for steel and other metals can be measured using different methods. The Rockwell test is the most basic method for determining the hardness of steel and other metals.
A steel ball or diamond indenter is used to make an indentation on the specimen surface under an applied test load. A minor load is applied to the indenter and is pressed on the specimen surface, after which the major load is applied. The major load is then removed keeping the minor load applied.
The indentation made is measured and converted into a hardness value. Brinell testing is the oldest method used for hardness testing of metals.
A steel ball indenter is pressed on the specimen under a large test load which can go up to 3000Kgf. It makes a clear indentation on the specimen surface, which is then measured and converted to hardness value using conversion charts. The Brinell method is used for metals which have a coarse surface for other types of hardness tests.
Vickers test is microhardness testing method used for measuring the hardness of thin and small specimens. This method uses a diamond-tipped pyramidal indenter to make an indentation on the specimen surface.
The indentation is measured using advanced optical systems which ensure better accuracy and reliability of hardness testing.
The Vickers testing method is costlier compared to Rockwell and Brinell methods. It is also time-consuming as it takes 30-60 seconds for measuring the hardness of the specimen excluding the time needed for surface preparation.
As a result, the Vickers hardness testing method is not suitable for bulk testing on the production line, rather it is ideal for laboratory testing.
What is Non-Destructive Hardness Test？
Non-destructive hardness testing refers to assessing the hardness of materials without making any changes to its characteristics.
The non-destructive method is important as it does not destroy or renders the test specimen useless after the hardness testing is completed. This ensures that the specimen can be used for other purposes after completion of hardness testing.
Non-destructive hardness testing also takes lesser time and is cheaper to conduct than destructive testing as the specimen is not destroyed and can be reused. Microhardness testing is a good example of non-destructive hardness testing. Most portable hardness testers are able to conduct non-destructive hardness testing of the specimen.
The portable hardness testers can measure the hardness of specimen surface on a variety of materials on flat, round, thin, or large surfaces.
Unlike the regular hardness testers, these portable testers use ultrasonic technology which allows you to measure the hardness of thin and small-sized specimen. These ultrasonic hardness testers can conduct non-destructive testing of the specimen.
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This feature allows the operator to conduct hardness testing of highly sensitive or finished parts.
The non-destructive nature of this hardness tester also allows the operator to do hardness testing without causing any type of physical or structural damage to the specimen which would have otherwise led to its disposal.
Rebound testing using the Leeb method is another type of non-destructive hardness testing.
Under this method, the impact device makes use of a spring to propel an impact body towards the surface of the specimen. The impact body rebounds after making an impact on the specimen surface.
The testing instrument determines the hardness by using the ratio of voltages and further analyzes their phases to offset for any changes in orientation.
Leeb testers are portable and can be used at different angles as long as they are perpendicular to the test surface.