The hardness tester is a device which is used to determine the hardness of a material.
The hardness is determined by measuring the effect of penetration on the surface of the specimen by a standard indenter.
There are different hardness testers available in the market like Rockwell hardness tester, Brinell hardness tester, Vickers hardness tester, etc.
Why Hardness Testing is Important for Product
Hardness testing plays a very important role in material testing, quality control, and ensuring that the final product meets the regulatory compliances.
It allows determining the suitability of materials for specific uses and the amount of treatment which they can be exposed to. It provides data pertaining to structural integrity and quality of components.
Hardness testing is very popular in R&D labs and industrial applications. It helps to ensure that the materials being used to manufacture products are suitable for doing so and meet the quality and safety standards for that particular use.
Hardness testing ensures that the final product is of the highest possible quality.
Industries that Use Hardness Testing
Hardness testing is widely used in many industries. Some of the major industries which make use of hardness testing are as follows:
Mechanical Engineering Companies
Hardness testing plays a very important role in the mechanical engineering industry.
It is widely used in different mechanical engineering companies ranging from the production of agricultural equipment, construction machine, or a machine tool, cranes and car wash installations, etc.
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Specimens on which hardness testing is done can vary widely from small applications like hardness testing of gear wheels in gear motors to large applications like testing of welded joints on components of a car wash or gear wheel for a wind turbine which may be several meters in diameter.
In the mechanical engineering industry, normally the same parts need to be tested for hardness, hence, the process is generally automated and can be unmanned also in case of total automation of the process.
Hardness testing finds frequent application in the automotive industry. In fact, the automotive industry has perhaps the widest range of applications for hardness testing.
There are many moving parts in an automobile which needs to be tested for hardness and reliability for ensuring complete safety of the passengers.
The test specimens in the automotive industry can range from small screws, door panels, engine components, transmission parts, welded joints, etc.
If a large number of same parts need to be tested daily or some in-process tests like CHD measurements need to be carried out, then the hardness testing process should be automated to save time and resources.
Primary Metals Industry
The primary metal industry also has numerous applications for hardness testing. Hardness testing is important in the primary metal industry as it helps to ascertain the metal’s suitability for specific applications and the kind of treatment which it can be subjected to.
It also helps to understand the examining the material structure of the metal and whether it complies with specific regulations.
Generally, a specimen is extracted or cut out from the center of the primary metal and tested for hardness. Hardness testing in the primary metal industry is conducted in the low-load, micro and macro ranges, as per the quality assurance required.
Foundries use hardness testing to control the quality of the manufacturing process during production. It is also commonly used to ascertain and measure the properties of cast metals and know their suitability for different applications.
Hardness testing is non-destructive in nature meaning it can be conducted on the cast metal directly instead of the test piece. Tensile strength of the metal is determined by using hardness testing.
Generally, the castings to be tested are large and heavy and the test forces involved are mainly in the macro range, which is mostly within the scope of Brinell tests.
Rockwell Hardness Tester
The Rockwell hardness testing method allows you to directly read the hardness value from the specimen.
Unlike Brinell or Vickers hardness testing method, there is no need for optical reading for determining the hardness reading.
The instruments working in compliance with the Rockwell method are more popular as they are not affected by the operator’s influence.
This method is also the least influenced by the surface roughness of the specimen. The testing cycle is short and quick under this method and it is also cost-effective.
The cost-effectiveness is mainly due to the fact that Rockwell hardness testers don’t need to be connected to sophisticated optical systems for hardness testing like in other methods.
This method of hardness testing is non-destructive and the specimen can be used for other purposes after testing.
The drawback of Rockwell hardness testing is that it is not the most accurate testing method. A slight discrepancy in measuring the depth difference can result in considerable deviations in the calculation of the hardness value.
The test location and specimen must be completely free from any contamination to derive accurate and meaningful results from this method of hardness testing.
This method is also known to be inaccurate as indenter has unknown effects on the results. If the indenter is worn out and is not sufficiently acute, then it may impact the test results.
Using the Rockwell method has another flipside that with increasing hardness, it becomes difficult to differentiate between metals.
Brinell Hardness Tester
The Brinell method for hardness testing can be used for testing non-homogeneous materials as the large ball comes into contact with many crystals which forms a mechanical average value.
You can get good precision, repeatability, and good representation using this method.
The impression or indentation made by the Brinell hardness tester is sufficiently large to provide a fair and good estimate of the hardness of the specimen.
It is not affected by soft spots and small hard spots. It can give you reliable hardness measurements even if the surface of the specimen is rough.
The Brinell method requires surface preparation unlike the Rockwell method and is a bit more complicated. The surface quality of the specimen needs to be good as the indentation is measured optically.
When the specimen is being tested with high test load, then there are good chances of deformation of the surface of the specimen when testing is done in the macro range.
This can lead to errors in the measurement of hardness value due to wall formation. The method has limitations in application to thin specimens of very hard materials.
The brinell method is slow compared to the Rockwell method, and it takes about 30-60 seconds to measure the hardness, excluding the time needed for preparing the specimen.
Vickers Hardness Tester
The biggest advantage of using the Vickers method is that it can be used to measure the hardness of all kinds of materials as the procedure covers the entire hardness range.
Due to this, the method is more commonly used in a laboratory setup.
Under the Vickers method, there is only one type of indenter which can be used to measure the hardness of different material types and specimen.
The hardness value derived from Vickers method has a proper meaning, as it corresponds to a specific load on an indentation having the same shape.
The Vickers method is non-destructive; thus, the specimen can be used for other purposes after testing.
The surface quality of the specimen to be tested should be good as the hardness is measured optically. This means that the test surface should be prepared, otherwise the readings will not be precise. The test area should be polished and prepared and the perpendicularity of the indenter axis plays an important role in measuring hardness as any inclination will give an irregular indentation. The Vickers method is also slow compared to the Rockwell method. Vickers method is not recommended for use in production lines.
Knoop Hardness Tester
The Knoop hardness tester can be used to measure the hardness of all materials from soft to hard as the procedure covers the entire range.
It is more accurate than the Vickers method as the Knoop measurement diagonal is longer for any indentation depth. The damage to the specimen surface while testing is less than the Vickers method which makes it a non-destructive testing method.
The Knoop method is ideal for testing small and longish specimens, very thin layers, and brittle materials like glass and ceramics.
The Knoop method requires good surface quality due to the measurement being done by optical methods. The use of optical systems makes the Knoop method slower and costlier compared to the Rockwell method of hardness testing.
Things to Consider When Choosing a Hardness Tester
Selecting the perfect hardness tester can be a difficult task even for the most experienced testers.
There are different types of the specimen to be tested and different testing procedures which make it difficult to narrow down on the best hardness tester.
The following considerations can help you when choosing a hardness tester.
Total Test Load
The total test load to be tested is a very important consideration for choosing a hardness tester. There is a general rule that the higher the test load, the better and more accurate are the readings.
This is due to the fact that hardness testing measurements are less sensitive to surface texture with higher test loads. However, it is also important to note that the indentation should not be deeper than 1/10th of the total thickness of the specimen or the hardened surface which has to be tested.
The homogeneity of the specimen is also an important criterion for selecting the ideal hardness tester.
You should consider the hardness range of the specimen you want to test before choosing a hardness tester.
Generally, if the hardness of the specimen is above 650 HB/30, then you should use a diamond penetrator. Hardness below these levels can be tested by using penetrators made from steel or other similar hard metals.
The Brinell method of hardness testing does not allow you to use diamond penetrators, thus, can not be used to measuring the hardness of specimen above 650 HB/30.
The Rockwell method, on the other hand, is more universally accepted as it allows you to use diamond cone and steel ball penetrators. The Vickers method only allows to use diamond pyramid penetrator and can be used to measure the hardness of any range.
Accuracy is a very important consideration for choosing a hardness tester. If the tester is unable to provide you accurate measures of hardness, then it can not be relied upon to take further decisions. You should always prefer hardness testers which are more accurate and reliable.
The precision of measurement while hardness testing is mostly dependent on the accuracy employed by the operator. This also includes well-ground surfaces, sufficient measurement periods, and frequent revisions of the testing device. Wherever possible, you should prefer static systems over dynamic systems.
Testing loads can also influence the accuracy of your hardness testing device. Using low testing loads can lead to inaccurate and unreliable measurements.
The flexibility of the Device
Before selecting a hardness tester, you should thoroughly analyze its flexibility with regards to different forms and dimensions of the specimen to be tested.
For hardness testing, either the specimen can be put on the device or the device can be put on the specimen.
The first instance relates to stationary devices which have sufficient capacity to hold the specimen for hardness testing. Stationary hardness testing devices are best for testing small and medium-sized specimens.
In case of large and heavy specimens, portable hardness testing devices can be clamped to the specimens. These portable hardness testing devices can only be dynamic when they are using high testing loads. When the testing load is smaller, then they can also be static.
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The costs associated with hardness testing is also an important consideration for selecting the ideal hardness tester for your business.
Different parameters like the cost of the testing device, the applicability of the testing to various situations, measurement period, and the technical expertise needed to operate the hardness tester should be considered before selecting it.
These considerations become even more important when specimens of varied forms and testing surfaces need to be tested. In most technical industries this is generally the case.
In industries which do their hardness testing serially, the quickness of the tests and the scope to deploy unskilled staff are also important considerations for selecting the hardness tester. In such situations, testers which do not require special clamping equipment are preferred.