Introduction: What is Brinell Hardness Testing
The Brinell method is one of the most popular and widely used techniques for measuring the hardness of a wide variety of specimens.
The method has been named after J.A. Brinell who was its inventor in 1900. Hence, the Brinell test is the oldest method of hardness testing.
The test loads applied under this method is variable which adds to its versatility and makes it ideal for different types of specimens.
It can be easily used to measure the hardness values of nearly all types of metals and also of other material types.
Under this method, a test load force of 3000Kgf and a spherical carbide ball of 10mm diameter is used as the indenter.
In case of measuring hardness for softer metals and alloys, the test load force can be reduced to 500Kgf and the indenter ball can be of diameter 5mm to 10mm.
In Europe, the Brinell test is often used for small applications wherein a test load force of 1kgf and a spherical indenter of 1mm is used.
The test makes use of small parameters in Europe and is often also called as Baby Brinell tests there.
Why Use Brinell Hardness Testing
Hardness testing is an important part of any engineering-based company.
It helps to determine the structural integrity and suitability of the specimen to be used for specific applications.
It enables companies to make use of the best quality raw materials after conducting its hardness tests and being sure of its suitability for the said application.
It helps to meet the safety compliances issued by a different government or relevant authorities by ensuring the best quality of products is manufactured.
Hardness testing also provides a fair idea of the degree to which a particular material can be subjected to treatment for being used for particular applications.
The procedure of Brinell Test
The Brinell method of hardness testing is used to measure the hardness of specimen which is generally too rough or course to be measured by other methods.
This method generally uses a higher test load of 3000Kgf and a 10mm indenter diameter.
The indentation made by using this method evens out the specimen surface and any other sub-surface inconsistencies.
The pre-determined test load is applied to a spherical indenter which is held on the specimen surface for a fixed time (generally 10 seconds) and then moved.
The depth of the indentation made on the specimen surface by this process is then measured with advanced optical instruments across at least two diameters.
A guide chart is then used to convert the averaged diameter to the corresponding Brinell hardness value.
The most common test loads used are up to 3000Kgf for ferrous alloys and steel.
For softer metals, test load force of 500Kgf is used.
Sometimes test loads as low as 1Kgf are also used, but it is a rare thing in Brinell hardness testing.
During hardness testing by the Brinell method, the depth of the indentation on the specimen should be measured very carefully.
Inconsistencies in measuring it are the biggest source of error while measuring hardness using the Brinell method.
There can be disparities by the operator in measuring the indentation which can lead to errors in hardness values.
To address this issue, automatic optical Brinell scopes have been developed which makes use of advanced optical systems to measure the indentation.
This helps to remove the human error and subjectivity due to operator measurements and standardizes the measurement of the indentation.
This provides more accurate and reliable hardness testing values.
However, using such optical systems also increase the complexities and costs associated with hardness testing using the Brinell method.
If the test surface is not flat or if it is covered with thick scale, then the hardness values will be unreliable.
Other instances where the Brinell testing method will provide erroneous and unreliable measurements are if the test specimen surface is too thin- less than 9.6mm.
If the specimen surface is too hard- in excess of 450 HBS for steel ball indenter and 650 HBW for tungsten carbide ball indenter-then also the hardness values will be unreliable.
HBS represents Hardness Brinell with a steel ball and HBW represents Hardness Brinell with tungsten ball.
The hardness reading should also be completed by providing the details of the applied load and the diameter of the indenter.
Hence, if a test load of 3000Kgf has been applied to an indenter ball of 10mm diameter, then the complete hardness reading should be 450 HBS 10/3000.
Advantages of Brinell Hardness Testing
Brinell method of hardness testing is one of the most commonly used techniques for hardness testing of different type of materials.
Some of the key advantages of using Brinell hardness testing method are discussed below:
Can be Used for All Metals
The Brinell method of hardness testing is more versatile than the Rockwell method.
The Brinell hardness testing method generally uses either a steel ball indenter with a specific diameter for making indentation marks on the specimen surface.
Due to this, the Brinell hardness testing method is ideal for testing the hardness of a wider variety of materials.
It can be used for measuring the hardness of nearly all types of metals.
As other methods use a fixed test load and indenter size, they are not suitable for testing the hardness of softer or more fragile specimen materials.
However, the advantage of Brinell method of hardness testing is that both the load size and size of the indenter ball can be varied, which makes it ideal for testing other more delicate and fragile specimens also.
the Brinell method provides you with a wider test space which helps to average the hardness measurements.
This is a more accurate way to measure the hardness of specimens and also provides a more reliable estimate of how the specimen may react to deformities in the real world.
Accurate than Rockwell Method
The results of the Brinell hardness testing method is more accurate and reliable than the Rockwell method.
The Rockwell method measures the indentation depth made on the specimen surface without the use of advanced optical equipment, which makes it prone to errors.
The operator’s influence can lead to varying test results considerably.
Brinell method of hardness testing also measures the indentation depth made on the specimen surface by the spherical indenter, but it provides good precision and representation using than the Rockwell method.
The indentation depth is studied and analyzed using advanced optical equipment which provides a better and accurate hardness value compared to the Rockwell method.
This method is not affected by soft spots on the specimen surface.
Even if the specimen surface is rough, the Brinell method of hardness testing can still provide you a more accurate and reliable hardness measurement compared to other methods.
Results are Force Independent
The results of the Brinell hardness testing method are independent of the applied force as the force can be adjusted for different specimen types and this will not affect the hardness readings.
This is due to the fact that the Brinell method uses a spherical indenter for testing hardness of specimens.
The sphere distributes pressure uniformly along its surface which is not the case when using points and conical indenters in other methods.
Due to the results being force independent, the Brinell hardness testing method is a more accurate and reliable method of hardness testing of various types of specimens.
Under the Brinell method, as long as the relationship between the force applied and the size of the indenter spherical ball remains constant, the results of the hardness values generated by this method will remain independent of the applied force.
Limitations of Brinell Hardness Testing
Despite having many advantages, the Brinell method of hardness testing also comes with few limitations.
The indentation depth made on the specimen surface needs to be accurately measured under this method.
Even a small deviation in measuring it can lead to a significant error in the measurement of the hardness value of a specimen.
The hardness values are measured by using optical equipment to assess the indentation depth made on the specimen surface.
Due to this, the specimen surface should be prepared well in advance before conducting the Brinell test.
Any form of imperfections on the specimen surface like dirt, roughness, etc.
will provide erroneous and unreliable hardness testing values.
As more advanced and sophisticated optical equipment is required for measuring hardness values in this method, the Brinell method is costlier compared to the Rockwell method.
The specimen surface needs to be prepared before conducting the Brinell test, hence, the Brinell method of hardness testing is time-consuming and not ideal for bulk testing.
It takes more than 30-60 seconds to measure the hardness of the specimen and this excludes the time needed for specimen preparation.
Brinell test is only suitable for hardness testing of flat surfaces.
If the surface is not flat, then it is very difficult to use this method.
General Precautions During Brinell Hardness Testing
There should be certain precautions which should be followed carefully to ensure that the hardness value derived from Brinell testing is accurate and reliable.
The first precaution which should be ensured is that the test specimen should be at least 8-10 times the planned depth of indentation to be made on its surface.
This will avoid any deformities to be extended to the opposite surface of the test specimen.
After the test is conducted, there should be no deformation on the surface opposite to where the indentation was originally made.
The specimen surface should be smooth and free from any kind of imperfections.
The specimen surface should be free from any type of dirt, contamination, foreign matter, particle matter, etc. to provide reliable and acceptable hardness values.
When testing, it should also be taken care that the indentation should not be made on the edge of the specimen surface.
The distance of the indentation should be at least 2.5 times greater than the diameter of the indentation.
This helps to avoid unnecessary concentration of stress.
The spacing between the two indentations should be at least 3 times the diameter of the indentation.
If successive indentations are made on the specimen surface which is too closely grouped, then it may provide higher than actual hardness readings due to work hardening of the specimen surface.
The test load should not be applied rapidly in Brinell test as it may reduce the plastic flow of a material which produces an effect on the size of the indentation.
The specimen surface should always be perpendicular to the indenter.
This ensures proper and visible indentations on the specimen surface.
If the indenter is not perpendicular to the specimen surface and there is a deflection of the specimen, then the hardness values derived will be unreliable and error-prone.
You can use anvils for keeping the specimen surface safe and securely in place while the indenter comes down on it perpendicularly.
The testing environment and the specimen surface should be free from any type of dirt and contamination.
The test personnel should carefully follow the operating procedures for measuring the hardness of the specimen.
If a hardness tester has not been used sufficiently in the past, then it should be tested many times before using it for testing.
The hardness tester should be kept clean and in good working condition to provide accurate hardness readings.
The indenter can also be coated with anti-rust oil or lubricant to prevent it from rusting and corroding.
The Brinell hardness value is dependent on the surface area of the indentation made on the specimen surface.
The indentation in the Brinell method may provide different characteristics which need to be studied and analyzed carefully for getting accurate results.
When the indenter is under the test load force and it presses on the specimen, it will make a large number of deformations under the indenter.
The surface near the indentation bulges out slightly due to the volume of metal displaced by the indenter.
In case of ridging type impressions made, the diameter of the indentation will be greater than the actual value of indentation and in sinking type impressions, the diameter of indentation will be lesser than the actual value of indentation.