Overview: Rockwell Hardness Test
The Rockwell hardness test is one of the most basic and effective methods of hardness testing of different materials. It was invented by Stanley P. Rockwell in 1919 for determining the accurate and speedy measurement of hardness.
The Rockwell test method applies a minor and major test load force through an indenter to make indentations on the specimen surface for which hardness has to be tested.
The indentation depth is then measured and converted to the Rockwell hardness value. Using the Rockwell conversion charts, the Rockwell hardness value is converted to the corresponding hardness measurement.
This method has been used to measure the hardness of different types of materials. Depending upon the hardness of the specimen, the Rockwell test method can either use a steel ball indenter or a diamond-tipped conical indenter.
Benefits of Rockwell Hardness Tester
Rockwell hardness testers are widely used for measuring the hardness of different types of specimen. Some of the key benefits of using a Rockwell hardness tester are as follows:
- The test is simple and fast. Due to it being non-complex in nature, it does not mandatorily require a highly skilled and trained operator for conducting it.
- The Rockwell tester is cost-effective and does not require advanced optical systems for measuring the specimen hardness.
- It is also a non-destructive test. The indenter makes only a small indentation on the specimen surface. The specimen is not destroyed and can be used for other purposes after the test is completed.
- It does not require a highly skilled operator for conducting the test.
- Rockwell testers provide quick hardness testing and are suitable for bulk testing of products on the production line.
Limitation of Rockwell Test
The Rockwell hardness test also has few limitations. It is not as accurate as other hardness tests as the measurement of indentation depth are dependent on the operator and no optical systems are used for it.
This can cause errors in the measurement of indentation depth which can cause significant deviations in hardness values. Any imperfections on the specimen surface like dirt or contamination can also lead to variations in hardness values.
Key Features of Rockwell Hardness Testers
Rockwell hardness testers help you to measure the hardness of materials using the Rockwell test method. It is important to understand that not all hardness testers are the same.
Generally, all Rockwell hardness testers can perform the Rockwell hardness tests in accordance with the prescribed standards and guidelines, but some of them are more suited to your specific needs than the others.
Hence it is important to assess and understand the different features of different types of Rockwell hardness testers before selecting the ideal one for yourself.
You should thoroughly consider factors like the accuracy of testing, repeatability, versatility in the testing cycle, speed of testing, compatibility with different Rockwell scales, specimen size that can be tested, etc. before buying a Rockwell hardness tester.
These hardness testers come equipped with many features and many features are being added with the advancement of technology. Some of the key features of Rockwell hardness testers are discussed below:
Most Rockwell hardness testers were designed to measure hardness using either one of the regular Rockwell tests or the superficial hardness test. This was so because both these test methods use different levels of test load force and varying resolutions of depth measurement.
However, over time and with improvement in technology, most Rockwell hardness testers can measure specimen hardness by using either the regular Rockwell test or superficial Rockwell hardness test.
These hardness testers are also known as twin testers or combination testers.
These modern hardness testers can measure the specimen hardness on all the different Rockwell scales. Some of these hardness testers can also be used for conducting hardness testing using other test methods also.
The earlier designed Rockwell hardness testers used to apply the minor load through the indenter by compression of a helical spring. The total test weight was applied by dead weights through force-multiplying lever systems.
The problem with this design is that with frequent usage, the helical spring used for applying the minor load and the knife-edges which support the force multiplying lever system become worn out. This leads to inaccuracies in hardness testing of the specimen.
With technological advancement and the advent of technologically controlled feedback systems, the Rockwell hardness testers have improved considerably.
The test load can now be applied by a screw-driven device which can be monitored effectively. this new technology makes the testing process and cycle controllable and removes any potential errors which may have resulted due to inconsistencies in helical spring and lever arm mechanism.
The new and improved Rockwell hardness testing machines make the hardness reading more accurate and reliable. The current trend is towards developing load-cell design hardness testers.
When the preliminary force or total test load force is varied, the Rockwell hardness value obtained for the same specimen material can be different.
When the preliminary force is increased, it increases the indentation depth on the specimen.
This causes the hardness value to reduce and results in higher hardness value. Similarly, a decrease in the preliminary force provides a lower hardness value for the same specimen.
In the earlier Rockwell hardness testers, dial indicating-gage was used for measuring the indentation depth and for calculating the Rockwell hardness number. This system was very simple and convenient and is still used today in some of the hardness testers.
This mechanism used to track the movement of the indenter through a multiplying lever system and on to the specimen surface. The dial is adjusted in a manner to indicate the Rockwell number which corresponds to the displacement of the indenter.
The problem with this type of Rockwell hardness testers was that with usage, the dial gages and lever system became worn out and misaligned. This resulted in faulty and unreliable estimates of the hardness value of the specimen.
Majority of the Rockwell hardness testers used currently have electronic or optical displacement measuring instruments for measuring the depth of the indentation made on the test specimen surface.
The signal from the measuring system is converted to the corresponding Rockwell hardness value and the reading is displayed digitally. This digital and electronic measurement of hardness values have greater accuracy compared to the earlier dial gauge and multiplying lever system.
The Rockwell hardness value is determined and is directly proportional to the depth of the indentation made on the specimen surface.
An error in measuring the indentation depth can lead to significant discrepancies. An error of 0.002mm in measuring indentation depth can lead to a deviation of 1 HR unit in measurement of Rockwell hardness value.
Mode of Operation
Since the inception of Rockwell hardness testers to a considerable length of time, the operator played a vital role in determining the hardness values.
They had to apply and remove the test load force which provided them a great degree of control over the testing process.
The problem with such a testing process was that the consistency in the testing cycle varied between different operators. The process also took much longer for determining the hardness value.
Over time, the Rockwell hardness testers became more advanced and started using motors for providing automation to the process and making it more repeatable.
Some hardness testers are completely automated which allows for the application of greater test load than was possible manually in previous hardness testers. The automation process makes the testing process faster and more consistent, removing the inherent bias of the operator.
Though the application of greater test load force and shorter dwell time has made the Rockwell hardness testing process more efficient, it also comes with few drawbacks.
It has been shown that such faster testing methods with lower dwell time can lead to poor repeatability of hardness measurement. It was realized that better measurement repeatability and control of the testing cycle was important due to varying plasticity of the specimen.
This led to the manufacturers of Rockwell hardness testers to make appropriate changes to the design for allowing the operator to control the testing cycle.
One of the most important precautions and best practice for the Rockwell hardness test is to ensure that the specimen is stationary and does not deflect during testing.
Even the slightest of deflections and movements in the test sample can lead to significant deviation in hardness value. The movement or deflection of the test specimen will most likely be reflected as a discrepancy in the measurement of the indentation depth.
When conducting a superficial Rockwell hardness test, even a slight movement of 0.01mm can cause a difference of 10 Rockwell points, which is significant and sufficient to make the test results unreliable.
Anvils should be used in Rockwell hardness testers to clamp the specimen and provide accurate hardness values. There are different types of anvils available for a specimen of different shapes and sizes.
Flat anvils should be used for measuring the hardness of flat specimen. If the specimen is curved, then V-shaped or double roller type anvil should be used.
There are some Rockwell hardness testers which apply a clamping force to the specimen which is greater than the Rockwell test force. Such hardness testers are useful for measuring the hardness of larger specimen.
It is also important to ensure that the anvil is clean and free from any scratches or dirt as a dirty and imperfect anvil can cause major deviations in the measurement of hardness values.
Repeatability of a hardness tester is important for getting reliable and consistent results.
Repeatability of a Rockwell hardness tester refers to its ability to obtain the same and consistent hardness measurement on a uniform specimen over a period of time when the test conditions remain the same.
If a specimen which is uniform and specially prepared for hardness measurement using the Rockwell test provides different hardness values, then the hardness tester is not consistently repeatable.
The degree of agreement in the hardness value of the same specimen over a short time period provides the degree of repeatability of the hardness tester.
Lack of repeatability is a randomly occurring error and will generally increase when the parts of the hardness testers are worn out, due to excessive friction while testing, etc.
It is also observed that the degree of repeatability for many Rockwell hardness testers vary between different Rockwell scales due to variances in force loads and types of indenters.
It may also vary at different hardness levels within the same Rockwell hardness scales, due to variations in the indentation depth made on the specimen surface.
The acceptable levels for Rockwell hardness testers for repeatability are assessed from the difference between the maximum and minimum hardness values provided by it.
Depending on the different Rockwell scales, the generally acceptable levels for repeatability range from 1 to 2 under ASTM(American Society for Testing and Materials) and 1.2 to 6.6 units under ISO (International Standards Organization).
Indenters are an important part of Rockwell hardness testers. They contribute majorly to errors in hardness measurements.
The indenter used in Rockwell hardness testers can either be spherical steel ball or diamond-tipped conical indenter. There can be various factors for differences in indenter performance.
It can be due to the indenter’s manufacturing process also wherein two indenters identical in each and every respect can produce different hardness measurements.
To overcome such problems, benchmarking the indenter against a master-indenter used in the past which produced accurate hardness measurements can be done.
In the past, one way of certifying or benchmarking Rockwell indenters was to make hardness tests on reference test blocks and the measurement was then compared to the block value.
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The generally acceptable tolerance levels for performance of diamond indenters are 0.5 to 1 Rockwell units from the test block value under the ASTM standards.
ISO(4) standards allow a tolerance level of 0.8 Rockwell units from the performance of the reference indenter.
There are no requirements or tolerance levels for the performance of steel ball indenters either in the ASTM or ISO standards.
Before conducting the hardness testing, indenters should be visually inspected for any type of damage. The indenters should also be cleaned regularly so that there are no imperfections or residue on the indenting portion of the tip.