Two common hardness tests are used in the industry: the Rockwell scale and the Vickers scale. Both test the surface of a material, but they are used for different purposes. The Rockwell hardness test measures the hardness of all metals except in conditions where the structure of the test sample would cause too much variations, or the indentations formed would be too large for application or measurement. The Vickers hardness test measures the hardness of a material using optical methods. This method is mostly used to measure the microhardness of very small parts or thin sections. Both are valuable tools for judging the quality of materials, but each has its own advantages and disadvantages.
This article will compare the two hardness tests to evaluate the most suitable method to use in your application.
1. What is the Rockwell Hardness Test?
The Rockwell hardness test uses a small steel or carbide-tipped ball called the indenter to measure the hardness of a material. The indenter is placed on the test sample and a specified minor force is applied to cause an indent on the test sample. This first indent is produced on the test sample by placing a force on the indenter. This force is also known as preload or minor load. Then another major load is applied to the test sample for a specified amount of time. This major load causes a larger indent than the first indent caused by the preload. The measurement of this larger indent is taken and recorded. After this, the major load is removed, leaving the minor load on the test sample. The measurement that remains after the major load is removed is used to calculate the hardness value of the test sample by subtracting the major load indent from the minor load indent. The resulting “Rockwell hardness” number is the average of multiple readings taken from different locations on the sample until a reading that is considered valid is obtained.
2. What is the Vickers Hardness Test?
The Vickers hardness test uses an optical measurement method using a diamond indenter of specified light loads. The diamond indenter is placed on the test sample, and a specified load is applied. The light load causes a small indent in the test sample, which is measured with a microscope. The measurement of the light load indent is taken and recorded. Then a second major load is applied to the test sample for a specified amount of time, causing again another indentation that must be measured with the microscope by taking multiple measurements from different locations within the indentation until an average value is obtained. This resulting measurement from this second major load application is taken as the hardness value of the test sample.The vickers test is mostly used to measure the microhardness of very thin materials such aluminum foil, and cross sections of thin pipes. This is because the Vickers hardness test uses an optical microscope to take the measurements of the indents.
3. What is the Difference between Rockwell and Vickers Hardness Test?
Test Force：
The Rockwell hardness test employs a force ranging between 15 kgf and 3000 kgf while the Vickers test employs a lighter force ranging between 10 g to 1 kgf although a macro load of up to 30 kgf.
Test Material:
The Rockwell hardness test is applied to a wide range of materials and can be used for non-ferrous materials as well. On the other hand, the Vickers hardness test is limited to very thin materials such as aluminum foils because it uses optical measurement.
Indenter Type:
The Rockwell hardness test uses a steel ball of specified weight or a carbide tungsten ball to test very hard materials, while a Vickers hardness test uses a diamond cone-shaped indenter to perform the hardness test.
Indentation:
The Rockwell hardness test produces larger indent compared to the Vickers test because of the major load applied to the test sample, while the diamond cone on the Vickers tester produces a very small indent which is measured using a microscope.
Test Time:
The Rockwell hardness test is quicker than the Vickers test which takes up to 60 seconds to perform a test on one test area of the sample.
Accuracy:
The Vickers hardness test is more accurate than the Rockwell hardness test because of the use of a microscope to measure the indentation.
Cost:
The Rockwell hardness test is cheaper and can be used on a wider range of materials, while the Vickers hardness test is expensive because of the optical measurement material used to take the measurements and the time required to perform the test. However, it can be used for very thin materials such as aluminum foils and other non-ferrous materials.
4. How to Convert Rockwell Hardness to Vickers?
The various hardness scales do not directly correlate in a way that is accepted worldwide. However, some national standards include relevant formula and hardness conversion tables. Numerous hardness readings discovered these, and a conversion was made once the results were plotted. BS EN ISO 18265:2013 and ASTM E140: 2012 are two examples of such regulations. These standards’ conversions, which incorporate the Vickers, Brinell, Rockwell B, and Rockwell C hardness scales and conversions for a select few material types, are not precisely the same but comparable. The standards also include a few other hardness scales, however these are less frequently utilized.
You must understand that the conversion between hardness scales is approximate rather than absolute, as is the case with all conversions.
Conclusion
The hardness test methods are fundamentally different in how they determine a material’s hardness value. The Rockwell hardness test uses a simple mechanical load application, while the Vickers hardness test uses an optical measurement method. The Rockwell hardness test is the most common type of hardness testing method because it is easier to perform and more accurate than other methods. In contrast, the Vickers method is good for measuring very thin materials such as aluminum foils and cross-sections of pipes due to its use of optical microscope measurement. Although these methods are similar, they are not directly comparable because each one reports different “hardness” numbers with varying accuracy and precision, depending on their specific application.