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BACKGROUND
Hardness is a fundamental property of a material which defines its resistance to indentation which is determined by measuring its permanent depth of indentation.1 Hardness testing has a powerful connection to find out the strength, wear resistance and many other properties of a metal or a non-metal, but it never gives a direct measurement of any physical properties.2 These tests are widely used because of its low cost and simplicity to perform.
Rockwell hardness test is one of the general methods to measure the bulk hardness of any metallic or polymer materials. This test was invented by a metallurgist by Stanley P. Rockwell. It has an advantage over Brinell and Vickers as Brinell’s indents are too big and Vickers experiment is very time-consuming. Rockwell hardness test provides accurate results on a variety of test particles in just a few couples of the second.
This test is defined in following standards:
1. ASTM E 18 Metals.
2. ISO 6508 Metals.
3. ASTM D785 Plastics.
In general, there are two types of Rockwell test, one is Rockwell itself, and the other one is Superficial Rockwell test. The difference between these two tests is based on the load. Rockwell test is performed under the minor test of 10 kgf and the major load of 60,100 or 150 kgf.3 On the other hand, Superficial Rockwell test is performed under a minor load of 3 kgf and the major load of 15, 30, or 45 kgf.
Rockwell Hardness test uses the preliminary load and the major load to measure the permanent depth of the indentation. Preliminary test force or preload is applied on a specimen using a diamond or a ball indenter which reduce the effects of the surface finish. After this preliminary load, the major load is applied to test additional requirement of force. This major load is held for a certain period to allow for elastic recovery and then released to the preload value. Then, the preload value released after some period and the final depth of indentation is measured.
The central objective of this experiment is to measure the hardness strength of a material by Rockwell testing methods.
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REQUIREMENTS
Materials: Rockwell hardness testing machine
Specimen of mild steel
Steel ball of diameter D
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PROCEDURE
First of all, insert the indenter ball of diameter D into the ball holder the machine. Then check out the specimen surface, if any dust, dirt, oil or grease is attached to it then clean it by using any piece of cloth. Then, by rotating the jack adjusting wheel, make contact between the specimen surface and the ball. After this, push the loading button and the load release lever. After a minimum interval of 15 seconds, the load will be gradually applied on the specimen. Now remove the sample from the provided specimen table and mark the point of indentation. Now, repeat the entire operation for 3-4 times to find the approximate hardness.
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CONCLUSION
The process of finding the hardness of a material through Rockwell Hardness testing method is discussed. It is a characteristic property which is used to measure the depth of indentation.
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REFERENCES
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George E. Dieter. “Metallurgy and Metallurgical Engineering Series,” in Mechanical Metallurgy, New York, TMH, 1961: 47-82.
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N. C. Y.-L Shen. On the correlation between hardness and tensile strength in particle reinforced metal matrix composites. Journal of Elsevier. 2000;297(1-2):35-48.
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T. K. Hyungs un Kim, “Measurement of hardness on traditional ceramics,” Journal of the European Ceramic Society. 2002;22(9-10):25-32.
