The Truth About Vickers Hardness Test
What is Vickers Hardness Test?
Vickers Hardness Test is the most widely accepted method for measuring the hardness of materials. Vickers test provides a continuous scale of hardness and it can be used for all metals - from the very soft ones to very hard metals. The Vickers Hardness Test is described in ASTM Standard E92-72.
Developed in the early 1920s by engineers at Vickers Ltd. in UK, the Vickers Hardness Test, like other hardness tests, gives a measure of hardness by assessing a sample materials ability to withstand indentation from a specific source. The uniqueness of Vickers test is that it virtually gives identical hardness numbers on the same material and that even with different loads. The unit of hardness given by the test is known as the Vickers Pyramid Number (HV).
How is the Vickers Hardness Test conducted?
Also known as the Diamond-Pyramid Hardness Test or DPH Test, Vickers test makes use of a right pyramidal diamond indenter with a square base. The indenter is fitted in the Vickers machine in such a way that it looks more or less like a playing card diamond, with the opposite faces making an angle of 136 degrees.
During the test, the sample material is placed on an anvil (attached to the machine) that has a screw threaded base. The metal strip is raised with the help of the screw threads so that the point of the indenter presses against it. When the start lever is activated, load is slowly applied to the indenter for some 10/15 seconds. (Different load settings in the range of 1 to 120 kilograms-force are applied). Thereafter, the load is released and the specimen is lowered. The entire process is controlled automatically.
How is Vickers Pyramid Number determined?
Vickers Pyramid Number (HV) is gauged from the ratio F/A, where F is the force applied in kgf and A is the surface area of the resulting indentation in sq mm.
Why is Vickers Hardness Test considered best?
In spite of its cumbersome floor standing model and high price tag, the Vickers Hardness Tester is considered ideal because it gives extremely accurate/correct readings. The device makes use of just a single type of indenter - a diamond-pyramid, which gives as precise readings for a soft material like ceramic as for a hard material like steel. Moreover, diamond being the hardest material, the indenter does not deform over time.
It is not just that the Vickers Hardness Test makes use of a consistent/reliable scale (unlike the arbitrarily changing scales of other devices), the deep indentation simplifies the calculation of the indentation area and that of the Vickers Hardness Number.
About the Author
Allen is a successful writer and publisher of Hardness Tester related issues, for more informative articles go to www.hardnesstesters.info
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