The Fundamentals of the Vickers Hardness Test Method
The test method for Vickers Hardness is defined in the test standards, ASTM E384 and E92. ASTM E384 covers microhardness testing including knoop scales in typical load ranges of 10 grams to 1 kg. ASTM E92 provides for using Vickers in a higher load range up to 30 kgs, sometimes called macrohardness.
The microhardness test method is used frequently throughout the world to obtain test results for small test samples of a cariety of materials - not just for metals. Macro Vickers is used frequently in Europe, often instead of using a Rockwell test, although there are increasingly frequent calls for its use worldwide due to globalization. (Note: Knoop is not used in the higher load range as its application is for placing indentations close to the edge of a specimen or close to each other - which would be accompished at higher load ranges by selecting a lighter load.)
The test method requires a square pyramid indenter made of diamond to be pressed into the test sample at a specified load. Then the indentation is measured from tip to tip in both axes. The average measurement is converted to a Vickers hardness value according to a formula or a chart based on the formula. Whether using micro- or macro-hardness ranges the selected test load should be the highest load which can be performed - barring a requirement for a specific test load. Higher loads create larger indentations which are more accurately measured. Higher loads provide more indentation resolution and more measurement resolution so the result is generally more reliable.
The micro vickers hardness test is usually required for testing very small or thin parts or in tight locations, close to specimen edges, etc. Case depth traverses are done on sectioned samples which have been polished. Generally, the main requirement for the test sample in Vickers hardness testing is a level, polished- finish test surfaceand loads applied for 10 to 15 seconds unless it is shown the lower times produce the same results. Samples also need to be securely mounted perpendicular to the indenter to prevent any rocking during the test. A result should be reported as 208 HV500gf for hardness Vickers. If a non standard time is used the time should be listed after the test force.
Another microhardness test method is the using Rockwell method together with micro range loads. This test method has the primary advantage of being strictly mechanical in the measurement process so no optical measurements used.
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