Microhardness testing requires care in order to achieve accurate and repeatable results. The key with microhardness testing is that it requires more sample preparation and operator skill as well as expensive testing equipment. And unlike Rockwell scales, microhardness values are affected by the respective applied load.
For example, a Knoop hardness value of 500 taken with a 500-gram load (HKN500 500) is not equivalent to a Knoop hardness value of 500 taken with a 50-gram load (HKN50 500). Hence it is crucial to report the applied load with the test result value. Consequentially, conversions to Rockwell scales produce errant values with lighter loads.
Common pitfalls associated with microhardness testing involve the following factors.
1. Component Size:The size and shape of a component can affect testing. A component may require the application of lower loads because samples are too small to withstand the typical testing load. A specimen may not be able to physically support the hardness test load without deflection. Samples of this type may need to be fixtured or reinforced. Also, microhardness-testing machines can accommodate only samples within a certain size range. Samples may need to be sectioned if they are too large or mounted and polished using standard metallographic techniques if they are too small.
2. Sample Orientation:The sample has to be flat so that the indenter is in contact with the surface evenly. Furthermore, the sample surface to be tested must be perpendicular to the indenter. Samples may need to be mounted and polished using standard metallographic techniques.
3. Sample Surface:The surface condition of the sample is also critical. The finish must allow the correct extent of indentation to be measured.
4. Vibration:Vibration of the testing machine can cause erroneous readings. For this reason, the tester should be isolated from vibration to the degree possible.
5. Multiphase Sample Materials:A microhardness test is often used to show hardness gradients within multiphase materials. For general microhardness testing, a homogeneous sample is optimal, but in the case of a multiphase alloy, it is often necessary to take multiple hardness measurements so as to obtain an adequate sampling of different phases (grains) within the alloy. The sampling scheme would be determined on the basis of statistical calculation and the model that is chosen.
6. Sample Preparation: The treatment to which the sample is subjected prior to testing can cause microhardness data to display extreme scatter or be skewed. Preparation of the sample is often the single biggest variable over which the operator has influence.
The next blog discusses how sample preparation can affect microhardness readings.
Microhardness Testing Tips
Dan Herring is president of THE HERRING GROUP Inc., which specializes in consulting services (heat treatment and metallurgy) and technical services (industrial education/training and process/equipment assistance). He is also a research associate professor at the Illinois Institute of Technology/Thermal Processing Technology Center. tel: 630-834-3017; e-mail: email@example.com; web: www.heat-treat-doctor.com
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