Our discussion of hardness testing now focuses on microhardness testing methods.
Microhardness testing (Fig. 1) is performed to measure the hardness of constituent phases, segregation effects and the carburizing gradients in a metal. Indenters for microhardness testing include Knoop and Vickers with applied loads ranging from 1 gram to 1,000 grams. The test for both methods is destructive since careful preparation of the metallographic sample is necessary to obtain accurate and consistent readings. The indentations are typically measured optically at magnifications from 400-1,000X. High-quality optics are critical, especially when magnifications of 500X and greater are used. Accurate measurement of the indentation boundaries, especially when using lighter loads, is mandatory.
Microhardness testing is ideal for precisely sampling a small region, a very thin or soft sample, or a material with either hard or soft particles you wish to include or exclude from the field of measurement. Knoop and Vickers indenters are different. Knoop indenters are an elongated diamond shape and better suited for more precise measurements of layers or for measurements at specific depths. Vickers indenters are a diamond-pyramid-shaped indentation and are more symmetric and better suited for particle hardness measurements.
Microhardness testing is sensitive to a number of variables such as indentation spacing, material segregation, inclusions, edge effects and cold working. The key to microhardness testing is that it requires more sample preparation and operator skill, and, unlike Rockwell scales, the applied load affects microhardness numbers. It is important to remember that ASTM E140 only recognizes microhardness conversions with minimum loading of 500 grams. Consequentially, conversions to Rockwell scales produce errant values at increasingly lighter loads (Fig. 2).
1. Stone, Alan and Daniel H. Herring, “Practical Considerations for Successful Hardness Testing,” Industrial Heating, April 2006.
2. Lysaght, Vincent E., and DeBellis, Anthony, Hardness Testing Handbook, American Chain and Cable Company, 1969.
3. Wilson Instruments Division, Instron Corporation, Norwood, MA (www.wilsoninstruments.com)
4. Midea, S., “Brinell Testing,” HOT TOPICS in Heat Treatment & Metallurgy, Vol. 2 No. 12, December 2004.
5. Midea, S., “TheBasics of Microindentation Hardness Testing,” HOT TOPICS in Heat Treatment & Metallurgy, Vol. 1 No. 2, December 2003.
6. ASTM Specification Nos. E3, E10, E18, E103, E140 and E384 (www.astm.org).