We continue our discussion of hardness testing by focusing on Brinell testing.
Brinell Hardness Testing Tips
Brinell testing (Fig. 1) generally employs larger indenters and heavier loads than other hardness-testing methods to minimize localized feature effects such as voids and chemical inhomogeneity. Brinell indenters are made from tungsten carbide with a specific chemistry and hardness. Smaller indenters (1, 2.5 or 5 mm) have their uses in certain applications. However, the standard indenter size for iron and steel is 10 mm (0.4 inch) diameter. As a rule, the indentation should be at least 24% of the ball diameter to avoid damaging the indenter and avoid problems with accurately measuring the indentation. The test indentation should be no more than 60% of the diameter of the indenter to avoid excessive plasticity around the indentation and reduced sensitivity of the measurement.
To measure the hardness, the ball indenter is pressed into the metal’s surface using a known force for a specific time. The force is then removed as well as the indenter. The resulting depression made in the metal is then measured (Fig. 2). The surface area of this depression corresponds to the hardness of the material.
Brinell testing tips and common errors include:
Specimen too thin
a. Test specimen should be 5x thicker than the depth of hardness indentation).
Incorrect measurement of the impression
a. the impression should be essentially round and measured in two positions approximately 90°apart.
b. The reported value is the sum of these measurements divided by two.
Incorrect eyepiece resolution
a. The micrometer eyepiece in the microscope should have divisions of 0.1 mm and be able to estimate to 0.05 mm.
Dirty sample surface
a. Parts should be clean and free of contamination.
b. Part can be ground or polished if necessary.
Curvature of the surface
a. Radius of the curvature should be no more than 2.5 times the diameter of the indenter ball.
Distance from edge
a. Indentions should be done 2.5 indenter diameters from the parts edge.
Distance between indentations
a. Indentions should be done 2.5 indenter diameters from the next indentation.
Indenter alignment with part
a. The test force should be applied at a 90° angle to the part, plus or minus only two degrees.
a. If the indenter pushes into the sample with more than 60% of the diameter penetrating into the surface, excessive material will dimple up around the indentation. This makes the reading less accurate and more difficult to read. Use a larger indenter or a smaller test piece.
a. Due to the nature of Brinell testing, conversion to the other hardness scales may be inaccurate.
Coming up, we begin a discussion of Rockwell hardness testing.
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., “The Basics 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).