Hardness testing is the most common quality-control check performed in the heat-treat shop. While perhaps one of the easiest tests to do, it can be one of the hardest tests to do properly. And since hardness testing often determines the success or failure of the heat-treatment operation, doing it properly is critical.
What is Hardness?
A common definition of hardness is the measure of the resistance of a material to an applied force, and it involves the use of an indenter of fixed geometry under static load. However, hardness can also refer to stiffness or temper resistance, or it can refer to resistance to scratching, abrasion or cutting. It is the property of a metal that gives it the ability to resist being permanently deformed (bent, broken or have its shape changed) when a load is applied. The greater the hardness of the metal, the greater resistance it has to deformation. The ability of the material to resist plastic deformation depends on the microstructure of the material. Therefore, the same material can have different hardness values based on its microstructure, which is influenced by the heat-treatment process.
Hardness is not a fundamental material property but rather a composite value that can be related to the strength of material. Hardness testing is considered a nondestructive test, meaning it does not significantly damage the part during the testing process.
Hardness Testing Methods
Brinell, Rockwell, Rockwell Superficial and microhardness methods are the most common indentation hardness testing methods used. The Brinell test is used primarily for inhomogeneous materials and larger parts such as forgings and castings, particularly cast iron. The Rockwell test is used for both ferrous and nonferrous materials that have been annealed, hardened, case hardened or tempered. Sheet material in heavier gauges and cemented carbides can also be tested. Rockwell Superficial is used where lighter loads are required for testing thin case-hardened surfaces, decarburized layers and sheet material in thin gauges. Microhardness tests are typically used for very small and thin parts, intricate shapes and case-depth determination.
Next time, we continue our discussion on the principles of hardness testing.
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5. Midea, S., The Basics of Microindentation Hardness Testing, HOT TOPICS in Heat Treatment & Metallurgy, Vol. 1 No. 2, December 2003.
6. Herring, Daniel H., Common Pitfalls in Hardness Testing, Wire Forming Technology International/Spring 2012.
7. ASTM Specification Nos. E3, E10, E18, E103, E140 and E384 (www.astm.org).