There are many types of tests that have been created to measure the physical properties of metals. Tensile tests measure strength, impact tests measure toughness and bend tests measure ductility. But how is the hardness of a metal alloy measured? This article goes in-depth on two of the most common types of hardness testing: Brinell hardness testing and Rockwell hardness testing.
What is Metal Hardness?
Hardness can be defined as the ability of a metal or metal alloy to resist plastic deformation, in a specific, localized location rather than in a general location. It is also defined as metal’s resistance to indentation, scratching, or abrasion.
Hardness is an important feature because the level of hardness that a metal has directly relates to its ability to resist wear. Hardness levels can vary within a given type of metal depending on the alloying elements, heat treatment, work hardening, and other hardening methods used.
Methods for evaluating hardness such as Brinell hardness and Rockwell hardness were invented in order to create a common understanding of hardness levels because of the variation of hardness among metals and even within a family of metals.
What is Brinell Hardness?
Brinell hardness is a scale used to provide a numerical value to the level of hardness of a material. The method for conducting a Brinell hardness test is defined fully in ASTM E10. In general, to conduct the test, a certified Brinell indenter is pressed against a metal under a predetermined load for a predetermined amount of time. All of this is specified to reduce the risk of experiment method variation affecting results. Typically, the indenter is a 10mm ball of hardened steel and the force is 3,000 kgf for steels and other similar materials.
For softer or harder materials, the test changes slightly. The indenter is removed after it is applied onto the metal with a force, and the resulting width of the indentation is measured using a microscope. The measurement of the indentation can then be converted into a Brinell hardness value using a Brinell hardness scale.
What is Rockwell Hardness?
Similar to Brinell hardness, Rockwell hardness is also used to understand what the hardness of material is in numerical terms. This is done through the use of a Rockwell hardness test and a Rockwell hardness scale. The exact method can be found in ASTM E18. Like Brinell hardness tests, Rockwell hardness tests are performed with an indenter of a specified size applied with a specified force for a predetermined amount of time. The measurement of the indentation is then converted to a Rockwell hardness value using a Rockwell hardness scale.
What is the Difference Between Rockwell and Brinell Tests?
While the two tests have similarities, there are several important differences, listed below:
- The Brinell hardness testing uses a 10mm hardened steel ball, while the Rockwell test uses either a much smaller steel ball (<4mm) or a diamond cone, depending on the material being tested.
- The Rockwell test measure the depth of the indentation, while the Brinell test measures the width of the indentation. Rockwell hardness tests use a preload to establish a zero position before the main load is applied. The main load is then taken away and only the preload remains. Then the distance traveled is measured by the Rockwell testing machine.
- It’s also important to note that the conversion scales for Rockwell hardness and Brinell hardness are not the same and should not be confused with one another.
Where are Brinell and Rockwell Hardness Tests Used?
The Brinell and Rockwell hardness tests are used in nearly every industry. They are critical for understanding what metals and other materials will provide adequate resistance to indentation, abrasion, scratching, and other forms of wear for a given application. Examples include the evaluation of materials for engine pistons, jet turbine blades, ship hulls, bronze fixturing equipment, railcar wheels, and many more components that might be subjected to conditions where wear may occur.
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