The Test Method of Brinell Hardness

The test method to measure Brinell hardness, is defined in the ASTM standard E10. Most commonly it is used to test materials that have a structure that is too coarse or have a surface that is too rough to be tested using another test method.

The reason for this capability is that the most commonly-used test load is very large, 3000 kg, and the indenter is 10mm wide, so the resulting indentation averages out a lot of surface and subsurface inconsistencies.

A result in Brinell hardness measures the permanent width of indentation produced by a carbide indenter applied to a test specimen at a given load, for a given length of time. Typically, an indentation is made with a Brinell hardness testing system and then measured for indentation diameter in a second step with a specially designed Brinell microscope.

The resulting measurement is converted to a Brinell value using the Brinell formula or a conversion chart based on the formula. Most typically a Brinell test will use 3000 kg load (force) with a 10mm ball. On aluminum, the test is most frequently performed with a 500kg load and 10mm ball. Brinell test loads can range from 3000 kg down to 1 kg. Ball indenter diameters can range from 10mm to 1mm. Most frequently the lower loads and ball diameters are used for convenience in “combination” testers, like Rockwell units, that have a small load capacity. The test standard specifies a time of 10 to 15 seconds, although shorter times can be used if it is known that the shorter time does not affect the result. There are other condition that must be met for testing on a round specimen, spacing of indentations, minimum thickness of test specimens, etc. For complete information on the test standard, please purchase a copy of the E10 standard from the ASTM website.

Typically the greatest source of error in Brinell testing by a good margin in the measurement of the indentation. Due to disparities in operators making the measurements, the results will vary even under perfect conditions. Less than perfect conditions will cause the variation to increase greatly. E.g. Frequently the test surface is prepared with a grinder to remove surface conditions. The jagged edge presents a great challenge to interpretation. Furthermore, when operators know the specifications limits for rejects, they will often be influenced to see the measurements in a way that increases the percentage of “good” tests.

Two types of technological fixes for the measurement error problem have been developed. Automatic optical Brinell scopes use computers and image analysis to read the indentations the same for all operators without being influenced to read in-tolerance results. Automatic depth measurement Brinell units, according to ASTM E103, measure the samples using Brinell hardness parameters together with a Rockwell hardness method. This method provides the most repeatable results (and greater speed) since the vagaries of optical interpretations are removed through the use of an automatic mechanical depth measurement. On the other hand, using this method the results may not be strictly consistent with Brinell results due to the different test methods – an offset to the results may be required for some materials. It is easy to establish the correct values in those cases where this may be a problem.

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