This is part 4 of David Pye's series on hardness testing.
Interpretation of the Hardness testing Result
The Brinell test is a very simple indentation test for determining the hardness of a wide range of materials. The Brinell test is the application of a constant standard selected load or force (500-3,000 kg) for a specified time (10-30 seconds) using a 5- or 10-mm-diameter tungsten-carbide ball on the flat surface of the workpiece being tested.
The load time period is required to ensure that plastic flow of the metal has ceased. After removal of the load, the resulting round impression made into the surface of the steel is measured in millimeters using a special low-power microscope.
Brinell indentations may exhibit different surface characteristics. In some instances, there is a ridge around the indentation that extends above the surface of the workpiece. In other instances, the edge of the indentation is below the original surface. Sometimes there is no difference at all. Figure 1 shows a cross-sectional view of some Brinell Indentations that are made based on the hardness of the steel (ASM Handbook Vol 8).
For accurate results, indentations should not be made near the edge of the workpiece. Lack of sufficient supporting material on one side will result in abnormally large, asymmetrical indentations.
In most instances, the error in Brinell hardness number will not be significant if the distance from the center of the indentation to any edge of the workpiece is more than three times the diameter of the indentation. Similarly, Brinell indentations must not be made too close to one another. The first indentation may cause cold working of the surrounding area that could affect the subsequent test if made within this affected region. It is generally agreed that the distance between centers of adjacent indentations should be at least three times the diameter of the indentation to eliminate potential and significant errors.
While Brinell hardness test instruments can be very large and in fixed locations, the Figure 3 illustrates the King portable Brinell hardness test unit, which is found in many heat-treatment shops. This is a very simple unit to operate, and the results can be read by using a Brinell hardness test indenter microscope.
The measured results, gathered by the microscopic reading, are then interpreted for the Brinell hardness tables in relation to the diameter measured by the hand-held microscope.