Hardness measurement is a topic that has not been addressed in this column before. However, when dimensional measurements can affect hardness results, it's time to focus on the issue.
Although there are a number of ways to define and measure hardness—including some new methods using ultrasonics—the most common form of hardness testing is static indentation, as measured on certain scales. The static indentation test centers on the idea that a set force is applied to an indenter to establish the resistance of the material to penetration. If the material is hard, then a relatively shallow indentation will result. If the material is soft, then a larger or deeper indentation will occur.
The main difference between the several types of static indentation tests lies in the shape of the indenter. It can be round, pyramidal or triangular, but the principle is the same: A force is applied by means of the indenter, and the size of the resulting mark is related to a hardness value.
There are many things that can influence hardness results aside from material considerations. These include differences in indenter shape; the relative consistency of the force being applied; the velocity of the probe; and whether the operator is using the test equipment properly to assure that the force is being applied square to the surface.
Another big source of error with some hardness testing is the surface quality on the test sample. This is where surface finish inspection starts to play a role. Contained within the literature of many hardness gages, a minimum surface finish requirement is specified in order to assure proper hardness gage function. A surface finish of 80 microinches or better average roughness (Ra) is often required to assure proper hardness measurements.
Surface finish—also known as profile—is composed of two elements: waviness and roughness. Waviness, or longer wavelength variation, is caused by macro-type influences, such as worn spindle bearings or vibration from other equipment on the shop floor. Roughness is the short wavelength pattern caused by tool marks from grinding, milling or other machining processes and is influenced by the condition and quality of that tooling. As the indenter is apt to be small compared to the waviness component of the surface, it is the latter, short wavelength roughness pattern that influences hardness values the most.
Ra is the most widely used parameter for measuring surface finish. It is an arithmetic average of surface irregularities measured from a mean line that lies somewhere between the highest and lowest points on a given cut-off length. Today, there are more than 100 additional means or parameters with which to calculate surface finish from the same measurement data. However, Ra is the most common and is available through most entry level surface finish gages.
It is easy to visualize what happens when the indenter of a hardness tester impacts a smooth surface: The point—whether rounded or sharp—penetrates, and the area of contact increases geometrically until it reaches the depth associated with the hardness of the material.
However, when that surface is rough, a number of different forces come into play. If the surface has a consistent pattern of roughness, then the indenter must work its way through a succession of more or less evenly spaced peaks as its force is dissipated. Therefore, besides the hardness in the part, the indenter is seeing increased resistance as the probe increases its area of contact with the material. On the other hand, if the surface has some big peaks in it, the indenter will hit these first, then work through lesser peaks before it gets to the "real" surface.
Either of these conditions—and many others—will add variability to the hardness measurement result. How do you reduce the variability? Make sure the print for the part calls out a surface tolerance. Verify that the surface call-out is less than the requirements needed by the hardness tester. Then, use a gage to verify that the surface is within specification prior to performing the hardness test.
This process is no different from any other dimensional measurement. When checking gage blocks, be sure each block is clean, contains no burrs and has reached the proper measuring temperature. With hardness, the measurement process specifications should require the operator to verify that the surface is clean and has no burrs and that a surface finish check is done to ensure it is within the tolerance of the part and of the hardness testing gage.
Once the surface finish is within limits, variation in the results won't make you as tempted to test the hardness of your own head.