Hardness testing is the easiest test to perform in our lab. but, no pun intended, it can be one of the hardest tests to do. Properly, that is.
Hardness is a property determined by measuring the resistance to deformation to the applied load perpendicularly indenting into the sample as expressed by the depth of penetration. The penetration is measured by a depth gauge and translated into a hardness number. It is crucial to realize that all conventional hardness testing methods involve sampling a volume of material. The amount of material actually sampled is a function of the indentor selected, the applied load and the material properties. If the sampled volume is limited by the physical size of the piece to be tested, then you may actually be sampling the underlying anvil or pushing out the edge of the sample.
Common indentors for Rockwell testing include diamonds and steel or carbide balls (though steel is being phased out). Applied testing loads range from 15kg for the superficial 15N and 15T scales to 150kg for the C scale.
Imagine a pen being pushed into a small cubic chunk of clay and then retracted. You are left with a hole whose depth is dependent on the force of your applied load and the resistance of that clay to the indentation. You may also see some bulging of the clay. Obviously, the heavily loads produce deeper indentations and sample more volume of material. Higher hardnesses are more resistant and result in shallower indentations.
1) Cube of soft clay
2) Effect of a small indenter and relatively light load. There is no observable distortion to the cube.
3) Larger indenter in early stage of indentation. The deformation is barely discernable.
4) Large indenter during indentation. The deformation is obvious.
5) Large indenter penetrating deeper. The deformation is pronounced.
6) Image processing highlights the edges of the original untested cube.
7) Processed image of the small indenter.
8) Processed image of the large indenter at full load.
9) Processed image of the cube after full load indenting with the large indenter.
This all sounds pretty simple. That is until you add some real world factors. Now what happens if there is a hard case over a soft core? Or a soft layer over a hard core (decarburization)? Or soft inclusions for machinability? Or hard particles such as carbides? What if the shape is complex and you don’t have flat parallel surfaces? Or you can’t get your indentor to the region of interest? The most common problem we see is that the material is too thin/soft/irregularly shaped to allow for rockwell testing or a test method that does not truly represent the hardness of the area of interest.
Indentors for microhardness testing include vickers and knoop with applied loads ranging from 1 gram to 1,000 grams. The indentations are measured optically at magnifications from 400X to 1,000X. We appreciate the superb optics of our Leitz VMHT. It uses magnifications of 500X and 1,000X and allows us to more accurately measure the boundaries of the indentations, especially when using lighter loads.
This is ideal for precisely sampling a small region of interest such as a very thin or soft sample or a sample with either hard or soft particles you wish to include or exclude. The knoop and vickers indentors have different aspect ratios. Knoop indentors are elongated and better suited for more precise measurements of layers or at specific depths. Vickers indentors are more symmetric and better suited for particle hardness measurements. The catch with microhardness testing is that it requires more sample preparation and operator skill, as well as expensive testing equipment. And unlike rockwell scales, microhardness numbers are affected by the respective applied load. For example, a knoop hardness value of 500 taken with a 500 gram load (HKN500 500) is not equivalent to a knoop hardness value of 500 taken with a 50 gram load (HKN50 500). Hence it is crucial to report the applied load with the test result value. Consequentially, conversions to Rockwell scales become more errant with lighter loads.
Our Leica VMHT microhardness tester. Measurements are made at 500X or 1,000X.
The loads range from 1 gram to 1,000 grams.
Knoop indentation used to measure the hardness of a layer of chromium plating. The elongated Knoop diamond is best suited for maximum sampling of thin layers or specific depths. This is especially useful for measuring case hardness, effective depth and near surface. While there are numerous micro fractures in the chromium plating, we sampled the best regions available in the sample.
Use of DIC (differential interference contrast) illumination illustrates the cold working resulting from the indentation. Heavier applied loads and larger indenters would result in more severe cold working.
We often find ourselves involved in disputes regarding hardness. The first issue to explore is to ensure that the specified hardness is appropriate for that material. We then investigate how the hardness was measured and if was it an appropriate method for that sample. While, there can be shades of gray and varying levels of uncertainty between labs we expect some level of consensus if the methods are correct.
ASTM guidelines states to report the rockwell readings results to the nearest integer. We have calculated our internal uncertainty for the rockwell C scale at 1 point over a ten year period., so we dissuade clients from conflicts of 1 rockwell point or less.
We don’t expect our clients to be experts in details of hardness testing, they leave that to us. However, it is important that the casual user understand the inherent geometric and equipment limitations of hardness testing as well as the repercussions of using a particular hardness testing method or scale where it may not be appropriate. This can lead to errant interpretations of the true material condition and properties.
Hardness testing labs should have the following ASTM specifications on file: E18, E3, E384 and E140. These specifications address proper sample preparations, selection of loads and penetrators, sample geometry, minimum sample thickness considerations, roundness corrections, spacing and edge considerations and conversions between scales.
We recently had an after hours voice mail inquiry for rockwell hardness testing. During a discussion with the client the following day, we learned that the samples were taken to another lab equipped with only a standard rockwell tester. Our services were still needed as the specification called for superficial rockwell testing using a 15N scale. Despite hardness certifications from the heat treater, one lot of parts were failing. After hearing a more detailed description of the samples, the combination of the specified hardness and part thickness did not meet the minimum thickness requirements per ASTM E18. Hence, we informed the client that microhardness testing was required for proper hardness measurements. We found good and failing parts to have the same hardness and that the actual hardness was out of their specified range. We took this a bit further as there can different microstructures exhibiting the same final hardness. Microstructural examinations revealed that in addition to the errant hardness, the offending lot was poorly heat treated (under austenitized).
Remedial action included revising the print to specify microhardness testing to verify the hardness and to restrict errant microstructures resulting from heat treating.