We continue our discussion on the principles of hardness testing begun in Part 1.

For the most accurate hardness measurements, proper alignment of the indenter to the sample surface is critical. The penetration is measured by either depth or area and then translated into a hardness number. It is important to realize that all conventional hardness-testing methods involve sampling some volume of material. The amount of material actually sampled is a function of the indenter 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 beyond the edge of the sample.

For example, imagine a pen being pushed into a small cubic chunk of clay and then retracted (Figs. 1-3). 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. A different sized or shaped pen (Fig. 4) produces a different type of indentation. As shown, the heavy loads produce deeper indentations and sample more volume of material. The higher the hardness of the material, the more resistant it is to penetration and the shallower the resultant indentation.

From this explanation, hardness testing sounds pretty simple; that is until you factor in real-world considerations. What happens if there is a hard case due to nitriding, carburizing or plating over a soft core? Or a soft layer over a hard core (e.g., 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 indenter to the region of interest?

One of the most common problems we see is that the material is too thin, soft or irregularly shaped to allow for Rockwell testing. Another common problem is that the individual or method specified fails to account for the three-dimensional aspect and the selected test method does not truly sample the appropriate volume of material.

Next time, we begin a discussion of Brinell hardness testing.

References

  1. Stone, Alan and Daniel H. Herring, “Practical Considerations for Successful Hardness Testing,” Industrial Heating, April 2006.
  2. Lysaght, Vincent E., and DeBellis, Anthony, Hardness Testing Handbook, American Chain and Cable Company, 1969.
  3. Wilson Instruments Division, Instron Corporation, Norwood, MA (www.wilsoninstruments.com)
  4. Midea, S., Brinell Testing, HOT TOPICS in Heat Treatment & Metallurgy, Vol. 2 No. 12, December 2004.
  5. Midea, S., “The Basics of Microindentation Hardness Testing,” HOT TOPICS in Heat Treatment & Metallurgy, Vol. 1 No. 2, December 2003.
  6. ASTM Specification Nos. E3, E10, E18, E103, E140 and E384 (www.astm.org).