Daniel Schaeffler Daniel Schaeffler

Limitations of Hardness Testing for Deep Drawing Applications

July 29, 2020

Your sheet metal certifications may contain a reading documenting the hardness of the received product. For 1.5-mm-thick mild steel, the hardness probably will range from the mid to high 70s on the Rockwell B scale. But what does this value really mean?

Hardness testingSimply, hardness represents a material’s resistance to indentation. Of course, this resistance varies among material types, but hardness-test results also depend on the size, shape and material of the indenter, and the amount of force used to push it into the sheet metal. The appropriate scale is a function of these testing parameters. The accompanying table displays the testing conditions associated with common hardness scales used in metal testing.

Determining Rockwell hardness values requires a two-step process. First, the test apparatus pushes the indenter (either ball- or cone-shaped) into the test-piece surface until reaching the desired preload (also called “minor load”) force—10 kg for the B and C scales, and 3 kg for the N and T superficial scales. This small initial penetration seats the indenter and provides a reference depth. Application of an additional “major load” results in deeper penetration into the sheet metal surface. Removal of the major load occurs, followed by a re-application of the minor load. The Rockwell hardness calculation defines the difference between this depth reading and the reference depth as “d” in the equation for the Rockwell B scale:

HRB = 130 – (d / 0.002 mm)

Per this equation, a hardness measurement of HRB 80 means that the indenter penetrated 0.10 mm into the metal, and 0.13-mm penetration occurrs on a metal with a hardness value of HRB65. Put another way, there is only a 30-micron difference in penetration depth between readings of HRB65 and HRB80. As a point of reference, the thickness of human hair is on the order of 100 microns. Another option for measuring hardness, the Brinell hardness test involves applying a specified load using a hardened-steel or tungsten-carbide spherical indenter of a specified diameter (typically 1 to 10 mm). To calculate the Brinell hardness number, we divide the load applied by the hemispherical surface area of the indentation. Measuring sheet metal hardness with Brinell tests is not common, due, at least partially, to the relatively high loads required and to the challenges of measuring a curved surface area.

A third option is the Vickers hardness test, and, as with Brinell testing, calculating the Vickers hardness number requires dividing the applied load by the surface area of the indentation.  However, compared to Brinell testing, a Vickers microhardness test applies significantly less force to the sheet metal. The Vickers indenter is a diamond with a square cross-section.  Built into the Vickers microhardness test machine is a microscope that allows for more precise measurement of the diagonal cross-sectional lengths.  By magnifying the surface, we can target specific microstructural constituents (such as martensite or bainite in advanced high-strength steels), or assess the quality of heat treating or surface-hardening operations.

Regardless of which type of hardness scale is used, making a deeper, wider impression allows for more accurate and representative readings. However, if the impression is too deep, then the hardness of the platform that supports the test piece, known as the anvil, will influence the result. According to ASTM Standard E18, Standard Test Methods for Rockwell Hardness of Metallic Materials, to avoid this so-called “anvil effect,” indentation depth must not exceed 10 percent of the total test-piece thickness. Using an inappropriate hardness scale with an excessive indenter force results in a shiny spot on the test-piece underside showing where the indenter pushed into the anvil surface (figure above). Creating such a spot indicates that the test has measured the hardness of the anvil, not the test piece, and that the test conditions will need to be changed in order to produce a smaller, shallower indentation. In cases where the test pieces are too thin for Rockwell B and C scales, technicians will use the Rockwell superficial scales of T and N, as these scales use reduced minor and major loads compared to those used for B- and C-scale tests.

Hardness Scales used in Testing of MetalsAn example of where this matters:  Using the measurements described earlier in this article, a test providing a Rockwell B hardness reading of 80 or less on sheet metal less than 1 mm thick violates the ASTM requirements. Why? HRB80 indicates a 0.10-mm indentation depth―with lower hardness readings associated with increasing indentation depth. Ten-times this indentation depth is 1 mm, and any greater penetration violates the 10x rule; you’ll likely see the influence of the anvil in the test results. The applied load on the Rockwell B scale is 100 kg. Should a shiny spot appear on the underside of the test samples, switch scales to produce a shallower impression, potentially to the 30T scale, with a 30-kg applied load.

Another consideration: We started this discussion stating that an incoming coil of mild steel has a Rockwell B hardness in the mid to high 70s. If we can make this statement without knowing anything else about the coil, what does that tell you about the usefulness of hardness testing of sheet products?  Hardness testing can distinguish if your coil is aluminum, mild steel or high-strength steel, but do not use hardness-test results to predict formability differences between coils of the same grade. Hardness measures the resistance to indentation, not formability.  MF

Danny Schaeffler is the technical editor for metallurgy and forming for the upcoming release of the Advanced High Strength Steels Guidelines. Contact him at ds@EQSgroup.com to contribute a case study to the Guidelines. We’re looking for lessons-learned, where things may not have gone smoothly at first. We will be giving full attribution where requested, including links to your company.

Industry-Related Terms: Case, Forming, Martensite, Penetration, Point, Rockwell Hardness, Scale, Surface, Thickness
View Glossary of Metalforming Terms


See also: Engineering Quality Solutions, Inc., 4M Partners, LLC

Technologies: Materials


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