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Hardness tests measure indentation resistance, not sheet steel formability

These tests are suitable when working with tool and die components

Hardness tests will not give you the information required to decide how the sheet will perform in a stamping press.

If you are using mild steel that’s about 1.5 millimeters thick, it probably has a Rockwell B hardness in the mid to high 70s. But what does this reading mean?

Simply put, hardness is a measure of the resistance to indentation. Of course, different materials have different performance, but the test result also depends on what kind of indenter is used (size/shape/material) and how much force is used to push it into the sheet metal. These testing parameters determine which scale is used to report results. Three scales are appropriate for typical sheet steels that are used for stamped parts: Rockwell B, Rockwell C, and Rockwell 30T (see Figure 1).

A light initial penetration known as the minor load seats the indenter and provides a reference depth. For the B and C scales, 10 kg/cm2 of force is used; 3 kg is used for the 30T superficial scale.. An additional major load is applied, which results in deeper penetration into the sheet metal surface. The major load then is removed, and the minor load is applied again. The difference between this depth reading and the reference depth is used in the Rockwell hardness calculation, and is d in the equation for the Rockwell B scale (see Figure 2).

This calculation (see Figure 3) shows that if a Rockwell B value of 80 is measured, there is a 0.10 mm difference in depth between the minor and major load, and for HRB 60, there is a 0.14 mm penetration. Put another way, the difference in penetration depth between readings of HRB 60 and HRB 80 is only 40 microns. As a point of reference, the thickness of human hair is about 100 microns (0.10 mm).

Anvil Effect Means an Invalid Test

Independent of the hardness scale used, a deeper, wider impression will allow for more accurate and representative readings. However, if the impression is too deep, then the platform that supports the test piece, known as the anvil, will influence the result. According to ASTM Standard E18 for Hardness Testing, to prevent this anvil effect, it is necessary to have the indentation depth no more than 10 percent of the total test piece thickness.

If your indenter or hardness test scale is not appropriate for the sheet metal, you’ll likely see a shiny or burnished spot on the test piece underside where it was pushed into the anvil surface. This means you are testing the hardness of the anvil rather than the hardness of your test piece. You’ll need to change your test conditions to produce a smaller, shallower indentation.

As an example of why this part of the specification is relevant, using the measurements in Figure 1, you would be in violation of the ASTM requirements if the Rockwell B hardness reading were 80 or less on sheet metal that is less than 1 mm thick. Why? HRB 80 means an indentation depth of 0.10 mm, and as the indentation depth increases, the hardness decreases. Ten times this indentation depth is 1 mm. Any greater penetration violates the 10x rule, and you are likely going to see the influence of the anvil in your results. The applied load on the Rockwell B scale is 100 kg. To produce a shallower impression, you should switch scales, potentially to a 30T scale, where the applied load is 30 kg.

Appropriate Use of Hardness Testing

Hardness testing is appropriate for use with tool and die components, where resistance to indentation can help you judge one of the potential component failure modes. In sheet metals, the failure condition usually is the beginning of a neck (also called a smile) generated in tensile conditions. When stamped sheet metal fails, that failure usually is unrelated to the degree to which the sheet metal can handle a hard ball pressing into it.

In the first paragraph, I told you that your mild steel had a Rockwell B hardness of about 75 to 78. If I could do that without knowing anything about your coil, what does that tell you about the usefulness of hardness testing of sheet products? The bottom line is that hardness measures the resistance to indentation, but is a poor predictor of sheet metal formability.

About the Author
Engineering Quality Solutions Inc.

Daniel J. Schaeffler

President

P.O. Box 187

Southfield, Michigan 48037

248-539-0162

Engineering Quality Solutions Inc. is a provider of practical solutions for sheet metal forming challenges.