You’ve probably noticed that hardness is sometimes reported on your sheet metal certs (and if you see it, you are likely paying for it, probably a few dollars a ton). If you are using mild steel that’s about 1.5 mm (0.060”) thick, it’ll probably be in the mid to high 70s if it’s measured on the Rockwell B scale. But what does this really 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 what scale is used to report results.
Rockwell hardness values are determined using a two-step process. First, the indenter (either ball- or cone-shaped) is pushed into the surface until the desired pre-load (also called “minor load”) is reached (10kg for the B and C scales, 3kg for the N and T superficial scales). This small initial penetration seats the indenter and provides a reference depth. An additional “major load” is applied, which results in deeper penetration into the sheet metal surface. The major load is then removed and the minor load is re-applied. 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:
HRB = 130 – ( d / 0.002mm )
This calculation shows that if a Rockwell B value of 80 is measured, there is a 0.10mm difference in depth between the minor and major load, and for HRB65, there is a 0.13mm penetration. Put another way, there is only a 30 µm 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 µm (0.10mm).
The Brinell hardness test involves applying a specified load using a hardened steel or tungsten carbide spherical indenter of a specified diameter (typically 1mm to 10mm). The Brinell hardness number is calculated by dividing the load applied by the hemispherical surface area of the indentation. Due at least partially to the relatively high loads and to the challenges of measuring a curved surface area, Brinell testing is typically not used for sheet metal.
Like Brinell testing, the Vickers hardness number is calculated by dividing the applied load by the surface area of the indentation. However, a Vickers microhardness test is typically done with significantly less force than a Brinell test, using a diamond indenter having 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, it becomes possible to target specific microstructural constituents (like martensite or bainite in Advanced High Strength Steels) or to assess the quality of heat treating or surface hardening operations.
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 avoid this so-called “anvil effect,” it is necessary to have the indentation depth no more than 10% of the total test piece thickness. If your indenter or hardness test scale is inappropriate, you’ll likely see a shiny spot on the test piece underside where it was pushed into the anvil surface. If you see this, then 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.
A brief example of the relevance of this part of the specification: Using the measurements shown above, you are in violation of the ASTM requirements if you are getting a Rockwell B hardness reading of 80 or less on sheet metal that is less than 1mm thick. Why? HRB80 means an indentation depth of 0.10 mm, and as the indentation depth increases, the hardness decreases. 10 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 more shallow impression, you should switch scales, potentially to a 30T scale, where the applied load is 30 kg.
Something else to think about … In the first paragraph, I told you that your Rockwell B hardness was about 75 to 78. If I was able to 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.
|Selected hardness scales||Indenter||Applied load|
|Rockwell – B scale||1/16” diameter ball||100kg|
|Rockwell – C scale||120° diamond cone with a 0.2mm radius spherical tip||150kg|
|Rockwell – 15T scale||1/16” diameter ball||15kg|
|Rockwell – 30T scale||1/16” diameter ball||30kg|
|Vickers||Square-based pyramid diamond indenter with a 136º included angle||Typically 10g to 1,000g|
|Brinell||Spherical indenter, with a diameter typically ranging from 1mm to 10mm.||Typically 1kg to 3000kg|