There’s a rack of bars made out of different types of steel, and you have to pick one so you don’t break it when you put it under some load. Now you understand why hardness testing might come in handy if you’re a designer or a machinist. Here’s where the Rockwell scale comes into play. It can immediately tell you about resistance level of your material. And it doesn’t take a microscope and a lab tech to do it.
It takes special equipment called an indenter that is pressed into test material at various loads. It then measures how deeply it goes in the material. And that reading tells you how brittle, or durable, the material is. This chart breaks down the Rockwell Scale to show you what each one mean for materials. Now you won’t make the mistake of using a scale meant for hardening tool steel and thinking it applies to aluminum.
What is Hardness Testing?
Depending on the material you’re testing, the load and type of indenter will vary. For super-hard materials such as tungsten carbide, HRA scale requires a diamond cone indenter. Because these materials are so hard, softer types of indenters will bend and not adequately measure sample. It’s a brute force method for the hardest materials out there.
On the other hand, at the opposite end of the hardness scale you have the HRB scale which employ a steel ball instead of a diamond point. This allows less penetration into softer metals such as copper and brass. Using a diamond cone on softer alloys would result in an indentation that was too large and inconsistent. That would ruin accuracy of the test. Not only does it matter how you perform the test but also what scale you select.
HRC, If you are working with hardened steels, which includes most of what HRC covers, then you will know about them best. The blades on your knives and the cutting edges of your drill bits falls into that 58-64 range. Higher = Better edge holding but more likely to chip out from hard use. Lower = Better bending/deflection properties, but the material start to deform/ bend when put under torque.
Look at the chart above which shows typical values for common objects such as drill bits made from high speed steel. See where 440C stainless lands? It is around 58 HRC. If you have a standard piece of carbon steel sitting there unhardened it may only top out at 30 HRC. This gives you a good idea visually where your heat treat landed and lets you make an estimate. You do not need to wait for a full metal analysis.
Testing accuracy is affected by surface prep. This is where many people get in trouble. They go too fast and have wildly inconsistent results from test to test. Surface has to be smooth and flat, roughness will give you an irregular contact area. That skews the indentation depth. Are you testing over a mill scale patch or a machined scratch? Now you’re testing the geometry of the defect. It is not the underlying metal hardness. Sounds trivial but messes up your consistency each time.
Also make sure that thickness of the piece is appropriate relative to size of the indentation. A thin sheet may spring back compared to a solid block of material. The lack of mass under it won’t resist the force as well. For parts that don’t like a full load, there’s an option with light scales. This reduces the load being applied and in some cases can go down to a mere 15 kilograms. For HRC the normal load is 150 kilograms.
For super-thin coatings, these scales are necessary for testing surfaces. You just want to test the outer skin of the material. Without these, you may find yourself testing the core of the material rather than the hardened case layer. That is what you paid to have. They separate these ranges out on the infographic because special equipment settings are required. If you do not follow these settings, you run the risk of testing the wrong layer.
The calibration blocks are like a safety net for your data. They allow you to trust your data over time, even though your best machines does slightly drift over time (whether from temperature changes or mechanical wear). By running a certified reference block prior to your testing session you can be assured that your machine is reading within tolerance. It only takes a half minute. It saves you from wasting hours of rework and wasted material down the road.
Do you want to scrap out an expensive batch of parts? Or know if it’s a meter problem or a metal problem? When we say hardness, we’re not just talking about one thing; it’s a set of behaviors depending on how hard you poke at it. Knowing which scale works for your material is like trying to turn a blindfolded, bound man into a precision instrument while working in the dark. Matching the material characteristic to the test method means getting meaningful answers.
Whether it be a mild steel bracket or a carbide insert, this holds true. Know there is a particular set of geometry and force behind each number that reveals the true strength of the material. You should of checked your settings first.
