It is solid. When you pick up piece of steel, it feels solid. You think you know what it will do for you. Often times that’s not true.
That misconception leads to broken molds and snapped off prototypes in short order. This results in failed tooling that is never use in production.
How to Choose the Right Hardness Test
The hardness isn’t something you can measure with a ruler. It’s a collection of properties that indicate a metal’s behavior under wear and stress. Knowing what those numbers mean help you obtain correct result.
These properties are mapped out in the chart above at five different scales. Not all materials can be tested with one test. Tungsten carbide wouldn’t be tested with a lead ball. Pure aluminum wouldn’t be tested with a diamond indenter. The chart breaks this down nicely.
You see that Rockwell B applies to softer alloys while Rockwell C apply to harder, hardened steels. Why? If you use the incorrect scale, it is nothing more than a guess. That guess makes you feel confident when you should of been.
Now think about the contrast of hardness or softness. The chart shows Brinell hardness of Annealed D2 Tool Steel is approximately 220. That means it can be shaped easy into intricate forms on a CNC machine or mill. But when heat treated, this same material reach a Rockwell C 60 hardness. That’s quite a leap!
Soft becomes rigid. It goes from being easy to shape to resisting deformation. And there’s the reason companies purchase their metal in the soft condition. It is shaped into form and then hardened for use at the end application.
There isn’t a way to get both extremely high machinability and extreme wear resistance in the same condition. It’s pick your moment.
The conversions from one scale to another are useful. But they’re dangerous when viewed as absolute. There are no hard, fast rules. The chart represent ballpark relationships. For example, it relates approximately 32 Rockwell C to about a 300 Brinell.
This isn’t gospel. Instead, this is an average drawn from standard data tables. Grain structure and alloy composition will alter these numbers by as much more than ten percent. Surface finish can affect results too.
Don’t trust a conversion table when making important decisions. Whenever possible, make measurements within the scale applicable to the family of materials you work with.
Another misstep with hardness testing can occur at the surface prep level. Test a heat treated part straight from the oven and your results won’t mean much. The oxide scale is either harder or softer than the metal underneath. This forms a buffer between the tester and real material and skews the indent.
Grind off a small amount of material until you get down to bare metal. Indentation placement is also significant. Place them too closely and the previous test will influence the next. Work-hardened metal increases the number.
These details define precision engineering.
The file test is one of those practical shortcuts many machinists employ. Take a file. If it skates across the material cleanly, the steel is probably higher than Rockwell C 60. If it bites its softer. There is no certifiable data here. It’s simply a fast means of separating hard from the rest.
Being able to know where your material falls on that scale allow you to choose appropriate cutting tools. You don’t want to go running carbide inserts on soft copper. And you sure as hell don’t want to go running high-speed steel on hardened tool steel.
Matching material behavior with a job requirement result in hardness. The materials used for conducting and forming is pure aluminum and lead. Medium-hard steels is used for general machining and structural strength. Tungsten carbide is an ultra-hard metal used for cutting other metals.
That’s how you match behavior to requirements. Understanding these bands makes you an engineer instead of a guesser.
Above is a chart of reference points. You make the judgment call. Choose the right scale, prep the surface and respect the material’s limits. That’s how to keep your parts in spec and your tools sharp.
