Don’t worry if your major was in something other than materials science, all you need to know here is that hardness isn’t one number. Hardness is a range. It is a spectrum. There are five scales for measuring hardness, to be precise. Those are graphed out on the chart above.
It’s a lot to take in until you remember that each scale have a different question. Is it going to get scratched by glass? (Mohs) Is it going to dent easily under a heavy load? (Brinell) Will it keep its edge after I hammer it around? That is the Rockwell scale. Knowing what test to believe helps you avoid purchasing a stock with the wrong properties.
How to Pick the Right Metal Hardness
So, at one end of the spectrum we have the softest metals: lead and tin. These guys are basically mush. They’re ideal for soldering or making a lead weight that sinks fast. You can even scratch them just by digging your fingernail in. And if you were to deform them, it’s easy because, that’s the point.
Up the scale, you come across metals such as copper and aluminum. These are workhorses of heat sinks and electrical wiring. They’re soft enough to be bent into shape without cracking but hard enough to retain their structure. Pure copper is actualy quite soft. It doesn’t take much force to leave a mark. That’s good for bus bars or gaskets when flexibility is more important than strength.
Next up, you enter the land of steel. And now this gets fun. As the graphic indicates, mild steel is right smack dab in the middle. It’s soft enough to machine readily and yet tough enough to make shafts and frames out of it. Mild steel can be welded up, tapped and drilled with no more than a few fits of fussing.
But there’s the rub. Steel isn’t all steel. No, my friend, it’s a chameleon. Depending on its treatment, the exact same alloy can be either hard or soft. That’s what most folks don’t catch, and that’s when heat treating makes a difference.
Take a bar of D2 tool steel and run it through an annealing process in a furnace until it’s softer than butter. Now you can machine it into any shape you want. Stick it back in a furnace, then quench it in oil, then temper it, and voila! It’s as hard as a rock. Its hardness increases three to five times. That’s metallurgical alchemy.
Now, let’s look at some specifics. Hardness is actualy measured in resistance to indenting. So you take a tool called a diamond cone (which is why we use the Rockwell C scale on harder materials) and push it against the metal and the metal pushes back. The softer it is, the deeper the dent will be. Because you wouldn’t want to do that with something harder than the metal you’re testing, right?
A ball would also distort the metal by denting it all over, so it doesn’t work well for softer metals where the steel ball won’t go far enough to damage the metal. But it doesn’t penetrate very far either and gets a good reading. Again, you have to test the right thing with the right method. Using the wrong scale is like measuring water with a ruler. It doesn’t work.
Some easy tests for hardness are available in the shop and one of them would be a clean file. You can rub the file across the metal and if it doesn’t bite at all, it’s probably very hard, perhaps past Rockwell C 60. If it bites into the metal fairly aggressively, then the metal is softer. It’s not a certified test but just a quick way to tell what’s hard and what’s not.
Keep in mind that thickness makes a difference. If you use only a thin piece of metal, the anvil below supports it, making the metal appear harder than it actually is. Make sure your sample is thick enough that the indent isn’t through to the back side.
The job determines which hardness band is best. What are you expecting the part to do? Will you be wearing it out? Will you be cutting it out? Forming it out easily? If so look to the higher numbers for the former and lower ones for the latter, with the middle as the compromise of both. Stainless and titanium fall somewhere in this middle (the chart), and they have good strength, good corrosion resistance, and aren’t the hardest metals.
So now when someone asks about material selection you can ask them what their application requires and point to a hardness number that represents “hard” and then another that represents “soft”. That’s what we call going from guessing to making decisions. Not breaking parts because you picked too hard a metal, or too soft a metal for your task.
Learn the scales; Metal has its rules, and metal is stubborn. Respect the heat, and test properly. Understanding these numbers is often the dividing line between success and failure in your parts.
Soft is a feature, not a flaw. Hard is a tool, not a virtue. Both have their place… Use wisely.
