Hardness is an important concept when it comes to evaluating steels used in knife making. In this video, knife maker Walter Sorrells explains the meaning of hardness in the context of knife making, does a Rockwell hardness test, and discusses what hardness can tell us about the quality and usefulness of a knife.
People talk about hardness of steel as if it’s the end-all and be-all of evaluating a knife, but according to Walter, it’s not. Harder is not always intrinsically better, but sometimes, it can be. He informs viewers that Rockwell hardness numbers don’t tell you everything you need to know about a particular piece of steel or knife.
What makes a knife made out of steel better than a knife made out of Playdough? Obviously, steel is harder. So if steel is harder, steel must be better. If it’s harder, it will hold an edge better and it will be tougher, stronger, harder to break, and harder to bend. The truth is that hardness doesn’t necessarily mean any of those things. “Hardness” in the metallurgy world means resistance to plastic deformation.
Walter goes on to explain the Rockwell hardness test and does a little demonstration. Rockwell is not the only way to test hardness. There’s also Brinell, Mohs, scratch resistance, etc. That said, the same two samples won’t always correlate exactly between different measures of hardness because each test measures a subtly different quality of steel.
Does that mean that one test is more accurate or better? Walter says no, because “hardness” is what the test says it is, no more no less. He also compares and contrasts hardness to “toughness” and says that the harder the steel gets, the more likely it is to break or the less “tough” it is.
According to Walter, hardness is just one quality exhibited by steel that gives some useful information, but every steel is different. This is why it’s important to do research and look at the totality of a knife’s qualities. Instead of looking for the hardest steel in the world, look for the hardest steel that suits your needs. And if a particular steel suits your needs for all intents and purposes, it might as well be the hardest steel in the world.