Keith Nix Knives FREE Learning!! Hardness, Toughness, Edge stability, Edge Retention. They are not the same, and in many instances are mutually exclusive. Try as they may, steel designers and metallurgists can't make a steel that scores a 10 in every category!
Hardness is a measure of the relative strength of a material. This is a little counterintuitive for me, my mind thinks the Charpy toughness test is about strength. But, the definition says hardness is directly related to a steel's "ability to resist permanently deforming". So I believe hardness is a contributor to edge stability. Hardness also increases wear resistance, so let's remember that too.
Now let's tackle toughness. Toughness is a measure of a steel's ability to resist catastrophic fracture. To left is a chart of various stainless steels and D2. The vertical graph labeled 5, 10, 15, etc, lines represent toughness as measured by the foot pounds of force required to break a standard subsize sample. The horizontal lines labeled 58, 59, 60, and so on represent hardness of the sample as measured on the Rockwell C scale. On of the first things I noticed is the harder a steel is, the lower its toughness. So there are two desirable properties that generally affect each other in a negative way. However, when it comes to edge stability, we need as much hardness/strength AND as much toughness/fracture resistance as possible. When you look at the chart above, you'll see why I chose AEB-l for my "House Stainless".
Carbides and Edge Stability-
We've established that edge stability is at least partially dependent on hardness and toughness, but there is another property at play in the game, and that is carbide size. Carbide size and volume negatively affects toughness and therefore edge stability, but POSITIVELY affects edge RETENTION. Here's why:
Carbides are combinations of carbon and one or more other elements found in the steel. There are iron carbides, Chromium, Tungsten, Vanadium, and on and on. These carbides are MUCH harder than the steel matrix that surrounds them, but they're also quite brittle. In the images above you'll see a steel cracking at and then through a carbide particle(Steel matrix is lighter gray, carbide darker gray). So we have another conundrum here. More carbides lead to better edge retention, but poorer toughness.
We can work around that a little by processing steels to have very small carbides. It has been proven through testing that smaller carbides are less detrimental to toughness and therefore edge stability, while still being in the composition for edge retention. So we want a steel with extremely fine carbides. Again, AEB-L.
Here are SEM images of D2 on top and AEB-L, bottom.D2 has very poor toughness, and the carbide size reflects that, while AEB-L has tiny carbides and great toughness. While other factors contribute, it is to a large degree the carbide size in D2 that makes it test so poorly in toughness, while also being a solid performer in edge retention. So steel selection is a tight rope walk of sorts, trying to balance the properties we've discussed. There are folks out there who look at toughness ONLY, or others who are willing to sacrifice all other properties for the very best edge retention, or stain resistance, or some other trait. My approach in knife design is to find the steel with the best BALANCE of properties, including the ones we haven't discussed, sharpenability, affordability, and ease of manufacture. All those tangible and intangible properties contribute to making you the best, most affordable knife I am capable of making. Additional Reading! The Great Steel Debate
How I Design and Make a Kitchen Knife Part 1
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