Post by drhenley on May 14, 2019 13:46:32 GMT -5
I had to do a little research to understand the role of chromium in knife blades.
The oft quoted facts about chromium:
It makes stainless steel stainless
It makes knives hard to sharpen
Knives with a lot of chromium won't stay sharp
I didn't quite understand how, if a knife steel with a lot of chromium is hard to sharpen, why it won't keep an edge.
It has to do with chromium carbides. Specifically, Cr23C6 (K1) and Cr7C3 (K2)
When a knife steel has more chromium than can stay in solution with the iron, chromium carbide particles precipitate. It is the chromium in solution with the steel that makes stainless steel stainless. Stainless steels like 440A with a LOT of chromium to improve corrosion resistance will have more chromium carbide particles and larger chromium carbide particles.
These chromium carbide particles do make the steel more wear resistant (which is why they make high chromium stainless steel hard to sharpen) but that doesn't necessarily translate into staying sharp. The reason is that the softer material around the carbide particles on the edge wears away leave high spots where the chromium carbide particles are. Now on a flat surface, these particles provide a hard contact surface, but on a knife edge, the chromium carbide particles are not supported as well as on a flat surface, and when enough of the surrounding material wears away, the chromium carbide particles break away. This leads to microscopic chips in the edge. Once the chromium carbide particle breaks off, the material left at the edge is the softer steel, and the process repeats, wearing away the edge prematurely.
To better understand, imagine a knife blade made of concrete. You've got cement and gravel. On a concrete surface, it is the gravel particles that give the concrete wear resistance. But on a knife edge, the gravel particles aren't supported well and will break off. Then the cement, which is not wear resistant will wear down until more gravel particles are exposed and break off.
The larger the chromium carbide particles, the more pronounced the effect. So you want to control the chromium carbide particle size. K1 particles (Cr23C6) are larger and softer than K2 particles (Cr7C3). This is the difference between 440C which has large K1 particles, and AEB-L which has small K2 particles.
The following two images are of the K1 particles in 440C, and the K2 particles in AEB-L
The oft quoted facts about chromium:
It makes stainless steel stainless
It makes knives hard to sharpen
Knives with a lot of chromium won't stay sharp
I didn't quite understand how, if a knife steel with a lot of chromium is hard to sharpen, why it won't keep an edge.
It has to do with chromium carbides. Specifically, Cr23C6 (K1) and Cr7C3 (K2)
When a knife steel has more chromium than can stay in solution with the iron, chromium carbide particles precipitate. It is the chromium in solution with the steel that makes stainless steel stainless. Stainless steels like 440A with a LOT of chromium to improve corrosion resistance will have more chromium carbide particles and larger chromium carbide particles.
These chromium carbide particles do make the steel more wear resistant (which is why they make high chromium stainless steel hard to sharpen) but that doesn't necessarily translate into staying sharp. The reason is that the softer material around the carbide particles on the edge wears away leave high spots where the chromium carbide particles are. Now on a flat surface, these particles provide a hard contact surface, but on a knife edge, the chromium carbide particles are not supported as well as on a flat surface, and when enough of the surrounding material wears away, the chromium carbide particles break away. This leads to microscopic chips in the edge. Once the chromium carbide particle breaks off, the material left at the edge is the softer steel, and the process repeats, wearing away the edge prematurely.
To better understand, imagine a knife blade made of concrete. You've got cement and gravel. On a concrete surface, it is the gravel particles that give the concrete wear resistance. But on a knife edge, the gravel particles aren't supported well and will break off. Then the cement, which is not wear resistant will wear down until more gravel particles are exposed and break off.
The larger the chromium carbide particles, the more pronounced the effect. So you want to control the chromium carbide particle size. K1 particles (Cr23C6) are larger and softer than K2 particles (Cr7C3). This is the difference between 440C which has large K1 particles, and AEB-L which has small K2 particles.
The following two images are of the K1 particles in 440C, and the K2 particles in AEB-L