What still confuses me is the grain refinement of the various forms of producing steel. Traditional D2, for example, has coarse carbides that make edge refinement difficult, reduce blade toughness and increase edge wear resistance. Thats ingot D2, one of the most popular and respected of the traditional knife steels.
The apex of a sharp knife edge is 1 to 2 microns wide. The carbides in D2 are 40 microns or so. A traditional steel like 12C27, Sandviks main knife steel, will have an average carbide size of 0.5 micron. The large D2 carbides are not evenly distributed. They can clump, reducing toughness. And they can rip out when the steel is sharpened. You can literally put a razors edge on 12C27.
My unsupported understanding is that the average grain size in D2 was about 10 microns, with the grain size of powder D2(CPM D2) being 5 microns. I realize that Me2 disagrees, saying the powder process has little effect on grain size, but neither of us has been able to present any evidence. I don't care who is right, I'd just like to know.
But we seem to agree that the carbides will be smaller and more uniformly distributed in powder D2 than in ingot D2, making the powder steel tougher and better able to be honed to a fine edge.
Now comes PSF27, which is Carpenters D2 made with the spray forming process. The alloy is the same as D2, but the spray forming process is supposed to create a very fine grain, reduce carbide size, improve carbide distribution, increase toughness and allow the blade to be hardened to a higher Rc. In other words, the steel will have much better edge stability (strength and toughness) and be able to sharpened to a finer edge. It's D2 on steroids.
But what happens to the grain structure?
You can "literally" put a razors edge on ingot D2 as well.
Please, folk, read back: having large carbides at the edge allows for MORE edge-refinement, and not just a little bit more but
10-100X more refinement. With its small carbides and volume, you can sharpen 12C27 down to 0.5 um apex diameter ... but you can achieve 0.05 - 0.005 um with WC-Co, diamond, and obsidian, i.e. carbide edges. Why? Because the carbides are harder (> 80Rc), less flexible or prone to deformation, so they can be shaped to a point/plane 10-100X narrower than low carbide 12C27 can achieve with any stability. 12C27 taken that sharp is LESS stable (strength & toughness) because it is too soft to resist deformation at such thin geometry, it rolls and flattens. If you could place ingot D2's large carbides in the apex and shape them appropriately, you'd have a knife 10-100X sharper in that section. If D2 is "toothy" at a microscopic level, it is from having some sections of sharper carbide (much sharper than 12C27
anywhere) and some sections of duller matrix ("dull" at 0.5 um like 12C27).
With 12C27 and other razor steels, they easily take a "dull" edge, but that edge (0.5um) is "shaving sharp" and relatively "tough" against impact stress meaning it with deform rather than fracture - lots of room on the stress/strain curve. With harder carbide, there isn't much room in the stress/strain curve for deformation hence it being "brittle" not in the sense that it is weak, indeed it is much stronger than the matrix, stronger than 12C27 at max hardness, especially at such thin geometries where it will actually hold its shape, but in the sense that when lateral or impact stress is applied it will NOT deform as readily and will fracture in short order. That said, it isn't usually the carbide itself fracturing but rather the weak bond with the matrix (grain boundary).
To improve steel toughness, industry works to reduce the size of these carbide-grain boundaries by reducing the size of the carbides themselves such that lateral and impact stress on a section of edge can be distributed to many small grain-boundaries rather than one large one more susceptible to fracture-growth.
But again, the 12C27 and other"fine" steels do not obtain a "finer edge" than others, they simply are able to obtain that "fine edge" (more of a weak "working edge" really) more easily than high-carbide materials, and that edge is tougher and easier to maintain (requires less careful technique, less specialized equipment).
When speaking in relative terms - fine, dull, tough, brittle, strong, weak - it is important to specify the measurement range you are describing. If comparing >80Rc to ~60Rc, which is weak? If comparing 0.5um to 0.05um, which is "fine" or "sharp" vs. "dull"?
Also, when discussing "edge stability", one should specify the geometries (edge thickness) and stress to which that geometry is subjected. Remember that abrasive wear is, at the microscopic level, impact stress and lateral loading, so an edge more resistant to that stress could be described as "more stable" ... but the conditions are very specific.
If someone understands these notions differently, please explain why as i am by no means an expert. Alright, now back to an awesome thread about the matrix grain size (rather than carbide grain size). Thank you for your patience.
