Thank you as always for getting down to the science of it Mr Jan! I love to hear about real metallurgy. I would never have thought about how the steel beneath the scratch is affected, so I have a couple questions.
First, when you say "scratch width is proportional to scratch depth", it makes me wonder about all sorts of things. It can't be the same for different types of abrasive, can it? In the most basic sense, a brand new belt works much differently than one that's even slightly used, so I have to wonder about the qualifications of scratch width being proportional to it's depth. I don't understand why a scratch needs to be about 8x as wide as it is deep.
From a traditional point of view, you're right, Mr Jan. I've always sort of accepted the theory that the coarser the scratch pattern, the weaker the edge. I'm not sure where that comes from, but it seems to be an accepted theory in general. Personally, I see a lot of wiggle room. What grit? What steel? How hard? What are you cutting? Etc.
I can remember when S30V first came out as the most revolutionary steel to ever hit the cutlery market. The goal was Rockwell 60-61, but at that hardness S30V was too chippy, so they had to back off a couple points. Might not seem like much, but it made a big difference. In particular, the consensus was that if you wanted your new S30V blade to perform, the edge had to be highly refined.
With S30V at RHC 58-59, it wasn't chippy, but now it wasn't strong enough to hold a keen edge. S30V garnered the reputation as a steel that was tough to sharpen, but it could hold a mediocre edge for a long time. Not exactly the performance they were aiming for, but it kicked off the powdered steel revolution, which is practically ubiquitous today. Now we have S35VN, a third generation powdered steel, which meets all the requirements S30V was supposed to deliver. I dare say it's probably the most popular high performance stainless cutlery steel in America.
What I'm driving at is the fact that the steel we use now probably doesn't behave like the steel in textbooks. Even the former highest performance carbon HSS, M2, is now getting clobbered by the powdered versions, CPM4 (M4) and CPM4V (4V), in the cutlery world.
With the advancements in steel, are we reviewing the aspects of the metallurgy? Are we re-writing textbooks? Mr Jan, is your textbook example applicable to cutlery abraded with very little pressure? It seems pressure must be a pretty big factor when you're talking about affecting the steel below the surface of the scratch.
Long ago I had some understanding that much of what was being taught in metallurgical classrooms didn't come to bear on cutlery. It seems like the gap can only widen with the advent of completely different, specialized cutlery steel.
I only have one knife in S30 that sees any use. It's an old Sebenza that I used to keep polished. I've polished about 25% of the blade off, so I have a decade of experience with polished S30V. I've only been using super tooth S30V for six months or so, but the difference is unquestionable IMHO. My disdain for S30 has been eclipsed, because it works fine at 120 grit.
If I do have to sharpen more often, it's way quicker for me to get toothy than polished, but YMMV.
I know I'm bucking conventional wisdom, and questioning some of the validity of basic metallurgy in regard to cutlery. I do hope you'll forgive me, Mr Jan. I promise I don't want to be argumentative. I always appreciate your educated positions to the matters at hand.
Indeed, there are still many riddles in relation to sharpening and cutting, which is puzzling. Why is this so ambiguous? Doesn't it seem odd that we can send a probe through 2,700,000,000 miles of -300*F space to study an entire planet in detail, but we can't figure out the best way to cut rope? Astonishing.
Hmmm... Looks like I'm leaking somewhere! 