Super Hard vs. Soft Edges
Or Are Very Hard Edges Really That Brittle

Cutting material for this(and follow-up) test was aluminum. To be precise, aluminum bottle caps for test #1, which by no means is a normal cutting job for any knife, but given the primary goal of the test, damage to the edge, I figured I had to use something harder, yet still manageable for some time to get some results. The aluminum cut in this tests was 0.14mm(0.0055") thick and about 25mm in diameter. Knives tested in this session: Benchmade 710 M2 HSSR and Calphalon Paring Knife.

Contender knife number one - Benchmade 710 HSSR, which has M2 high speed tool steel blade, originally it was 59HRC, and later on, in April 2009, Phil Wilson rehardened it as per my request to 64HRC. Initial edge on the Benchmade 710 knife was approximately 13°-14  per side. I've measured using 2 different methods, height from the sharpening stone divided by blade width, and calculating arcsin from that, and by measuring bevel width and edge thickness behind the edge, calculating arctan of the divided value. Since I have already done some wire (copper, aluminum and steel) cutting with the rehardened Benchmade 710, without any damage to its 64HRC edge, I went with the harsher test. I cut the tops of the caps from the bottles. It's quite harsh on the edge, because the edge, at least one side of it comes in contact with glass, which is harder than the metal and can easily deform the edge. Obviously I was being cautions not to hit the glass directly. Overall, I cut 8 of those caps, before I run out of them. The diameter(d) of the caps is ~25mm, so overall length of the aluminum that was cut with 710M2 blade was 8×π×d, which comes about 630mm, or ~25". The resulting edge did loose original shaving ability(opposite direction, no skin irritation). Based on my past experience, I didn't try plain leather, since it doesn't work well on the edges that hard, instead I went with 0.25µm loaded leather strop. Although, in retrospect, probably more fair thing to do was to use steeling with borosilicate rod instead of stropping on abrasive compound, however, given the very low number of strokes I had to perform to restore the sharpness, it is rather irrelevant.

Anyway, with 10 strokes per side, 64HRC M2 steel blade regained pretty much all of its original sharpness. Jumping ahead, I did make another set of cuts with M2 blade, after I realized secondary edge, i.e. restored edge, was a lot less durable on Calphalon. Second run included 3 discs cut in quarters, on diameter, total of 150mm, and the loss of sharpness compared to the first M2 test was less, but still detectable. To restore sharpness I needed only half of the effort, 5 strokes per side. Well, may be it'd work in 3 too, but I didn't check the edge until I made 5 strokes per side ;) This concluded testing of the M2 steel knife. One thing worth mentioning here is that, at no time during first and second cutting sessions I felt that I needed to apply greater force, or the cutting became harder. Obviously, human feelings are not the most precise measurement, but since I couldn't tell physically the change in force, it was quite small for practical purposes.

Now, let's examine the damage, or the absence of it. The second and third micrographs attached to this paragraph are BM 710 HSSR M2 blade before and after cutting all that aluminum. It is considerable trouble finding the exact spot with microscope twice, but markers help, unless you have to cut something wet in between photo sessions. Anyway, this time there was a clearly visible scratch on the bevel, which I've used as a guide, and the "worst" damage the edge sustained was there. I use the word worst in quotes, because as you can see there is no real damage to speak of. The two slightly brighter spots in the middle of the third image are the rolled, but not chipped(!) sections of the edge and that's all that happened to 64 HRC M2 steel. For the record, the photos are taken right after the first cutting session, i.e. 630mm aluminum cut. I did examine the edge after second, 150mm long cutting session, but even under microscope I didn't find anything worth photographing. I can't say I was surprised, based on past cutting experiments.

Contender knife number two - Calphalon paring knife. Unknown steel, unknown hardness. Making an educated guess, 440A or X50CrMoV15 class steel, hardened to 54-56HRC. Given the small size of the knife, most likely it is closer to 56HRC, although I might be wrong on that one, however, generally that is the case with mainstream kitchen knives, large blades are closer to lower end of the official hardness range, unfortunately often below that too, and the smaller knives are a little harder. At least within the spec hardness range. That calphalon was given to me as a present and I had different experiment planned for it, but this was a good opportunity too. I'll do the rest of the planned experiments later anyway. The original edge was your average factory kitchen knife edge. ~40° inclusive angle, rather rough finish, judging by the size of the scratches on the bevel, the final abrasive used on it was below 800 grit. As mentioned above, I've used the edge-pro sharpening system to grind precisely 15° edge, which is 30° inclusive angle. That's quite a bit thinner than the factory original, but to make the test results more fair in terms of comparison, I had to thin it down. As a sidenote, 30° inclusive angle works just fine even on the soft steel knives like that Calphalon. That is, if you use it for its dedicated purpose and don't try to cut through the bones with it, plus you get significant increase in cutting ability.

Cutting with calphalon paring knife was conducted using the same pattern. Cutting aluminum, edge examination, attempted restoration, repeat from step 1, total of 3 sets. What was different with Calphalon, and what in fact gave Calphalon knife an advantage over BM 710 M2 knife was the fact that the aluminum was already in disc forms, and Calphalon knife never had do go anywhere near the glass of the bottle, plus in addition to that Calphalon's 15° was slightly more obtuse than the one on BM 710 M2. First run was cutting 2 discs in quarters, which comes to total of 100mm. Examined the edge, and the section I was using to make those cuts, about 25mm(1") long was dull, it lost shaving ability completely and I could see rolls reflecting the sunlight. I've used smooth steel, well, borosilicate rod to be precise, and to my surprise, about 10 strokes per side brought back shaving sharpness, mostly. There were several small sections, that were damaged bad enough that steeling, at least 10 strokes per side couldn't bring them back to alignment. I took another aluminum disc and cut that in 4 pieces, another 50mm that is. The restored edge was gone again, and I felt it was too quick too. It was at this time when I went back to Benchmade 710, cut another 150mm with it and tested sharpness to see if it would loose restored sharpness as quick as Calphalon did. As you already know, the answer is no, rehardened M2 steel stood up a lot better to all that abuse. Next, I've used a smooth steel again, this time it took 25 strokes per side to bring shaving sharpness back to the 1" test section, but I couldn't restore all of it. There were few spots on the edge that were still rolled, clearly visible by their reflections of the light. Disc #4 was also cut into 4 pieces, but I couldn't cut disc #5 into 4 pieces. The edge became so dull that cutting became practically impossible, I had to exert too much force and given the small size of the discs I wasn't in the mood stabbing or cutting myself on the slipped knife or the jagged edge of the disc leftovers.

Final attempt to restore the edge started with the same smooth steel, or borosilicate rod, but after well over 50 strokes per side I understood that I was achieving nothing, and I've picked up 1000 grit ceramic stick, which for the record, is much more coarse compared to 0.25µm(~100 000 grit) abrasive strop I've used to restore M2 steel edge, and tried to sharpen the maimed edge of the poor Calphalon parer with that. I did get some improvement, but overall, as far as removing the rolls and restoring the shaving sharpness goes, no, I couldn't do it. That concluded the testing of the calphalon knife that day. Now let's see the carnage inflicted onto the soft steel. Unfortunately, due to processing error, I've lost the original edge micrograph, but no biggie, there is one from the second test, and you have my word, it looked exactly the same when pristine ;) So, the #2 image is the edge on Calphalon before testing, images #3 and #4 are consequently 65x, 120x magnifications of various damaged sections of the calphalon edge, after I've stopped cutting with it. As you can see there is no edge to speak of left on the knife. The edge is rolled, twisted, small chunks are missing altogether. So much for the durability and ease of restoration, but about that later, in the conclusions section. Although, I have to say, I never expected the extent of the damage to be that large.

After posting the results and micrographs on bladeforums, ensuing mini discussion brought up another idea, to test ZDP-189 steel in a similar fashion. It is one of my favorite steels, but in the knife world, many consider it too "chippy" for practical use, to which I wholeheartedly disagree. Yes it's not a good choice for a heavy chopper, but for kitchen knives and generally light cutters, ZDP-189 at high hardness is one of the best choices. Thus, when I thought the experiment was over, I got more knives to test.