The Importance Of Knife Blade Hardness

Part II

I'll discuss the importance of Rockwell Hardness for the knife performance, based on the true story :) That is, whatever experience I have gained from my own knives and hardness testing.
Gator, 04/10/2009

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Benchmade 710HSSR RC Test Indentation Benchmade 710HSSR Edge

I went to the place next morning, and brought nice bottle of scotch with me, for the tester guy. First test was for the 710 BC blade. Testing was done near the pivot area. First test showed 58.8HRC! I really couldn't believe it was below the advertised range of 60-62HRC, although that happened few times with other knife companies, but not with Benchmade. Therefore, I've asked the guy to run the test again. Different spot, identical result 58.8HRC. That really sucked. I have to mention here that different Rockwell Testers have different margins of errors. In general error can be up to 1HRC point. More precise testers have 0.1HRC margin of error, or accuracy. So, at best the blade could've been 59.8HRC, at worst 57.8HRC. Although, knowing that factory knives are always close to the lower end of the advertised hardness I didn't have much optimism. In the end I can't blame Benchmade for the low HRC, details below, but it's related to BC coating process. To make matters worse, after the testing I've accidentally dropped BC 710 blade on the concrete floor. Second picture attached to this paragraph shows the result of that drop. The first picture is a magnification of the two indentations left by Rockwell tester diamond penetrator. Pretty smooth, tiny marks.

Tadatsuna Usuba RC Test Indentation

Next up, Tadatsuna Kamagata Usuba. After seeing low results for BM 710, frankly I've lost hopes for any decent results here too. Testing Usuba was little bit different. It is a kasumi or clad blade. in other words there is a soft layer of steel(jigane) over hard steel core(hagane). Hagane is what I was interested in. First test went wrong, we hit jigane, but that was interesting itself too. Jigane tested 15HRC :) I could see that visually, the penetrator tip went real deep into the steel, making a bigger hole compared to 58.8HRC hard M2. Since the Rockwell testing calls for the flat surfaces we had to get the incline holder attachment and do the second test with it. The result was 62.3HRC. Now that was a lot closer to the target. I have never seen Tadatsuna's advertised hardness for their white steel, but the word on the street is that it should be 63-64HRC. If we go by that, then it's 0.7HRC below the lower bound of the range. However, in the absence of the official statement, it's nothing to judge. 62.3HRC is still ok for white steel, but I'd much rather have 64HRC or more. I'm pretty sure then I could grind at least 10° edge on this knife.

I've mentioned above that I can't blame 58.8HRC hardness of the second BM 710 blade on Benchmade. The fact that I've had it recoated with Boron Carbide at BodyCote could have affected blade tampering. Boron Carbide requires heating the blade to high temperatures, as far as I know. So, if the temperature was high enough it could have softened the blade, granted that the blade would be simply let to cool down without proper procedures. On the other hand, I am not certain whether or not it was BC coating. Fact is that Benchmade themselves were using BC coating on their 154CM knives, but not on M2 knives. May be they knew that was the side effect? Eventually Benchmade dropped Boron Carbide coating altogether. Which, also can be an indication of the problem. Either way, I'm not trying to find the guilty party here. Whenever you customize your knife there is always a chance something will go wrong. Lots of people detamper their knives using electric sharpeners after all.

Rehardened Benchmade 710HSSR and Kershaw Shallot CPM S110V

Next step was trying to fix all that. I already knew custom knife maker Phil Wilson was rehardening production folders for a reasonable price. So, I've asked him if he would take the job. Even though he never really worked on M2 steel Phil agreed :) I've also sent another blade, Kershaw Shallot, made out of CPM S110V. Crucible Metallurgy recommends 61-63HRC for this steel, while Kershaw hardens it to 58-61HRC. That's pretty low if you ask me. So, all three blades went to Phil, and few days later I got them back. I got lucky with timing, because as usual Phil has 2 week turnaround time for rehardened knives, but I managed to get in the right window, so for me it was just 3-4 days. Now, about the results. Mr. Wilson did retest Rockwell Hardness for all 3 blades for me. BC coated 710 blade showed 59HRC, which was rounded up from 58.8HRC. BT-2 coated M2 blade tested 60HRC, which was in line with expectations, and Kershaw Shallot CPM S110V blade tested 59HRC, which is also within the advertised 58-61HRC range, but way below 63HRC, the upper limit as per Crucible recommendations.

Rehardened Benchmade 710HSSR and Kershaw Shallot CPM S110V

Rehardening was kind of live process. Phil contacted me in the morning, and informed me that the blades were already quenched and at the time he was tampering them. Quenched M2 blades cam out 64HRC. After tampering they might loose some of the hardness, but I was hoping for the best ;) By the evening all 3 blades were ready. Rehardened blades tested as following: BC coated M2, which by the way, lost all the coating in the furnace at temperatures well above 2000F, came out 64.5HRC, thus it went from 58.8HRC to 64.5HRC. BT-2(formerly) coated knife went from 60HRC to 64HRC, and Shallot CPM S110V blade went from 59HRC to full 64HRC. Now, for M2 blades 64HRC is just fine, I know some makers, like Alvin Johnston take M2 all the way to 66HRC, but by the book heat treatment came to 64HRC, which should do a lot better compared to 59-60HRC it was. 64HRC for CPM S110V is 1HRC higher than the recommended upper limit. However, I am confident that for light cutting blades such as Shallot folder 64HRC will be fine. For one, Crucible in general is pretty conservative with their recommendations for their steel hardness. Second, I already have a few blades that are hardened above the Crucible recommendations, such as Phil Wilson CPM-10V Utility Hunter knife and Phil Wilson CPM S125V Meadows Semi Skinner. Both knives can and did take a lot of use and abuse, never had any chipping problems whatsoever. Given that CPM S110V has improved composition, I mean addition of Niobium, and initial input from Phil Wilson, I expect it to perform even better.

Lessons learned

- Well, I've learned quite a bit out of all this. The most important and practical was that single point on Rockwell Scale can make very significant difference. So much that the edge can be ground 10° less (or more). I don't think or state here that Rockwell hardness is single and most definitive parameter to judge knife edge holding and cutting performance. Not at all. But, given everything else equal, for light cutters it is indeed one of the most important indicators. What was really important to me in all this, is the fact that from my side it was 100% blind testing. I just had two blades, from the same steel and the same maker. For all I knew, they had equal hardness. The initial edge on both knives was ground using Edge-Pro Apex sharpening system, 30° included angle. That excludes any significant variations in the starting edge angle and sharpness. I've carried and used them for identical tasks during those years. Haven't changed neither my job nor habits. used the same knife sharpeners to maintain them. So, the use was quite uniform for both knives. Yet, eventually I've had to thicken the edge on the softer blade all the way to 40°. Even though the difference was just a single point on the Rockwell Scale. So, to me that clearly shows the importance of hardness for light cutting knives.

In general, for most of the steels higher the hardness, the higher the steel matrix strength will be, this wear resistance increases. The only exception I know is A2 tool steel, which increases toughness until it hits 57HRC, then there's a drop and then goes back up at 60HRC. However, the increase is not linear, which could explain dramatic performance difference between 58.8HRC and 60HRC. On the other hand, with the increase of the hardness, knife becomes more brittle. So, if taken too high, the edge will degrade due to micro chipping, instead of rolling and folding. Actually, for very hard blades it'll be macro chipping, not micro. Extreme example being the glass. It's not a steel, but it's very hard, keeps incredibly sharp edge, but useless for any utility cutting in general, the edge chips because glass is so brittle. Similar things happen to the steel in the knife blade edge. So, the perfect blade has to balance toughness and wear resistance. Also, that perfect spot highly depends on the designated use for the knife. For the kitchen knives and light, or even medium cutting knives harder blades are more important than toughness, because those are designed to cut soft materials. For chopping and harsh use knives such as Himalayan Imports Kukris or Busse Combat Knives matters are more complicated, since toughness is pretty much as critical as wear resistance. Thus the balance is harder to find ;) To hold the good edge the knife must have both, toughness and hardness in other words.

What next?

- Well, I have had 3 rehardened blades as I wrote this article. All of them looked pretty messy. First thing to do is to get nice satin finish on those. The plan was to polish one of the M2 blades myself, the one with 64.5HRC hardness. The other two went to one of the guys that do those kind of jobs and probably regrind as well. After that I'll be very happy to test the new, or renewed knives. So, finally, by the end of June 2009 I got two polished up blades back from Richard J. Read next.

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