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The Importance Of Knife Blade Hardness

Part I

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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Generally, Rockwell Hardness is pretty much the only numeric indicator telling the buyer something about knife blade properties, besides from the steel of course. Quite often knife manufacturers don't specify neither steel, nor its hardness, and most of the time those knives should be avoided. I've discussed that aspect in the article Kitchen Cutlery Steel Overview. So, in short, either the maker is using cheap, low grade steel, or it's a well known maker, using his proprietary steel. For this discussion, neither is relevant, so we skip that altogether.

So, when the makers specify both stele and hardness, we get minimal information and certain expectations how the knife should behave and what kind of performance one could expect from the given knife. However there are still numerous variations to the equation. How the steel was heat treated, i.e. whether or not it achieved required temperature and then austentize temperature, cryogenic treatment or not. In short, the knife can have given hardness value, but still the behavior will vary. Basically, for those who like precision and numbers (myself included) the overall picture is quite blurry and messy, sadly of course.

Nevertheless, for a given steel, from the same maker HRC or Rockwell Hardness value does give quite a bit of information and even 1 point of difference can affect knife performance significantly, which is what I will discuss below. I emphasize the fact that we're talking about same steel, from the same maker, which in general means same heat treatment procedure. Although, in general heat treatment is more or less well defined set of rules, and mainly it's the matter of precision and care from the maker. How well they follow the rules, how precisely they maintain temperatures and timings and such. As Phil Wilson told me during of the email exchanges, even 30° drop of the austentize temperature resulted in 1pt HRC drop of the final result. Significant, especially that temperatures we're talking about are in hundreds of degrees if not thousands.

Benchmade 710HSSR BT-2 coated Benchmade 710HSSR Boron Carbide coated

The story actually begun long time ago, in 1999, when I got my first Benchmade 710HSSR McHenry & Williams folder. Officially the hardness of the M2 steel on those folders was 60-62HRC. I've begun using the knife back then and happily used it for few years. That's the knife on the top picture. Later on, I've bought several Benchmade 710 folders. In both ATS-34 and M2 versions. Makes an excellent present after all :) Anyway, the way it happened, I've sent second BM 710 blade, for re-coating with Boron Carbide(BC) coating to the BodyCote, which back then was doing coatings for the knife blades for individual customers like me. You can see coated knife on the second picture. For the record, BodyCote was coating knives with Boron Carbide for Benchmade knife company. Later on they've stopped taking individual orders. All that was happening in the period between December 2001/January 2002. Since then I was carrying my BC coated 710 McHenry &Williams as my Every Day Carry(EDC) folder, sometimes changing with another 710 that still had Benchmade Teflon based BT2 coating.

During next 7 years that was the case. Along with my Scott Cook Lochsa folder, I always had one of those BM 710s in my pocket. That is except the cases when I was visiting the areas with blade length restrictions. Then I'd have to drop back to 3" long blade folders like Camillus CUDA EDC D2, Benchmade 707 Sequel, or one of those Benchmade 773 Osbornes.

Eventually, I've started noticing that BC coated Benchmade folder was holding up noticeably worse than the one with the original BT-2 coating. For the record, I grind identical 30° included angle edge on all of my Benchmade 710 folders made out of M2 steel. Actually I've ground 30° angle on the Benchmade 710-01 folder too, which is D2 steel, but that's besides the point. Thus, whether I wanted or not I had to thicken the edge on BC coated BM 710. That wasn't something I was happy with, but the edge kept folding, while identical folder with the same steel held up fine with a thinner edge.

Benchmade 710HSSR Edge Benchmade 710HSSR Edge

To understand better what kind of folding I am talking about, take a loot at the pictures attached to this paragraph. Both pictures provide approximately 32x magnification of the original edge. You can clearly see the folds and dents on the cutting edge. The deformation is clearly plastic type, and can be easily (more or less) fixed by stropping, however that can be done only so many times, eventually the metal breaks away due to metal fatigue. Besides, I wasn't expecting to see that picture for 60-62HRC blade. On the other hand, even 62HRC isn't full hardness for M2 steel, which(full hardness of M2) is closer to 66HRC for industrial applications. Anyway, the point is I've had 2 knives, theoretically both 60-62HR hardness range, and one was holding up fine with 30° edge, while the other had problems with the same angle and eventually I had to thicken it all the way to 40° included angle. That's pretty much what factory knives are ground in 99% of the cases, but in my opinion that is a waste of knife performance, and the edge that thick is geared towards the knife users who don't sharpen their knives, thus thicker edges. They seem to last sharp longer ;) Except they're not that sharp to begin with. It was obvious that BC coated blade was softer. The interestig part was how much softer.

The second, but not the final part of the story took part on March 24, 2009. The day before, I have contacted local steel heat treating company to find out if they would do Rockwell Testing for me. Initially I was quoted 50$ per blade, which was quite a bummer, as I was planning on testing hardness of a dozen or so knives. However, when the guy heard that I was just a knife collector he offered to test 2 knives for free. Which was nice. So, I had a choice, out of 250+ knives I currently own, pick two, and test them. Given the circumstances, BC coated 710 was an obvious choice. Second choice for testing became Tadatsuna White Steel Kamagata Usuba. Tadatsuna also had folding problems, which was due to extremely low angle on the edge, 6°-7°. Once I brought it to 15° included angle it worked very well, but I still wanted to be sure.