Preamble - Once you get into the knives and start picking up basic knowledge about steels, Rockwell Hardness, edges and angles, inevitably you come across the statement that hard(i.e. high on Rockwell C scale) edges are brittle and softer edges are, well softer, but tougher, in that they take less damage and are somewhat more durable, overall at least. You don't have to go to far, Knife Steel FAQ is right here. The statement is generally true, but it is rather generic. And as with many general statements, there is always a room for misinterpretation, exaggeration and so on. Several times there were discussions on the bladeforums about hard vs. soft edges, and a lot of people have an opinion or belief that soft edges are preferable over hard for several reasons, specifically:
- Soft edges are easier to sharpen, especially when dedicated sharpening equipment is not available, e.g. using a flat rock you happen to stumble upon on your hike;
- Soft edges are more durable and resistant to damage;
- Hard edges are very difficult to sharpen;
- Hard edges are very brittle and they chip easily. As an extension of that statement, hard steel knives break easily;
About Hardness- First, a few words about hardness and how hard is hard knife in particular. Metal hardness in modern metallurgy is measured using several different scales, Knoop, Brinell, Vickers, and the most popular in knife industry - Rockwell C scale. For the record there's also Rockwell A and B, but those aren't used for the alloys used in knifemaking, therefore in knife world most of the time you see Rockwell hardness, or Rockwell scale, but it refers to C scale. Anyway, the point is, the Rockwell scale is used to measure metal hardness, or its strength, i.e. resistance to deformation. Rockwell test is based on measuring the force required to make a predefined size and shaped indentation in a given metal. Higher the number, stronger the metal.
In theory, stronger is better, but unfortunately it's not as simple. Harder the metal, the more brittle it becomes, extreme example would be the glass, although not a metal, it's very hard on hardness scale, but has pretty much zero plastic deformation, so once it reaches the limit, it shatters, at least in room temperatures. In today's knife industry hardness from 52-56 HRC would be considered low range, 56-60 HRC would be medium and everything above 60HRC is hard. Although, I personally, don't consider 60-62HRC range as really hard, for me 62 is lower bound of hard blades. There is also alloy composition to consider. Many steels can not get very hard, and just because a steel can reach very high hardness, doesn't mean it will be a perfect knife steel either.
Much more correct would be to talk about the steel hardened close to its hardness upper limit, than to talk about numbers in general, without specifying alloys. E.g. AISI 1095 carbon steel, you can see this alloy in various hardness levels, anywhere from 56HRC, all the way to 65HRC by few
custom makers. Ideally, for my test I should've used identical blades made out of the identical steel, at different hardness. Sadly, I do not have that capability for now, all of
my softer M2 steel knives were already rehardened, but overall, strength is still strength, so I think different alloys suffice. After all, what I wanted to see was hard vs. soft
Experiment Goals- To specifically test the damage sustained by the hard and soft edges, in the similar conditions, using identical edge angles, identical cutting mediums. Also, to test the ease of restoring the damaged edges using basic techniques, such as steeling and stropping.
Testing Conditions- The test was performed in two parts, part one took place on 05/08/10 and part two was commenced on 05/22/10. Total of four knives were tested. Benchmade 710 M2 HSSR, Kershaw - 1840CBZDP Shallot, William Henry B12 Spearpoint and Calphalon Paring Knife. Former three representing the hard steel knives, being in the total range of 63-67HRC, and the Calphalon represented the softer steel knives side. More details about each knife in their respective reviews. As for the rest, each knife was sharpened to 30° inclusive angle. First three folders are my user knives and they were sharpened on the waterstones earlier, at various times, and the 30° edge is approximate, in all 3 cases (BM 710, WH B12 and Shallot) the edge in fact is lower, more like 13°-14° per side, but the calphalon was sharpened on the Edge-Pro Apex sharpening system, since I wanted to have a precise angle. Besides, that gave a little advantage to the soft steel knife. So, I can't be accused of bias towards the hard edges in this tests :) In each test the same material was cut with test knives, and after the edge lost initial sharpness I've attempted to restore sharpness with either borosilicate smooth rod, 0.25µm loaded leather strop, or a plain leather strop, or a combination of those two. The results were photographed using a microscope, connected to the Canon 50D DSLR.
Last updated - 09/01/11