Rockwell Hardness and Bushcraft Knives

Rockwell Hardness and Bushcraft Knives

If you’ve spent any time at all cruising the blade forums, you have undoubtedly encountered the concept of Rockwell hardness and have been forced to contemplate why anybody would every buy a “soft” blade — especially for any type of heavy duty use that bushcraft often demands.

People serious about their knife use care about a couple of things: how easy the blade is to sharpen and how much abuse that blade can take before it needs to be sharpened. Honestly, everything else is just aesthetics and personal preference.

Shouldn’t a harder knife surface automatically make it a better knife? Well, unfortunately when we’re talking about a quest for your best bushcraft knife the answer is “it depends.”

Ahhh, Rockwell. How quickly we’re drawn to your sweet allure. The shocking truth (yes, there’s even controversy in the world of knife steel) reveals that Rockwell hardness scales can be either devious or worse yet — sort of irrelevant!

This makes it all the more necessary to understand what the hardness scale is telling you and what it’s NOT telling you. Don’t just punch the “I believe” button on the hardest blade you can find before reading the rest of this article.

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Hardness testing

Enough already, what is Rockwell Hardness?

But, before we can really get into the pros and cons of blade hardness, let’s back up and get a basic understanding of what Rockwell hardness is and what it means for the world of bushcraft knives.

Here’s the straight wikipedia.org definition of Rockwell hardness:

The Rockwell scale is a hardness scale based on indentation hardness of a material. The Rockwell test determines the hardness by measuring the depth of penetration of an indenter under a large load compared to the penetration made by a preload.[1] There are different scales, denoted by a single letter, that use different loads or indenters. The result is a dimensionless number noted as HRA, HRB, HRC, etc., where the last letter is the respective Rockwell scale (see below).

When testing metals, indentation hardness correlates linearly with tensile strength.[2] This important relation permits economically important nondestructive testing of bulk metal deliveries with lightweight, even portable equipment, such as hand-held Rockwell hardness testers.

Uhhhh…. yeah. Okay thanks, Wikipedia.

Let’s break this down, kids.

First: Indentation hardness is basically pressing something with a specially calibrated machine until it produces an indentation on the surface. Aha! Okay, now we’re getting somewhere.

Second: Tensile strength is the amount of stress a material can take before it breaks.

A Rockwell Hardness Tester

History of the Rockwell Boys

Leave it to two Americans during the Industrial Revolution (Hugh and Stanley in 1914 to be exact) to invent a machine that would measure the hardness of bulk metal without destroying the metal. Of course, this is during a period where the quality of steel being sold was more and more critical, so buyers and sellers needed a portable system that could indicate the relative hardness of their product.

Awesome. One hundred years later, we get to bicker about the finer points of Rockwell hardness on the Internet as a result! Everybody wins.

These relative measures of hardness lead naturally to a scale by which we can compare different material. Since there are many different types of metal and material combinations, we’re primarily concerned with the “C” scale (or HRC).

Very hard steel used as knife quality will generally fall between HRC 55 and 65. The best blades will have a Rockwell hardness between 57 and 62. The hardest blade I have personally come across is made by Fallkniven — the Idun Northern Light Cowry X. But, at about $1500 a pop, you can tell this is a very niche capability in a knife, and I suspect is more about being a luxury, scarce item than actual utility. Not sure… I’ll have to review that bad boy some day!

fallkniven-f1-review

Sharpness vs. Hardness

It’s easy to confuse how hard a blade is with how sharp it can get. Consider that I could sharpen a pewter spoon on my grinder sharp enough to do surgery (brain surgery, of course), but that doesn’t mean it’s going to STAY sharp for very long. No… this is where the Rockwell scale is most useful.

Similarly, a piece of glass can be insanely sharp (as I found out when my two year old broke a snow globe this morning), but it’s also usually insanely brittle — meaning, it has almost no flexibility. These properties are also important when searching for our own best bushcraft knife. If you simply head out and drop a few hundred dollars on the hardest blade ever made, this may well suit you, but it may end up breaking the tip the first time you come near a rock.

The whole art of knife making comes down to finding that perfect combination of toughness, hardness, and flexibility. This is truly a science that has evolved for several thousand years and is still evolving. Since we’re able to blend steels, heat them to volcanic temperatures, then cool them in controlled conditions, the chemistry of the steel can yield very surprising results.

serrated knife sharpening

Heat treats… don’t get me started

The soul of the knife is going to end up in the heat treat. If you ever have the pleasure of making your own knives, you will know the joy of becoming obsessed with heat treating knives. I will NOT be getting into it here, except to say that the very best knife makers are thinking of HOW the knife is intended to be used as they are making the knife. This may lead to a more or less aggressive heat treatment and tempering of the blade which takes much of that stress out of the knife.

The short story is, if you hear the term, “differential temper” it means that the knife blade actually has a variety of Rockwell hardness surfaces on the same blade! This is also why many knife makers present a range of hardness (example: HRC – 57-59).

They’re letting you know that the spine of the blade may be softer than the leading edge. This is generally a good thing, depending on the intended use.

All this means, you’re pretty much screwed.

I’m just kidding. Don’t panic…. Rockwell hardness scale is our friend.

I believe that many of the super hard blades should either have a differential temper or be made with laminated steel. That being said, there are outstanding blades that may be very hard, but the thickness of the blade may more than make up for any inherent brittleness. In other words, sporting a 1/4″ full tang may make a hard, brittle blade no problem.

That’s sort of why I’m here: to help you judge for yourself based on your intended use.

In the end, there is no “best” bushcraft knife. There’s only the knife that is going to do the best job that you can anticipate. I am, however, committed to only discussing or promoting a knife that you can safely bet your life on because bushcraft is all about being resourceful with limited resources. This is not a time when you want to be sweating Rockwell hardness – you just want your knife to take care of you when you need it most.

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