The Rockwell Scale measures the hardness of metal alloys. This scale’s broken up into different categories: A, B, and C, continuing all the way to V. The Rockwell C Scale measures the hardness of blade steels. Ratings are called “C Ratings,” or they’re abbreviated “HRC,” or “RC.”
Table of Contents
- How a Knife’s Rockwell Hardness is Measured
- The Best Hardness Ratings For Knives
- Which Blade Steels Are The Hardest?
- How is a Blade Hardened?
- Blade Hardness Isn’t All That Matters
How a Knife’s Rockwell Hardness is Measured
To determine a blade’s Rockwell Hardness, a diamond tip is pressed into the surface of a knife’s blade using minor force, forming an indentation. This is called the “reference level.” The load is then increased, pressing the diamond tip further into the surface of the blade.
The added force, the increased depth of the indentation in the blade, and the final depth of the indentation once the added load is removed, are all used to calculate the blade’s C Rating.
A Harder Blade Isn’t Always Better
Harder blades can hold a sharp edge longer, but increasing a knife’s HRC isn’t always better. Harder blades are more brittle and they’re more difficult to sharpen. Softer blades are tougher and more resilient to striking. Their cutting edges are less likely to chip, and they’re easier to sharpen. But if your knife’s steel is too soft, it won’t hold an edge long and will provide poor performance when cutting.
The Best Hardness Ratings For Knives
Regardless of its intended use, any good knife should have a hardness rating between 52 HRC and 64 HRC.
Super-hard knives are great for kitchen duties where accurate, thin slices must be made regularly. Softer knives are better for chopping and heavy-duty work, where resistance to breaking and chipping are more important than sharpness. Most general-purpose knives will fall between 55 and 59 HRC.
The guideline below describes optimal C Ratings depending on a blade’s intended use:
Choppers, Machetes, and Field Knives: 52 to 55 HRC
Between these C Ratings, a blade’s edge is unlikely to chip or crack. Instead, it will roll and dull after repeated heavy use. Sharpening at this hardness rating is easy, too — an important advantage for a large blade with a long cutting edge.
Everyday-Carry Utility Knives: 54 to 56 HRC
Pocket- and belt-carried EDC blades benefit most between these C Ratings: They’ll hold an edge long enough to cut the average cardboard box or string daily, and they can still take some abuse when striking and stabbing. Their cutting edges are small enough to prevent resharpening from becoming a time-consuming chore when they dull.
Pocket Knives / Differential-Hardened Blades: 57 to 60 HRC
At these C Ratings, a blade will hold a sharp edge after repeatedly cutting and slicing through stubborn materials, like rubber and corrugated fiberboard. Heavy striking and stabbing isn’t a great idea, as cutting edges and blade tips can start to chip instead of roll. That means re-profiling your edge and doing some extensive resharpening and honing.
Popular knife brands like Spyderco, Cold Steel, CRKT, Kershaw, Buck, Boker, Benchmade, and Emerson tend to produce blades within this hardness range.
Kitchen Knives and Japanese Blades: 61 to 63 HRC
A C Rating above 60 means you’ll be focused solely on making plenty of fine, precise cuts through organics or soft materials with infrequent sharpening. Blades this hard can chip easily and need to be cleaned after use.
Japanese kitchen knives and authentic Katanas, Wakizashis, and Tantos, undergo a differential heat treatment to obtain an edge hardness in this range. The differential hardness is obtained by quenching and tempering the edge hotter than the spine of the blade. The spine and cheek of these blade contain less carbon, and are hardened to obtain a much lower C Rating, between 50 and 54 HRC.
Which Blade Steels Are The Hardest?
Reference our chart below to compare the typical hardness ratings for the most popular blade steels on the market today.
How is a Blade Hardened?
Once the rough shape of a blade is hammered out, it’s heated in the forge to between 1,050°C and 1090°C (1,922°F to 1994°F). It’s then rapidly cooled (quenched) by dipping the blade in a vat of oil. This hardens the blade, but it also causes it to become incredibly brittle.
To eliminate brittleness, the blade is tempered (stress-relieved) by being heated to a much lower temperature of 175 to 350°C (347°F to 662°F) for around two hours. This softens the blade enough to provide resiliency against cracking, without compromising the hardness obtained from quenching. The higher the tempering heat, the softer the blade will become once cooled.
What Determines a Blade’s Final Hardness?
The amount of carbon that’s present in a steel determines how hard or soft a blade will become after it’s been quenched and tempered. The higher the carbon content, the harder the blade. Most stainless and high-carbon steels contain between 0.6% and 1.2% carbon. Newer powdered steels can contain up to 3% carbon. They’re capable of obtaining incredible C Ratings 64 HRC or higher. These are highly specialized blades that require expert sharpening techniques.
Blade Hardness Isn’t All That Matters
Other factors, like edge thickness and apex angles, help determine how well a knife performs at a given task. Learn more about those other factors with our other guides: