June 10, 2019
BY ELLIOTT BELL
The Rockwell hardness test was originally created to measure ball bearings.
Stanley P. Rockwell created a way to measure the steel ball bearings his company created, but the test was easy to apply to any type of metal or non-metallic material.
The scale that bears his name determines relative hardness of metal by measuring the depth of an indentation after a heavy object impacts a piece of metal. Diamond is the hardest natural substance known to man. The most commonly used version of the Rockwell test uses a conical diamond to impact the metal. The amount of force and depth of the cone-shaped indentation is then measured. The test is performed twice.
Initially only a minor load of pressure is applied, and the measurement is taken. The second time — in the exact position — the pressure is increased to a major load of about 300 pounds of pressure, and then the indentation is measured again.
The difference between the depth of the first and second indentation determines the hardness. The test is performed at least twice on a piece of metal to produce an average. Manufacturers usually offer a hardness rating range because steel’s hardness will vary slightly.
The Rockwell test is not the only way to measure the hardness of a material. The scale is created by comparing resistance to indentation. The test is most commonly associated with hardened steels, but it can be performed on practically any material — from plastic to concrete.
There are 30 different Rockwell scales, and each uses a unique combination of test forces and types of indentation. The scale follows the letters of the alphabet. The important scale for knife steel is the Rockwell C Scale, often shown as HRC.
Because it’s fast and reliable, the Rockwell method is the most popular hardness test for the stainless steel used to make knife blades.
In the case of the Rockwell hardness scale, what’s really being measured is the steel’s resistance to permanent distortion. Basically, how quickly will this steel dull. You end up with a Rockwell hardness number value measured at the microscopic level and expressed in degrees. When it comes to kitchen knives:
Anything below a 52 HRC rating would be too soft for a kitchen knife. For example, the average axe has an HRC of about 50, so the sharpened edge can withstand the impact of being hurled into a solid piece of wood without snapping off.
An HRC rating of 52-54 is soft but would make a reasonable, inexpensive kitchen knife.
Professionals and experienced home cooks look for kitchen knives with an HRC rating of 55 and above.
Most premium steels are in the 58-60 HRC range. A rating above this number indicates extreme brittleness. It would make an unreliable knife because you could easily snap the blade while using it.
Steel’s hardness is most often altered by the addition of carbon.
Harder steel holds its sharp cutting edge, so a high Rockwell scale hardness number is your goal when choosing a kitchen knife, right? It’s a logical assumption, but there are tradeoffs. The Rockwell hardness test allows knifemakers to find a balance that promotes blade sharpness and helps the owner keep it that way.
Generally, a knife with a Rockwell Hardness Scale rating of 58-60 will hold an edge better than a blade that has a lower HRC rating. Japanese-style knives tend to have HRC ratings of 60 and above.
Softer steel is more durable. A rating in the low- to mid-50s will mean that the knife blade will not hold its sharp edge as well, but it will be easier to sharpen and require less skill to maintain. German-style knifemakers, like Zwilling J.A. Henckels and Wüsthof, use stainless steel with an HRC rating of around 55.
All knives will need to be sharpened, and thankfully there are tons of easy ways to get your knife professionally sharpened for low costs these days. Misen for example, offers a mail-in knife sharpening service that will professional restore any knife without you having to lift a finger.
Rockwell hardness is just one way to look at a knife. It’s certainly not the only factor. How comfortable a knife is to hold, the type of steel used, and distinct shape and sizes are also critical factors when picking a new knife.
The HRC rating is a measure of quality for you, but for knife manufacturers, it’s more of a way to maintain quality control. Keep HRC in mind, but place more importance on the style of knife you prefer.
Now you know what those Rockwell Hardness HRC numbers mean, and how they can help you as you shop. It shouldn’t be hard — or difficult — to buy quality kitchen knives.
