Every sort of technical field, discipline, professional pursuit, and hobby has its own unique vocabulary. Electricians talk about resistances and ohms, political scientists discuss voting blocks and the Overton Window, and content marketers muse over content strategy and PPC efforts. As for knife makers and aficionados of bladed weapons, they like to wax poetic over metal toughness, edge retention, handle material types, and methods of blade deployment. Additionally, another term you’ll hear bandied about is “hardness,” a word that conjures up all sorts of varying associations from person to person. Fortunately, you won’t have to wonder what hardness means when discussing knives! This article will explain the technicalities of the term, the scale used to measure it, and how hard you can expect various materials to be.

The Definition of Blade Hardness

Put simply, the hardness of a knife blade is its ability to resist various forces such as stress or heat. However, a wealth of research and practical experience has gone into the testing of hardness and all of the practicalities it entails. The story starts in the early 20th century with an Austrian named Paul Ludwik. While Ludwik started out his career as an engineer at Maschinenbau-AG, he soon shifted his professional focus to academics and began to work for a professor at Vienna Technical University. He would perform quite literally pioneering work in metal deformation, creating the field of technological mechanics. Ludwik would study how metals reacted with put under certain specific stresses such as compression, torsion, and tension. He also hypothesized a way of measuring a material’s hardness.

Other theoreticians were studying similar matters, including Swedish engineer Johan August Brinell, who measured hardness by pressing a 10mm ball made out of iron or tungsten (the selection depended on the material itself) with 3,000 kiloponds of force into the material. While technically the first standardized hardness test and able to evaluate materials as diverse as wood, lead, aluminum, steel, and glass, it had several significant drawbacks. For instance, it cannot be performed swiftly and substantially damages any material upon which it’s used. It would take a pair of Americans to make a more workable model.

Hugh and Stanley Rockwell created a machine, the Rockwell hardness tester, that used an impeller to press against the material with two different forces (namely a major and minor load). The first force provided a zero point while the second served as a differential measurement. The hardness of the material depended on the depth of the indentation. Unlike the Brinell test, which left significant damage, the so-called Rockwell scale left only a minor mark.

The Rockwell Scale

In truth, calling the Rockwell scale a scale (note the use of the singular rather than the plural) is a misnomer. It’s true that the equation remains the same no matter what material is being assessed, but the factors placed into it change from material to material. Without delving into the actual arithmetic, understand that the range of Rockwell scales — with scale name initially listed and its abbreviation in parentheses followed by material type — runs as follows:

A (HRA): Tungsten carbide, thin steel

B (HRB): Brass, aluminum, soft steel, soft iron

C (HRC): Steel, titanium, hard iron

D (HRD): Case-hardened steel

E (HRE): Aluminum and magnesium alloys, thermoset plastic

F (HRF): Soft sheet metals

G (HRG): Phosphor bronze

H (HRH): Lead, zinc

K (HRK): Tin, hard plastic

L (HRL): Bearing metals

M (HRM): Thermoplastics

P (HRP): Bearing metals

R (HRR): Thermoplastics

S (HRS): Bearing metals

V (HRV): Bearing metals

Each of these categories have different set values for the major load and different scale factors. You can expect to receive a Rockwell scale number within a certain range, and that number should clue you in to the characteristics of a specific material.

While you can draw various inferences from a Rockwell scale number, exercise caution. Harder materials tend toward brittleness, although certain formulations may buck the trend. Newer steels in particular exhibit a certain degree of toughness uncommon amongst particularly hard steels. When in doubt, remember that hardness is only one measurement.

The Hardness of Different Kinds of Steel

Knife-making steels fall into the HRC scale, and their Rockwell scales range from 52 to 66. Any steel below the 52 HRC threshold are simply too soft to function effectively. The following categories are somewhat arbitrary, and you may find steels that straddle the line between a couple of them. Still, they function as general guidelines for what you can expect from different knife-making metals.

52-54 HRC

This HRC range tends toward softer, less expensive sorts of steels, such as 420. This common steel won’t hold its edge for very long, but it’s relatively easy to sharpen. Cheap and able to take a beating, it isn’t a metal intended for high-end knives.

54-56 HRC

Another common steel, 1095, falls into this range, although it can flex up to 60 HRC. A good all-around metal, 1095 is a steel that’s fairly hard, simple to sharpen, and holds a decent edge. It is somewhat susceptible to corrosion, though.

58-60 HRC

This is the HRC level at which premium steels start to appear, such as CPM S90V (a metal high in carbon and molybdenum) and CPM 154 (powdered steel) start to appear. The well-known tool steel D2 falls into this range and the next given that it can measure as high as 62 HRC. Vanax — an incredibly balanced steel famously created solely for the crafting of knives — also resides in this category.

60-62 HRC

Even more luxury steels land here, a testament to the fact that it takes metallurgical magic to circumvent the brittleness that tends to creep in at higher hardness levels. The Japanese steel VG-10 is so prized in its home country that its producers intended its second letter to stand for “gold,” as in “the gold standard.” CPM M4 is a common and preferred metal in many custom knives. And M390 has earned the designation of “super steel.”

63-66 HRC

The knives on this end of the HRC scale exhibit wonderful edge retention. However, they are generally only used in kitchen settings since they’re vulnerable to chipping or breaking if improperly handled. ZDP-189 has quite a following amongst Japanese knife makers, and CPM-M4 is a powdered-steel formulation with great abrasion resistance.

Browse our premium EDC knives where we list the technical specifications for all of our knives, including their HRC scores.