Hardness and Toughness: Why One HRC Number Isn’t Enough

Whoever sells you a kitchen knife will almost always quote one number. Fifty-eight HRC, sixty-one, sixty-five. The number is neat, the buyer cannot check it, and it leaves the full impression that everything about the steel has now been said.

About half of it has been said. And not the important half.

What Rockwell actually measures

Rockwell hardness is a measure of material hardness defined by the corresponding test. “Hardness” there is understood narrowly: as resistance to localised indentation. A steel or carbide ball, or a diamond cone, is pressed into the surface of the sample and the relative depth of the indentation is read off.

The result is a dimensionless number on one of several scales, labelled with letters from A to V. For hard steels, scale C is widely used; hence “61 HRC”, meaning 61 on the C scale of Rockwell hardness. Of the other scales, R and M come up often. Within one scale, the higher the number, the harder the material.

That is all. The test says nothing at all about what will happen to the edge when it meets a chicken bone or the rim of a plate.

What hardness is good for

It is useful in a very concrete way. For every two points of Rockwell hardness, a knife holds its edge roughly twice as long. The difference between 52 and 62 HRC is the difference between “sharp for about a week” and “sharp for about twelve months” under regular use.

That accounts for the whole gap between the European and Japanese schools. European knives — Sabatier, Gude, Zwilling, Henkels and Wusthof, Messermeister — use softer steels with low carbon content but good impact toughness. This is compensated for by the working routine: in a professional European kitchen the knife is steeled daily or several times a day. Cheap Chinese knives, the ones sold in sets with a block, come in around 52 HRC: they hold an edge worse and need sharpening more often.

The Japanese side lives on different numbers. The best known Japanese maker of chef’s knives, Global, uses Cromova steel at 58 HRC — relatively soft by Japanese standards. The spread among Japanese knives themselves is wide, but comparing them with European brands below 56 HRC is already pointless: these are different tools with different maintenance routines.

Why hardness alone is not enough

Steel that is too hard, without the additions that provide impact toughness, makes no sense: it chips very quickly. The edge does not go blunt — it breaks away in pieces. A good knife steel means high hardness and high toughness at the same time, and that combination comes dear.

The mechanism is known. Wear resistance is set by three things: the hardness of the steel itself, the hardness of the carbides in it, and the amount of carbides. More carbides and harder carbides mean the edge wears more slowly and holds its sharpness longer. But carbides are hard and brittle, and they make it easier for a crack to start. The more of them there are, the more readily the edge chips and the more easily the tip snaps.

This is the fundamental dichotomy: more carbide means better wear resistance and worse impact toughness. There is no way around it, only a choice of a point on that scale. This is exactly why steel makers keep looking for the optimal balance of hardness and toughness, and why complex alloys and exotic additions mostly serve to give back the toughness lost in the chase for hardness.

From this point of view, steels like Aogami Blue and ZDP-189 look ideal for kitchen knives: they deliver both at once.

The number on the price tag and the number in the blade

Hardness is a property of a particular blade, not of a grade. The method of forging and tempering affects it noticeably, so the same steel gives different results with different makers. The textbook example is Shirogami White #1: a traditional, very clean Japanese steel, not especially strong on its own, but able to take an extremely aggressive quench, which sends hardness wandering from 60 to 65 HRC.

The same route is the easiest way to end up with a knife you did not pay for. Many makers today ship steel in from Japan — usually laminated, with VG-10 in the core — and turn it into chef’s knives at cheap Chinese plants. The price comes out very interesting. The finish, the polish and the handle material often do not, and quality is inconsistent. But the worse part is elsewhere: through poor knowledge of forging technique, such blades are often heat-treated incorrectly and ground on wheels without water cooling. The steel overheats and Rockwell hardness drops. VG-10 in a Japanese knife gives 60–61 HRC; in a Chinese one it can come out far lower, and then the knife is no harder than an ordinary one for half the money.

A similar confusion arises over origin. Knives at 61 or 66 HRC sold under the German name Henkel in the Miyabi line are made in Japan. Cold Steel, Fallkniven and Spyderco — makers of pocket knives — also produce their best models in Japan.

Who all this is for

It is worth understanding that a real professional cook in a real kitchen is unlikely to be working with a knife made from Aogami Blue or ZDP-189. All these “professional knives” in exotic alloys are a domestic luxury item, and their purpose is different: to give pleasure to the owner, not to earn a cook a living. And to enrich the manufacturers, of course.

This is exactly why expensive knives are made with such attention to design and looks, sometimes to the point of absurdity. Finding a knife in expensive Japanese steel with a normal European handle and without secondary frills — a deliberately rough-finished spine and other decoration added for “authenticity” — is close to impossible. A plain cook who spends the whole day on his feet, in heat and damp, has no use whatsoever for a knife you must not drop, whose handle must not get wet, and which is easy to steal.

The HRC number on the price tag answers exactly one question: how often you will have to sharpen. Whether the edge will survive the way you handle a knife, it does not answer at all.