What the Rockwell Hardness Scale Can Tell You About a Kitchen Knife

Some steel is actually too soft to make a practical knife.

• Some steel is too brittle to be used for knife making.
• Softer steel makes knife maintenance easier.
• A knife made of harder steel will stay sharper longer.   

How spicy is hot? It’s relative unless we have precise measurements for comparison, which is why, in 1912, Wilber Scoville decided on a scale to rate the pungency of chilies and peppers by measuring their capsaicin content. It was originally determined by diluting the pepper extract in sugar water until a panel of five people could no longer taste it.

The jalapeño pepper earned a Scoville rating of 5,000 because it needed a dilution ratio of 5,000-to-1 before it could no longer be tasted. Today, we can be far more precise by using methods such as high-pressure liquid chromatography. And it’s how we know that the hottest chili pepper in the world — the Carolina Reaper — has a rating of 1,641,183 Scoville units.

Precise measurements are also helpful when you are looking for a quality kitchen knife. There are so many types of steel, it can be difficult to determine what the best steel for kitchen knives is — unless you have a system of precise measurements. Enter the Rockwell hardness scale. Think of it as a Scoville scale for steel.

The Rockwell hardness scale important to your search for the perfect kitchen knives because harder steel holds its sharp cutting edge better than softer steel. But how hard is hard?

The Rockwell Hardness Scale

The Rockwell hardness test was originally created to measure ball bearings.

Seven years after Wilber Scoville decided to get serious about the precise measurement of spiciness, a metallurgist working at a New England ball bearing plant invented an accurate and repeatable way to measure hardness. 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.

More Than One Rockwell Test

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.

There are other tests that may be more suitable to determine a material’s hardness. For example, the Brinell hardness test is used on materials that are too coarse to capture the tiny indentation made by a diamond tip.

At the other end of the spectrum, the Vickers hardness test method is used on thin metals, as well as composites and ceramics. The test uses a square-base, pyramid-shaped, diamond indenter. Similar to Vickers is the Knoop method, which uses a rectangular-shaped diamond and measures hardness near the edge of very thin softer materials.

Because it’s fast and reliable, the Rockwell method is the most popular hardness test for the stainless steel used to make knife blades.

What Hardness Testing Tells Us

In the case of the Rockwell hardness scale, what’s really being measured is the steel’s resistance to permanent distortion. 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 59-64 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.

Rockwell Hardness Value Ratings and Kitchen Knives

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 58-62 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.

But steel that’s too hard can make a knife blade brittle and lower its tensile strength, so the knife will be easy to damage if misused. Harder steel may also take more time and effort to sharpen, but then, of course, it will remain that way for longer periods of time.

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.

The Sweet Spot

It’s impossible to have hardness, flexibility, and toughness. Knifemakers search for that optimal spot between a high HRC and a low HRC that offers an acceptable amount of edge retention without creating a knife that’s so brittle it will be easily damaged.

Steel’s hardness is most often managed by the addition of carbon. A blade’s hardness increases when more carbon is added. Knife makers heat the metal for their blades to specific temperatures. It causes the steel and carbon atoms to reorder themselves, which will change the characteristics of the metal. Other elements besides carbon can be added to contribute various properties to kitchen blade steel.

Like carbon, chromium adds hardness and helps the knife retain its sharp edge. Chromium is also the element that makes steel resistant to rust and discoloration. Manganese, vanadium, and molybdenum can contribute to hardness as well, helping the steel blade to retain edge sharpness.

The harder material allows for a thinner blade with a finer cutting edge. Knives manufactured with these harder metals are sharper and will carry a higher price tag. A kitchen knife with an HRC above 60 rewards you with excellent edge retention, but it will take more time and effort to sharpen when honing no longer restores that razor-sharp edge.

A knife with an HRC in the high 50s offers higher tensile strength, meaning it can flex without breaking. That’s an important consideration when choosing a paring knife.

Don’t Let Rockwell Be Your Top Decision-Maker

Like a fancy sports car, a knife with an HRC rating well above 60 may be expensive and hard to maintain.

You probably wouldn’t use a Maserati as your commuter car, just as you wouldn’t want to purchase a high-end Japanese-style chef’s knife with an HRC above 60 for everyday use in your nonprofessional kitchen. Both the automobile and the knife require additional attention and potentially expensive maintenance.

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.