What is Rockwell hardness?

December 7, 2022

Rockwell Rating is a standard index showing the hardness of a material. In knives, the Rockwell Hardness Rating is crucial for determining the principal qualities of a steel, such as strength and the ability to hold a keen edge. A high Rockwell rating does not necessarily reflect an overall high quality of a steel, although steels with a high Rockwell rating are graded as ‘premium’ steels for their superior hardness and strength.

UNDERSTANDING ROCKWELL RATING

When looking at knife specifications, you might have come across numbers with “RC” or “HRC” indexes. What do they mean and how do they affect your blade’s properties? We unpack these and related questions in our brief Rockwell guide. 

rockwell hardness test

What is Rockwell rating?

“RC” or “HRC” stands for the Rockwell Hardness Rating – a scale used to measure the hardness of a material. Hardness of knives is indicated by way of the Rockwell hardness test as the industry standard. The Rockwell rating is normally listed as a range of values indicating possible variations of hardness that might occur in the steel after the hardening process. In a well-controlled setting the range is never greater than two. 

What does “HRC” stand for? 

There are a number of Rockwell scales of hardness for different materials. Part C refers to steel and, hence, is used to measure hardness in knife steels. Part C uses a load of 150 kg/c provided by a diamond tip with an angle of 120 degrees. Rockwell Hardness Scale C is often abbreviated as HRC or just RC.

Testing procedure

The Rockwell test inflicts little damage to the steel being tested to determine the extent of hardness and durability of the steel. A diamond-tipped indenter is impressed into the flat surface of the steel at high load, and then the penetration depth is measured. The extent of the penetration depth shows how hard the metal is. 

Rockwell Hardness Testing

Is Rockwell hardness test destructive?

The Rockwell hardness test is a non-destructive test. This method involves applying a constant load via a rounded or pointed object on the exterior surface of a material. Only a tiny portion of the surface is impressed with an indenter. In case of knife blades, the test is usually performed on the tang portion of a knife, in order to further hide the small dot dent at the point of contact with the indenter under the handle.

Is Rockwell B or C harder?

The Rockwell C (HRC) test is used for harder metals, so, definitely, a Rockwell C rating means a metal is harder than one measured with the Rockwell B scale. 

The Rockwell B scale is used for softer metals (such as aluminum alloys, brass, and metals of similar hardness). The Rockwell C scale is used for harder metals such as steels. The difference between HRB and HRC is that the HRB system uses a steel ball for testing, with a load of 100 kg, while the HRC test uses a diamond cone with a 150 kg load. 

What do the HRC numbers mean in practice?

It is crucial to understand that there can be no single HRC index for a steel: the same steel can be hardened to either a low HRC index, or to a high HRC index. Steels possess different properties when hardened to various hardness values, so it is important to consider at which range a steel will perform best. A very hard steel will have great edge retention (i.e. you will have to sharpen it less often). However, a very hard steel tends to be brittler – it may shatter at a sudden impact. 

What Rockwell hardness should a knife be?

It all depends on your preferences and prospective blade use. For example, you may need a very hard blade (over 59 HRC) in a knife intended for slicing (as impacts to the blade are low and sharpness is crucial). However, you will need a much less hard (below 54 HRC) blade for an axe or machete you use for chopping (as a huge extent of impact will ruin your blade, unless it scores quite low on the hardness scale).

It all comes down to choosing an optimum solution for the primary purpose of your knife, considering the extent of impact it will be subject to. If you want a knife capable of retaining a sharp edge for a long time that will be used for delicate kitchen operations (such as a slicing knife) – look for a blade with a higher Rockwell index. If you want a blade with extreme toughness that will survive hard work (such as a hunting knife) – you definitely need a steel with a lower Rockwell rating. 

Milder steels

Milder steel, in the 52 to 56 HRC range, will be fine for many applications where you need a rough and resilient blade. These steels need sharpening more often, but they are much easier to sharpen than harder steels. Chipping or cracking is not such a hazard as with harder steels. Large knives, such as survival knives and bushcraft knives, axes and other hard impact blades are usually made of softer, tougher steel. A balance between thickness and steel hardness is also crucial, because these big blades must be able to withstand harder use and brute force without breaking. 

Harder steels

Harder steels start from 57 HRC. Smaller everyday carry knives, intended to be sharp enough for a long time, will have a hardness of about 57–59 HRC. A steel that hard provides perfect performance of a tool meant for delicate everyday operations. Very hard steels, also known as ‘premium’ steels, range from 59 to 66 HRC. Some of these steels are claimed to be the best all-rounders, and some feature superb peculiar properties. For example, VG-10 is renowned for the ability to hold a very fine, thin edge for a long time.

Metallurgy is constantly advancing, and most recent powdered steels are capable of reaching Rockwell indexes so high that they require diamond sharpeners to impress such steel. But how much hardness is enough? Where should you stop to get a quality that is sufficient for your use?

 

What about steel quality?

If you have a steel over 59 HRC, you are dealing with a super-hard, ‘premium’ steel. However, it is often wrong to judge the quality of a steel based on the Rockwell number. Higher HRC numbers do not always indicate higher quality steel, and a steel is not better than another simply because it is harder. A harder steel is brittler and, under impact, may shatter because it is too hard. When you’re dealing with a Rockwell range of about 58, you already have superior hardness, and going further will just make the steel brittle.

Does everyone need a premium steel for every use? Of course, no. For example, if you just need a basic knife to carry around, you don’t need premium steel. You definitely don’t need premium steel for a knife that will be exposed to rough operation. Once you have an optimum hardness of a steel, other important blade properties may be considered. Durability and rust resistance are crucial, and affordability is important for many consumers. 

Rockwell testing machine

How to use Rockwell practically

The Rockwell test is important for manufacturers and consumers alike, as it indicates a qualification of a steel, that is how a blade will perform when subject to a certain use mode. Blades that are frequently subject to abuse and rough operation – such as axes and survival knives – normally have low HRC indexes. On the contrary, folding pocket knives usually have a higher HRC rating, as they are expected to hold a sharp resilient edge and are not subject to gross impacts or heavy striking.

And, if you are a knifemaker, remember that there is always an option when heat-treating a new blade – you can harden it to a higher or a lower HRC rating. A good bladesmith heat treats a knife to a Rockwell hardness range that balances edge retention and toughness with due account for a normal range of use for that individual knife.

You can choose a handmade custom knife here. 

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