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Understanding Rockwell Hardness in Knives

By | 2016

If you're looking at buying a quality knife, you may have seen something like "this knife has an HRc rating of X", where X is the rating value. Have you ever wondered just what that rating means? If so, you're not alone!

The Rockwell test of hardness

HRc refers to the Rockwell Scale of Hardness, part C. The Rockwell scale is widely used by metallurgists to define just how hard a piece of steel is: the higher the number, the harder the steel. A particular metal's rating is important to the knife maker because a harder steel will hold an edge better than a softer steel.

There are several different Rockwell scales; each one is used for a different material. Scale C is specifically used for rating the steel used in knives.

The highest RC rating is not necessarily the best

A harder steel will generally hold an edge better than a softer steel, but it's also more likely to crack or fail. In fact, if it's really hard, it can shatter just like glass on concrete!

The steel used in making a knife also has a great deal to do with how well that knife will hold an edge. Each different steel alloy has its own optimum range that balances hardness with performance and the intended use.

So, why does a knife's Rockwell rating matter? What is a good rockwell hardness for a knife?

The hardness of a knife is very important as far as its performance and durability. For instance, a harder steel with a RC of 58-62 will hold an edge better than a softer steel. However, that same harder steel is less durable and more prone to chipping or even breaking. Some kitchen knives with high hardness require extreme care so that you do not damage the thin edge.

The softer steel is more durable, but won't maintain an edge for as long as the harder steel. Most axes and chisels use a softer steel that can withstand the impacts they encounter in their everyday use.

Since pocket knives and hunting knives aren't generally used for chopping wood, they benefit from a harder steel that will take, and keep, a good sharp edge for slicing and cutting.

On the same note, a survival knife that you are going to put to extreme, rugged, brutal, near-abuse would benefit from a rockwell hardness of 55-58. A knife that could chop trough bone, hard woods, dig and pry needs to be durable first and foremost. The knife with a lower hardness might dull more quickly but it is more likely to survive intact.

Because hardness varies based upon intended use, there is no one "good" or "best" hardness for all knives.

Getting what you've paid for

The Rockwell test helps knifemakers balance the three most important factors that can affect the quality of their finished product: hardness, flexibility, and toughness. Having these three factors in proper balance allows them to produce a knife that will take a sharp edge and then keep that edge under a range of conditions without damage to the knife.

Buy a high quality knife and you'll experience the lasting quality and pleasure of use that comes from a knife with that properly executed balance.

Lots of abbreviations, but just one scale

There are several different abbreviations a knifemaker may use when referring to the scale: HR, HRc, HR C, RC, Rc, C on the Rockwell Scale, Rockwell Hardness C Scale, Rockwell C scale... No matter how it's written about knife steels, they all refer to the same scale (c). It can get a little confusing, but just know that the ratings themselves are the same - no matter how the knife maker abbreviates the scale!

History of the Rockwell scale

Stanley P. Rockwell was a metallurgist at a ball bearing plant in New England in 1919. He developed his hardness scale in order to measure the hardness of the bearing races in a way that was quick, accurate, and repeatable.

Manufacturers of everything from watch springs to train wheels had long needed such a test and were quick to apply Rockwell's scale to all kinds of steel, as well as other metal, parts. Eventually the test was adapted to test non-metallic materials - even plastics.

How is Rockwell hardness measured?

The Rockwell scale measures the relative hardness of a metal. It's based on how deep the resulting indentation is when a heavy object impacts it. So, how do they go about testing metal?

First, the metal needs to be heat-treated and perfectly flat. Otherwise, the test results won't be accurate.

One method is by using a diamond-tipped cone to forcibly impact the metal. Testers then measure how deep the cone penetrated from a given amount of force. Finally, that measurement is converted into a scale that shows the various metals that were tested and how they all relate to each other.

One small drawback for testing a knife blade is that it leaves a small pinpoint indentation on the blade surface, which some might think is a flaw or defect. The rockwell testing mark can be concealed if the test is done in a area that is hidden from view by the handle.

Adding numbers to that scale

The Rockwell test actually consists of two tests. During the first test, only a minor amount of pressure is applied, using a diamond-tipped point on something that looks like a pencil in a drill press. This ensures that the test area is completely flat and provides a target for the major pressure test. After that first measurement is taken, the test is repeated at the exact same point. The pressure is dramatically increased for this second test with approximately 300 pounds of pressure bearing down on that diamond tip.

The difference between the amount of pressure used for the first test and the second test is the Rockwell hardness number. Two (or more) tests on the same piece of metal will produce an average for that particular piece.

Why a range of RC numbers?

Because Rockwell testing is done on only a small area of the metal, it's possible that a nearby area could yield slightly different numbers. Also, the Rockwell test only tests the surface of the material. It's possible that the hardness of the interior could differ from the surface results.

For that reason, manufacturers will typically list a range of numbers for their knives. Having a range of numbers allows for a margin of error within the test results. The actual results for the entire object will lie somewhere within that range.