The Rockwell hardness of any knife blade is the best indication of edge retention and sharpening ease.
In many of these columns you will see the phrase “Rockwell Hardness” when commenting on actual blade hardness. Invented by metallurgist Stanley P. Rockwell in 1919, the Rockwell Testing Instrument is able to determine the hardness of a wide range of materials, as well as many non-metallic materials. The instrument typically uses a cone-shaped diamond applied under pressure to penetrate the object tested. When a knife blade is tested, it is secured in a fixture and the diamond impressed into the blade under pressure. Initially, the first application of the diamond is used to establish a zero-reference point. Then, the diamond is applied once again for a specific period of time (dwell time) and the depth of penetration from the zero reference point is measured and converted to a specific value. Conceptually, the top end of the measure scale would be 100, but in reality most knife blades test in the 48 to 65 ranges. Not only is the Rockwell Hardness test quick, accurate and repeatable, the test can be performed by hand one-blade-at-a-time, or as part of a mechanized assembly process.
To learn the importance of this test, it must be understood that a knife blade cannot be both easy to sharpen and offer superior edge retention at the same time (despite what is often repeated in a typical retail sale pitch). Blade edge degradation is a process of erosion caused by abrasion and during the usage such erosion naturally occurs. To restore functional blade integrity, the edge angle must be reestablished, which is also a process of erosion. Therefore, if knife blade hardness is in the lower portion of the testing criteria (52-56) it is easily eroded and will not hold its edge for an extended period of time. However, since such a blade has little resistance to erosion, one testing in the same range will be easy to sharpen. Conversely, a blade manifesting higher test values (58-65) is far more resistant to edge erosion and will offer superior edge holding, but at the same time it will be more difficult to sharpen.
Blade hardness is dependent on both the heat treatment process and the formulation of the steel itself. Inadequate heat treatment will not produce consistent hardness throughout the entirety of the blade. Most production cutlery firms utilize computer-controlled heat treatment for blade-to-blade hardness consistency. Smaller shops, as well as individual knife makers, often send their blades to an off-site heat treatment facility, or perform in-shop heat treatment one-blade-at-at-time. Either the production firm, or the knife maker must Rockwell test each blade, or a representative thereof, to insure heat treatment integrity. In addition, alloying elements such as vanadium and molybdenum added to the steel formulation provides increased hardenabilty and strength. Blade steel hardness is a fine balancing act between heat treatment and steel formulation, the end product of which is a combination of edge retention and sharpenability. And no steel known to man can offer both extended edge retention and sharpening ease at the same time. It’s either one, or the other!
What that all means is that when you shop for a knife, realize that both the steel formulation and the Rockwell hardness are objective considerations. Opinion, either of a friend or a sales person is totally subjective and provides little, if any, relevant information. Be your own blade steel expert, don’t let a sales person steer you in the direction he or she wants you to go. The Internet is full of solid data on blade steel, including the Rockwell hardness of any steel you’re interested in. A little time spent on the computer can provide all of the understanding you’ll need to make an informed knife buying decision.–Durwood Hollis