Every knife blade possesses two inherent qualities—hardness and toughness. Flint and obsidian knives used by native North American aboriginals were far harder than steel and could be made extremely sharp, but lacked in toughness and would easily break. The opposite of this can be found in the table knives we use at home. These utensils are very tough and can be flexed to some degree without fracturing, but their ability to hold an edge is limited.
Hardness is imparted to blade steel by the heat-treatment process. Toughness is achieved by subsequent tempering, or the drawing off of some of the hardness that came about through the initial heat treatment. The end result is a blade that possesses both hardness to stave off edge erosion and toughness to handle everyday use. The degree to which final hardness is achieved can be measured by the use of a Rockwell tester.
Invented in 1919 by metallurgist Stanley P. Rockwell, this instrument can accurately and repeatedly measure the hardness of a wide range of metals, as well as many non-metallic materials. To do this the Rockwell tester employs a special conical-shaped penetrator (a diamond is used to test blade steel), which is applied under pressure to the material being tested. The hardness value is then read from a dial on the instrument, with harder material producing a higher number value.
Rockwell hardness values are a combination of a hardness number and a scale symbol. There are a number of different scales but the one used for testing blade steel is the Rockwell “C” scale. The top end of this scale would conceptually be 100. However, in reality, the hardness range of most heat-treated blade steel is from 48 to about 65. Blades that fall into the lower end of this range will respond to sharpening quickly. However, once sharpened, edge erosion will take place equally as fast. Correspondingly, blades that fall into the upper hardness range will offer extended edge retention. When the need arises, however, sharpening difficulties will be experienced.
The Testing Process
The actual hardness test is quite simple. The first step is to secure the heat-treated blade in the instrument. Next, the conical-shaped diamond penetrator is applied under minor downward pressure at a chosen test location on the blade. The reason why only minor pressure is applied at this stage of the test is to ensure that the test area is absolutely flat and for the establishment of a zero reference point. After this initial step, major downward force is applied to the penetrator for a specific period of time (dwell time). The depth of penetration into the test material from the zero reference position is measured and converted into a Rockwell hardness value. Even though the actual depth of penetration is only a fraction of a millimeter, this is sufficient to provide a hardness value.
The simplicity of the Rockwell test is the reason why its use has become so universal. The test can be done by hand or as part of a mechanized assembly line. Hardness values are displayed on a dial, which eliminates any need for special calculation. And the test provides a quick, accurate and repeatable measure of blade steel hardness at multiple points.
When looking at Rockwell test data keep in mind that the test does have its limits. First of all, the actual hardness value is only valid at the point of penetration. This may not be as important when blades are heat-treated in a computer-controlled environment that allows complete heat saturation. In that setting a random sample will suffice for the entire batch. However, when a blade is hardened by the old “hand and eye” method of heat-treatment used by many individual makers, the test will provide more accurate data when it’s performed at several locations along the length of the blade. Secondly, the Rockwell test only provides data about the hardness of the blade at the actual point of penetration. It’s entirely possible to have a blade with a hard edge and a much softer spine; in fact, many knifemakers consider this desirable. Even with these limitations, the Rockwell test is an extremely reliable indicator of blade hardness. And its quantitative numerical readings offer clear insight into functional performance.