Blade enthusiasts ask a lot of questions before settling on purchasing a “decent” knife. However, what really makes a “decent” knife blade? There are many answers to that question depending on the application of the knife and how it will be used; a knife might be used for commercial, industrial, home or hunting purposes. There stands one element that defines the steel hardness of a knife blade and its ability of edge retention: Rockwell Hardness Number.
Most knife manufacturers advertise that their blades conform to a certain hardness number on the “Rockwell C Scale.” But how can one measure a blade’s worthiness by just looking at its Rockwell hardness values? Is it possible to differentiate between two different blades based on their Rockwell scale and their hardness values? The answer is yes.
Factors Of Steel That Make A Good Knife Blade
A knife blade’s worth is measured by two qualities: the ability to form a sharp edge and the ability to hold that sharp edge under harsh use and abuse. Three main factors influence these qualities: hardness, flexibility and toughness.
A material need not be hard to be sharp; a paper or a thin piece of wood is an example of this fact. However, soft materials blunt easily when brought in contact with a material that is harder than them. Retrospect the children’s game “Rock, Paper, Scissors” — a rock crushes scissor, which in turn cuts paper, but a paper just wraps around rock to beat it.
There are only two ways to make a blade that has a sharp edge: use a material that is already hard, such as glass, or utilise a material that can be sharpened, like steel. A glass knife can be made as sharp as a steel knife, maybe even sharper; breaking a piece of glass produces a very sharp edge. However, glass is brittle and doesn’t flex under stress.
An example of a glass knife is a blade made of volcanic glass, also called obsidian; Native Americans used obsidian blades as disposable utility knives, owing to the fact that they break easily. In short, any hard material can be made into a blade easily.
Steel poses a different problem however. There are many types of steel alloys with different textures and colours, but there is no certain way to tell them apart just by looking at them; one can never know whether a steel has been hardened or not at a glance. Even if the steel hardness is known, it is difficult to tell how hard it really is, or whether it has been uniformly hardened across all its dimensions.
Measuring The Hardness Of Steel
Traditionally, a metal file or a sharpening stone was placed at the edge of a material to gauge its hardness. Nevertheless, this procedure was highly subjective and yielded uneven results every time. There was a need to devise a procedure to test hardness with accuracy and repeatability.
Stanley P. Rockwell came up with an effective solution to this problem in 1919 while he was working as a metallurgist in a plant that manufactured ball bearings. Rockwell wanted to measure the uniformity and hardness of the races (inner and outer rings with grooves) on which the ball bearings rolled. He designed an apparatus that could measure the hardness with accuracy and repeatability in a quick manner.
The Rockwell testing apparatus proved equally effective at finding the hardness of other steel alloys, metals and even non-metallic materials. The range of materials that could be tested for hardness with this apparatus was phenomenal, and hence a need arose to have different Rockwell scales to test different materials.
If accuracy and repeatability of the results needs to be preserved, then the correct Rockwell scale must be used. The American Society for Testing and Materials (ASTM) has standardized a set of scales for testing Rockwell hardness values. Each scale has been designated a letter and it corresponds to a different group of materials.
Here’s a breakdown of the different Rockwell scales and which materials they’re applied on:
- Scale A – Cemented carbides, shallow case hardened steel and thin steel
- Scale B – Soft steels, malleable iron, copper alloys, aluminum alloys, etc.
- Scale C – Steel, titanium, deep case hardened steel, hard cast irons and other materials harder than B 100
- Scale D – Thin steel, pearlitic malleable iron and medium case hardened steel
- Scale E – Aluminum and magnesium alloys, cast iron and bearing metals
- Scale F – Thin soft sheet metals and annealed copper alloys
- Scale G – Beryllium copper, phosphor bronze and malleable irons
- Scale H – Zinc, aluminium, lead
- Scale K, L, M, P, R, S, V – Very soft and thin materials, includes plastics
The procedure carried out for Rockwell testing is very simple and quick, though the equipment is costly. It usually takes less than 10 seconds to get it done manually, whereas automatic measuring devices can carry out the testing process in under a second per part.
The entire testing process requires just 4 steps after the testing piece has been placed on the Rockwell testing apparatus: minor load application, major load application, removal of major load, measurement of indentation caused due to major load. The penetration caused due to the major load applied is transformed into the Rockwell hardness number; the smaller the penetration, the better the hardness, and hence the bigger the Rockwell hardness number.
Rockwell Hardness In Relation To Knife Blades
Knife blades and other steel hardened tools are made from materials which correspond to the Rockwell C scale. Most knife manufacturers display the hardness number as “HRC xx,” where “xx” corresponds to the Rockwell hardness number. Very few blades measure over HRC 70, while most functional knives have a rating somewhere between HRC 56 and HRC 63.
As a rule of thumb, blades with a lower HRC number don’t hold their sharp edge for long under demanding use, but are easier to sharpen, whereas blades with a higher HRC value remain sharp for long and are difficult to sharpen. For example, stainless steel has a HRC greater than carbon steel, but it is also difficult to sharpen than carbon steel.
Keep In Mind…
A Rockwell hardness number has many limitations that are overlooked by consumers and even the marketers. Most blades are tested at only one point for hardness, whereas they should have been tested at several different points. Also, Rockwell hardness test only reveals the surface hardness of a material and not its internal hardness.
Apart from the Rockwell hardness number, other qualities of a knife blade that should be considered to determine the quality of a blade are: metal composition, handle construction, grind type, edge bevel, sharpening requirements, handle construction, blade shape, length of cutting edge, thickness and weight.