Knife steel information
Almost all knives are made of steel. However, pieces of steel differ. To understand the differences, we have listed some basic facts about tooling steel below.
Steel vs. Iron
Steel is a material that mainly consists of the chemical element iron (Fe). Iron is the name of an atom, and steel that consists of 100% iron is not suitable for daily use.
Never use the term iron unless you are talking of the iron atom.
Tooling steel
Tooling steel is steel suited for tools like knives, chisels and other tools.
The main element added to steel is carbon. The addition of 1-2% carbon makes it possible to harden steel.
If the tooling steel must be stainless steel, 8-30% chrome is added to the steel. Nickel is not used to make the tooling steel into stainless steel, as that hampers the hardening of the steel.
Besides carbon and possibly chrome, other elements are added sometimes to improve the quality of the steel on specific points. The best known elements are Vanadium and Molybdene. The influence of those elements is not described in this article as it is of a very complex matter.
Structure
It is very important to know that the composition of a type of steel in itself does not say anything about the characteristics of the steel.
Steel consists of iron crystals, that exist in different shapes (by shapes we mean the order of the atoms) and sizes. Additions like carbon can be present in the crystals or in different shapes between the crystals. Carbon can be present as balls or discs between the crystals but can also bind to the iron atoms (carbids).
The structure of the steel therefore not only depends on the composition but also the treatments the steel underwent. Think of heat and cold treatments and welding (= shaping steel in a solid shape by remodelling).
Hardening
When you slowly heat liquid steel (with 1-2% carbon) the atoms will order differently under different temperatures. That is because the atoms start to vibrate more strongly and need more room to move under increasing temperatures.
Above 200 °C the atoms can take on another order. It is not possible below 200 °C and the structure is frozen as it were.
Between 200 °C and 700 °C, ferrite crystals are formed (ferrite is a name for the order of the iron and carbon atoms, just like austenite and martensite).
In ferrite crystals the iron atoms are close together and there is no room for the carbon atoms between the iron atoms. That’s why the carbon attaches to the edges of the ferrite crystals.
Ferrite steel is soft and tough. It is used as construction steel, but is not suitable as tooling steel.
Above 700 °C the iron atoms arrange themselves differently. Austenite crystals are formed.
In austenite crystals there is room for carbon atoms, between the iron atoms. If you keep the steel at a temperature above 700 °C for a while, the carbon atoms move from the edges of the crystals to the space between the iron atoms in the crystals.
By quickly cooling off the steel from this phase to below 200 °C (by for example immersing in water or oil) the carbon atoms cannot leave the crystals. Because the carbon atoms are in the way, the iron atoms cannot re-arrange into ferrite crystals. Martensite crystals are formed instead.
In that format the steel is considerably harder and is called hardened steel.
The story sounds simple, but many parameters affect the result. Think of: the structure of the steel before it is hardened, the exact temperature, the duration of heating, the speed of cooling and the influence of other elements added purposely, or unintended as impurities in the steel.
Tempering
After a piece of steel has been hardened, it is too brittle to be used and is full of inner tensions. The method to remove that brittleness and tensions is called tempering.
During tempering the steel is heated for a few hours at 200-300 °C. In that temperature the crystals re-arrange themselves which decreases the inner tensions and brittleness.
The duration is critical here because the steel will become progressively softer.
Hardness
Hardness is the resistance of the steel to permanent distortion at microscopic level.
The hardness of the steel is measured by pressing a pointed object with a certain force against the steel and measuring the size of the imprint.
The hardness of tooling steel is expressed in degrees Rockwell C, often abbreviated as HRC.
Hardness of knives steel
The table below gives an overall overview of the characteristics of knives steel with different hardness levels.
- Up to 52 HRC: Too soft for making knives.
- 52-54 HRC: Quite soft steel, reasonable quality.
- 54-56 HRC: The hardness of many French chefs’ knives. The steel is hard enough for kitchen use, but regular use of a sharpening steel is required to keep the knife sharp. Knives of this hardness are usually easy to sharpen.
- 56-58 HRC: Hardness applied for professional German kitchen knives. Knives of this hardness remain sharp long enough for kitchen use, can be sharpened on a sharpening steel and are reasonably easy to sharpen.
- 58-60 HRC: Hardness you usually find in better quality pocket knives like Spyderco, Cold Steel and Buck, and kitchen knives from Japan, like Global. Those knives remain sharp considerably longer than cheaper knives, but are a lot more difficult to sharpen.
- 60-62 HRC: Knives of this hardness remain sharp for a long time, but they are at risk of becoming brittle and the knives are often difficult to sharpen. These disadvantages are quite easy to suppress with modern steel types, but the quality depends on the quality of the whole production process.
- 63-66 HRC: Currently knives of hardness up to 66 HRC (Twin Cermax of Zwilling J.A. Henckels) are available. These are no knives suitable for the majority of users, but rather a specific group of amateurs. Knives of such hardness have the disadvantage that they become brittle, resulting in breakage of blade parts when used carelessly and often a low resistance against corrosion. In other words: clean immediately after use.
Other characteristics
Hardness is not the only characteristic of steel. Other mechanical characteristics are a.o. the draw strength, sturdiness (impact resistance), and cold brittleness.
Corrosion resistance is also an important chemical characteristic.
Hardness alone is not the be all and end all.
Influence on the price of a knife
A knife in a good, hard steel type will cost a lot more than the same knife made from a lesser steel type.
The difference in price is only due to the cost price of the steel for a small part. The steel processing leads to the high difference in price.
Sharpening modern hard steel types like S90V are much more time consuming and more demanding on the used materials than the sharpening of 440C for example.
Hardening and tempering is a lot more critical with modern steel types and must sometimes be entrusted to specialised companies.
The steel type has an important impact on the price of a knife. If you want a really good knife you must be prepared to pay the price for it. But you will get an excellent piece of equipment in return.
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