Steel used for cutlery is mostly corrosion-resistant martensite steel. This steel is very tough and made of a high percentage of chromium, carbon and other elements that have interesting properties (molybdenum, vanadium). Knife blades should sharpen well, be easy to for the maker to shape and should have a good cutting resistance. These properties cannot be obtained simultaneously, it is necessary to find a compromise based on the use of the knife. For the hard structure called martensite, the blade must undergo suitable heat treatment (annealing, quenching, tempering).

Best Steel ...

A Compromise ...



The choice of steel is a compromise between:

- The ease of sharpening the blade

- The cutting edge of the blade and its behavior in time

- Its ability to resist corrosion

- Its ability to be polished and its resistance to scratches

At Forge de Laguiole , we add two criteria namely:

- Its origin (we require a French origin.)

- Its ability to be forged


-  A large chromium content increases the corrosion resistant power of steel.

-  A significant carbon content increases the cutting ability of steel, however, if the rate is too high it cancels the corrosion-resistant power of chromium. Indeed, the carbon will react to form chromium carbides (M3C , M7C3 , M23C6), the chromium content in solid solution will be smaller and it will be more difficult to form a protective oxide layer.

The addition of steel elements and their properties



Each element has a function in the alloy, here is a list of the main components and their functions in the alloy:

Iron (Fe ) is the predominant element in steel. In steel, the finer the grain, the harder the alloy. Iron carbides are not interesting for creating cutlery because they are larger than other carbides (about 50nm ) . There are other ways to harden steel. They are detailed a little further.

Carbon ( C ) : determines the hardness and traction strength, but it weakens the resistance to corrosion. Indeed, a large amount of carbon causes the formation of carbides which can oxidize ( localized corrosion). Carbon improves the cutting ability.

Chromium (Cr ) : allows the steel to be corrosion-resistant , it is an essential alloying element of the martensite steel.

- Silicon ( Si) : allows the steel to be corrosion-resistant particularly in the presence of strong oxidizing acids.

Nickel (Ni): resets the martensitic structure when steel has ferrite tendencies. Nickel increases use, formatting and forging abilities of steel.

Existing Steel

The most widely used steel for blades in French cutlery is Sandvik 12C27, Swedish steel that usually is never forged .

At Forge de Laguiole we use two different sorts of French steel by Bonpertuis to make our knives with T12 blades (Steel made in France and forged on the Forge de Laguiole site.)

Specifications : Choices and tradeoffs

We defined the specifications of our blades as such:
• Power to cut is important (long thin wire)
• Abrasion resistant wire
• Has to sharpen easily (if the knife sharpens easily we compensate for the low wear resistance )
• It polishes easily
• Good resistance to oxidation (in normal, daily use )



All these characteristics cannot be obtained in the same grade of steel, so there is a tradeoff : have maximum hardness (hardness depends on the rate of carbon.)  without being fragile or increasing risk of oxidation.

Dive into the heart of the T12 matter.

The T12 blade has a relatively homogeneous structure, primary and secondary carbides. The heart of the blade is a sorbitol.

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The following table gives the average grain size , hardness and number of carbides per unit area for each blade:

nuance

T12

12C27

taille grain moyen (µm)

3,0

8,3

dureté

58

58

nombre de carbure (carbures/µm²)

0,19

0,16

 

forgée

laminée


 To be hard, the blade must have a very fine grain and a lot of carbides along the grain. A T12 Forge de Laguiole blade has a finer grain than a 12C27 laminated blade.

These microscopic images confirm the very good quality (fine grain size, homogeneity, martensite structure) of T12

The T12 is a very good compromise : it is structured with fine grains, many carbides, and a homogeneous martensite structure which gives it a good ability to be polished, sharpened and have good cutting resistance.

Forging: forming a blade and refining the grain

Forging has a significant impact on the structure and hardness of steel (for a finer grain increases the hardness of steel). The unitary forging operation is important.

Different heat treatments for manufacturing a blade.

Steel, when bought, does not have the desired characteristics. It is therefore necessary to subject it to heat treatments. This allows to refine the grain, to harden steel, to increase its usage

We practice several types of heat treatment :

Annealing : it aims to erase all traces of the various manipulations previously incurred by the metal. It also allows for bringing the steel to soft condition for usage or for subsequent heat treatment.

Austenitization: heating in the austenitic range. The temperature and time control of austenitization.

- Good "surrender" solution carbides for low alloy steels.

- homogeneity of the distribution of carbides in high alloy steels.

- The austenitic grain size. It has a considerable influence on the working properties of steel such as toughness, fatigue or wear .

Quenching : it is sudden cooling after austenitization . Quenching is responsible for martensitic transformation . There are several types of quenching, we practice oil quenching

Income : it is intended to soften the structure ( usually very hard and brittle after quenching ) . The tempering temperature is generally between 100 and 300 ° C. Income in order to precipitate a portion of carbon in solid solution ( carbon in the interstitial sites which makes it very hard structure )

It is very important to know and control the temperatures and times of each heat treatment. These parameters are specific to each grade of steel.



A laminated blade is less expensive to manufacture than a forged blade for forging operations and annealing are not done. However, forging has an impact on the microstructure of the strip : the grain is thinner which provides a better cutting quality and a better hardness .

When the blade is forged, it is quenched in the air to harden its structure. Then we must anneal the blade to make it more ductile, it can then be shaped, drilled and punched. Then the blade is brought to its austenitizing temperature and T12 blades are hardened by oil that allows for very rapid cooling to obtain the martensitic phase. Then we practice income to soften the structure. This income precipitates some of the carbon in solid solution.

 

T12 trempé non forgé

T12 trempé et forgé

dureté HRC

   

moyenne

50,55

59,35

écart type

0,869

0,707