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The blade

The blade

There is no ideal steel for knifes, but a wide range of grades which are more or less well suited to the knife’s purpose. By definition, steel is an alloy combining iron and carbon.
When no other element is added, it is commonly called carbon steel. However, this combination is very sensitive to corrosion.
To compensate, at least 10.5% or more chromium is added. This steel is generically known as stainless steel. The added chromium creates a superficial and extremely thin oxide layer, in other words, a passivation layer which protects the blade from corrosion. 
The higher the carbon content in steel, the harder it gets through heat treatment. The hardness guarantees a long wearing cutting quality. It is therefore quite tempting to use a high carbon content grade.  
There are however two limitations to the increase in carbon content:
The harder the blade, the more fragile it becomes. This is true for both types of steel, whether it is carbon steel or stainless steel.
A high carbon content in stainless steel reduces its resistance to corrosion. Carbon tends to combine with chromium to form chromium carbides. As a carbide, chromium loses its protective property against oxidation.
It is important to note that, for a similar hardness, a stainless steel containing chromium carbides will be more resistant to abrasion. This property provides long wearing cutting quality.
We aim for 57 to 59 HRC on the Rockwell hardness scale for our blades.
Selecting a steel grade is always a compromise between mechanical strength and resistance to corrosion. From our years of experience, we have chosen two main grades.
 

Carbon steel

When it was first designed, the OPINEL blade was made from carbon steel. We still use a similar grade today, with an approximate carbon content of 0.90% which is still better than stainless steel. Our carbon steel is first produced in Germany and finalised in France before being worked by OPINEL.
The grade can be sharpened very easily, so with regular maintenance, its cutting power is always perfect. When cutting a soft abrasive material such as wood, paper or cardboard, the cutting edge gets worn and tends to resharpen when in contact with the soft material. On the other hand, the cutting edge easily deteriorates when in contact with hard surfaces such as ceramic plates. This grade is also cheaper than stainless steel.
A carbon steel blade is more sensitive to corrosion and requires more maintenance than a stainless steel blade. It should not be used in a damp or acidic environment. 
 

Stainless steel

Stainless steel appeared in the forms that we know at the beginning of the 20th century. 
The stainless steel grade for the OPINEL blades has been optimised to get the best compromise between resistance to corrosion and mechanical strength properties. Its carbon and chromium content are respectively 0.5 and 14.5%. Our 12C27MOD stainless steel is produced and finalised in Sweden by the SANDVIK group, internationally renowned in the cutlery industry.
This grade provides strong resistance to abrasion (wear) which allows the blade to be regularly in contact with hard material such as ceramic, without needing to be resharpened. However, the sharpening requires specific know-how and the appropriate tools.
Another advantage of stainless steel is that it requires no specific maintenance under normal conditions of use, unlike carbon steel. Nevertheless, it will likely meet its limits when in prolonged contact with an aggressive environment (acidic, salty water, detergent, etc.).
 

Heat treatment

Heat treatment is one of the key steps to the quality of a blade and is critical in obtaining the required mechanical characteristics.
Steel is worked when “annealed”, i.e. when its hardness is very low. To reach this stage, the blade is subjected to a two-phase heat hardening treatment. The first step is called “quenching” and involves heating the blade to temperatures above 800°C before cooling rapidly. The blade becomes extremely hard but also too brittle. The additional process is called “tempering” where the blade is heated again to a lower temperature than previously then cooled slowly.
 

Convex profile

The convex profile of the OPINEL blade provides greater resistance to cutting when compared to a flat ground blade of similar height and spine thickness. The contact between the sides of the blade and the cut material is minimised which significantly reduces friction and therefore the cutting power required.
 

Cutting edge

The sharpening angle is also a compromise.
If the angle is too sharp, the cutting edge is fragile and wears rapidly. If it is too obtuse, the cutting becomes too difficult. The OPINEL sharpening angle is approximately 40°.
The sharpening operation is the last step in the knife manufacturing process. The blade is hand-sharpened using grindstones for which the precision in diameter is to the 1/100 mm in order to guarantee a regular cutting angle.
 

Food grade

The different steels we use for the manufacture of our blades, whether carbon or stainless steel, is suitable for contact with foods. Samples are regularly taken during production by an official organisation for analysis and to check their compliance with various standards.
We meet the regulatory requirements with regards to food grades according to:
- Decree dated 13/01/1976 on stainless steel materials and objects in contact with food,
- French standard NF A36-714 on stainless steel without packaging – Flat steel products intended to come into contact with foodstuff for humans and animals – uncoated steels (not stainless steel),
- EC 1935/2004 Regulation dated 27/10/2004 on materials and articles intended to come into contact with foodstuff.
The blade