Materials
General comments about stainless steel
Stainless steel is a steel alloy with at least 10% chrome content in its mass.
Among its qualities, attention should be drawn to its beauty, malleability and durability. It is very easy to maintain and has features that only a fine, modern material is able to offer.
Stainless steel was discovered in 1913 by the Englishman Harry Brearley, when he was searching for steel alloys that would be suitable for gun barrels. During tests, he was able to ascertain that most samples that had been discarded some months beforehand had rusted, but not the one that contained 14% chrome.
Ordinary steel rusts because it is easily combined with oxygen from the air and this produces reddish iron oxides. Aluminium, nickel and chrome, in addition to other metals, react in a similar way, although their oxides form an impermeable layer. In the case of chrome, this layer means that the material possesses high resistance to corrosion as, despite being invisible and stable, this film adheres well to steel, defending it from the action of more aggressive media, making it inert and giving it great durability.
Stainless steel may be folded, cut, tooled and welded, adapting to different applications, and also offers a great variety of surface finishes ranging from opaque to mirror-like. It lasts a very long time and is very easy to maintain, which ensures the good value for money and functional nature of the products.
Stainless steel is a totally recyclable material and its reuse does not interfere with its quality. It permits total hygiene and ensures quality in terms of asepsis – an essential requirement in many applications.
Classification of stainless steel in accordance with the AISI standard
Stainless steel is classified as follows according to its crystalline structure:
• Martensitic steel
• Ferritic steel
• Austenitic steel
Martensitic stainless steel
This is the first branch of stainless steel, known simply as chrome, and was the first to be developed on an industrial level. It is used in blades of hafted knives and for monoblock knives.
It has relatively high carbon content from 0.2 to 1.2% and 12 to 18% chrome. The most common types are AISI 410, 420 and 431. The basic properties are: great hardness (this can be increased by thermal treatment) and is very easy to machine. In contrast, it has moderate resistance to corrosion.
Martensitic stainless steel is characterized by being magnetic and by being able to be hardened by thermal treatment – only after tempering will it become very hard and not very ductile, although it is precisely this condition that will make it resistant to corrosion.
The most used is Type 420. In its annealed state, (ferritic structure), it does not perform well against atmospheric corrosion, which is the reason why it is used in knives only after tempering treatment. Its great hardness and resulting resistance to wear determine the applications for which this material is used.
Ferritic stainless steel
This is also considered to be simply chrome. Its content varies from 12 to 18%, although the carbon content is low at <0.2%. The most common types are AISI 430, 409 and 434. The basic properties are: good resistance to corrosion and not very great hardness, which can be increased by thermal treatment. It is also magnetic.
Generally speaking, ferritic stainless steel contains a larger amount of chrome than martensitic steel. This increase in chrome improves resistance to corrosion in different media, but partly sacrifices other properties, such as resistance to impact.
The most used ferritic stainless steel is Type 430, which contains 16 to 18% chrome and up to 0.12% carbon. It is applied in cutlery, generally in low ranges. Servicios de Mesa de Guernica does not use this type of steel in any of its products.
Austenitic stainless steel
This is the most used owing to its wide variety of properties. It is obtained by adding nickel to the alloy, whereby the crystalline structure of the material is transformed into austenite, from where it takes its name. The chrome content varies from 16 to 28%, nickel content from 3.5 to 22% and molybdenum content from 1.5 to 6%.
The most common types are AISI 304, 304L, 316, 316L, 310 and 317.
The basic properties are: excellent resistance to corrosion, an excellent hygiene-cleaning factor, easy to transform, excellent weldability, does not harden with thermal treatment and can be used both at very low and very high temperatures.
Austenitic stainless steel is not magnetic and cannot be hardened by thermal treatment. It is very ductile and features excellent weldability.
The most popular austenitic stainless steel is Type 304, which basically contains 18% chrome and 8% nickel, with limited carbon content up to 0.08%.
There is a formula for rating resistance to corrosion:
PRE (Pitting Resistance Equivalence) = %Cr + 3.3x%Mb + 16x%N
It should be taken into account that while the percentage of chrome is around 18% (in AISI304), the molybdenum and nitrogen content is almost negligible, which is why the most important value for determining the stainless nature of steel is chrome.
