The characteristics determined by the American Iron and Steel Institute for AISI steel are necessary, as they identify the different types: according to the World Steel Association, more than 1,800 million tons are produced every year, hence the need for a standard to be met. Let’s see what are the differences between the various types of steel.
Steel types: recognition series
There are several types of steel, designated by a three-digit acronym plus a possible letter. The first digit determines the strength class, while the second pair of digits is necessary to distinguish the types of materials.
AISI steels are divided into:
- 2XX: austenitic steel, consisting of chrome, nickel and manganese
- 3XX: austenitic steel, consisting of chrome and nickel
- 4XX: ferritic or martensitic chrome steel
- 5XX: martensitic steel, medium chromium format
- 6XX: Chrome precipitation hardening steel.
The letter, where present, adds further information, for example the percentage of carbon, titanium or sulfur present in the alloy.
The AISI nomenclature further subdivides the types of steel into specific series: carbon steels (1000 series), low-alloy steels (four-digit series), high-alloy steels (200, 300 and 400 series). The latter series, in particular, indicates stainless steels.
Stainless steels according to the AISI classification
Stainless steels are defined as AISI, although it is inaccurate: the classification itself is perceived as a synonym, to indicate the family of steels containing at least 11% chromium, the element that increases the resistance to oxidation of the material.
Stainless steels are resistant to corrosion, thanks to the phenomenon of passivation, reacting easily with the surrounding environment: the elements of the alloy, in particular the chrome, form a thin protective layer against subsequent corrosive attacks.
Some of the most commonly used stainless steels are:
- AISI 304 stainless steel: austenitic and non-magnetic steel, composed of chrome between 18% and 20%, nickel between 8% and 11%. It is the most used stainless steel in Italy for the production of kitchen utensils, it is in fact the famous steel 18-10
- AISI 430 stainless steel: ferritic type, it has a higher thermal stability and a very high melting temperature. Its characteristics make it ideal for the construction of furnaces and combustion chambers.
- AISI 316 stainless steel: austenitic alloy composed of chrome, nickel and molybdenum, a key element to increase the resistance of the material to electrolytic and intergranular corrosion, even after welding. It is widely used in the food industry and, under a certain temperature, in the chemical industry.
There are two simple tests to identify which stainless steel is involved: on the basis of the length of the spark when the material is brought close to the grinding wheel; using a simple magnet as a magnet, to understand if it is ferromagnetic steel.
Stainless steel welding
Today, stainless steel welding is one of the most common practices. Being an easily weldable metal, both to another stainless steel and to a carbon steel, stainless steel is used in many braze-welding practices.
However, in order to weld the steel correctly, it is necessary to take into account the higher thermal expansion coefficient of the material and the lower thermal conductivity: the temperature must be adjusted perfectly to avoid deformation of the metal.
The recommended welding processes are:
- TIG welding, in an inert atmosphere and using the tungsten electrode
- MIG welding, under gas intercept and with metal electrode
In conclusion, stainless steel is a precious metal, given its great durability: the AISI classification helps manufacturers in choosing the right steel for each process.