Generalities on steels
Steel can be defined as an iron-carbon alloy with a carbon content less than 2.06%. In practice, however, other elements are always present in non-negligible quantities, so to constitute a complex alloy.
Steel finds applications in various fields and there is a great number of types of steels, which differ according to the characteristics of mechanical or corrosion resistance (for example) required according to the application.
It is therefore extremely complex to regroup steels into a few categories and to classify them. Among the possible classifications may be mentioned those:
- According to the quality requirements obtained through the control of production techniques (basic steels, high-quality steels, special steels);
- On the basis of chemical composition (carbon steels only, alloy steels);
- According to the physical properties and chemical-physical properties (corrosion resistance, electrical characteristics, etc.).
- Depending on the application (steels for general use, special structural steels, tool steels, etc..).
There are a number of methods used for the designation of steels, based on different criteria. Among these are:
- The rules established by the unification authorities of various countries (UNI for Italy, AISI, SAE, ASTM, UNS and AA for the United States, DIN for Germany, ANFOR for France, etc.).
- The rules established by the entity responsible for control of specific sectors (such as API, American Petroleum Institute);
- The rules established by individual manufacturers.
In cutlery steels one of the most common designations is the AISI (American Iron and Steel Institute).
This is based, for carbon steels or low-alloy steels, on a numerical system formed by four or five numbers, the first of which identify the class of the steel and the last two (or three) the carbon content multiplied for 100 (for example the 10xx series indicates carbon only steel).
As regards the alloy steel, in particular stainless steels, they are typically indicated by a code of three numbers (with the possible addition of some letters), the first of which identifies the class (for example 4xx indicates ferritic or martensitic stainless steels chrome).