Steel is a metal alloy consisting of iron and carbon with a carbon content of less than 2.06%, above this value, and up to a maximum of 6.67%, it is called cast iron; the carbon is always present as cementite, a substance also known as iron carbide Fe3C, while the presence of a significant percentage of other chemical elements gives the product particular properties.

Classification of steels

Steels are classified into common carbon steels and special steels according to their chemical composition.


These are steels made up only of iron and carbon, where the content of the latter is between 0.2% and 1%; as the percentage of carbon varies, the mechanical characteristics of the steel also vary, in particular, increasing its percentage will increase mechanical resistance, hardness, hardenability and resistance to wear, while brittleness increases and malleability, ductility, toughness and weldability decrease.

Depending on the carbon content of the steel we can have:

  • extra-soft steels: C < 0.008%;
  • mild steels: C between 0.008% and 0.1%;
  • semi-sweet steels: C between 0.1% and 0.4%;
  • semi-hard steels: C between 0.40% and 0.55%;
  • hard steels: C between 0.55% and 0.80%;
  • extra-hard steels: C between 0.80% and 2.06%.

These types of steels represent the bulk of production, resist static loads well and are used in a variety of fields such as: in the manufacture of knives and scissors, in the production of lifting ropes, in the production of high voltage cables, in the production of automobile and mechanical parts in general, etc.


These are steels that are often defined as alloy steels because in addition to carbon they contain other chemical elements that give the product special properties; the elements usually added are: nickel, chrome, molybdenum, silicon, copper, manganese, tungsten, etc.

Special steels are used in all those applications where properties superior to those of common steels are required and the type of alloy varies depending on the type of use for which it is intended; in general the presence of Chromium (Cr) increases hardness and resistance to oxidation, for these reasons they are used for example in the manufacture of bearings and parts of thermal and chemical plants.

The presence of Silicon (Si) increases the elasticity limit, which is why they are used in the manufacture of springs.

The presence of Manganese (Mn) increases the penetration of hardening effects, therefore hardness and wear resistance, for this reason they are used in the manufacture of parts for which high mechanical properties are required in very deep areas of the part.

The presence of Tungsten (W) increases the hardness, which is why they are used in the manufacture of tools for turning, milling, etc..

The presence of Lead (Pb) increases the machinability, i.e. the ability of the material to allow itself to be machined and produce shavings, which is why they are also called automatic steels and are used in the manufacture of mass-produced parts on automatic machines.