What are different types of carbon steel

The ferrous materials can be classified according to different characteristics (ferrous materials = umbrella term for all iron-carbon alloys - including steel). The ferrous materials can be classified as follows:

  • according to carbon content
  • according to chemical composition (steel)
  • according to usage requirements / quality classes (steel)

The following diagram shows the classification of ferrous materials graphically.

Classification of ferrous materials according to carbon content

The most important classification of ferrous materials based on their carbon content is the distinction between cast iron and steel. An iron-carbon alloy (= iron material) is called steel if the carbon content is between 0.002% and 2.06%. If the carbon content (mass%) is higher than 2.06%, it is called cast iron.

Based on their carbon content, steels are further subdivided into structural steels (0.1 to 0.5% C), quenched and tempered steels (0.25 to 0.8% C) and tool steels (0.5 to 2.06% C). In addition, a distinction can be made between hypereutectoid and hypoeutectoid steels. A hypereutectoid steel is used if the carbon content is above 0.8%. Steels with a carbon content below 0.8% are called hypoeutectoid.

The following table shows the classification of ferrous materials based on their carbon content.

Carbon content [mass%]Ferrous material
0.0 to 0.1Pure iron
0.1 to 0.5General structural steel
0.25 to 0.8Quenched and tempered steel
0.5 to 2.06Tool steel
2.06 to 6.67cast iron
0.0 to 0.8hypoeutectoid steel
0.8 to 2.06hypereutectoid steel

Material properties

Cast iron (with the exception of GGG) has a moderate strength, it is very brittle and sensitive to impact. Cast iron is, wg. its structure made of Ledeburit, not thermoformable. In contrast to this, steels can be thermoformed as well as partially cold-deformed, hardened and tempered with suitable heat treatment. Over-eutectoid steels acquire high strength through hardening and quenching and tempering, but, like cast iron materials, are brittle and sensitive to impact. Sub-eutectoid steels have a lower strength, but are more malleable.
In general, the strength increases with increasing carbon content, while deformability decreases in parallel.

Classification of ferrous materials according to chemical composition (steel)

Furthermore, the steels can be divided into unalloyed and alloyed steels based on their chemical composition. Whether a steel is considered alloyed depends on the mass fraction of foreign substances - i.e. the mass fraction of the alloying elements. This must exceed a certain limit for the steel to be considered alloyed.
The following table lists the limit levels for various elements.

Limit levels for alloying elements in steel

elementLimit content [mass-%]
Al aluminum0,10
Co cobalt0,10
Cr chrome0,30
Mn manganese1,65
Mo molybdenum0,08
Ni nickel0,30
Si silicon0,50
W tungsten0,10

Within the alloyed steels, one can further subdivide between low-alloy and high-alloy steels:

  • Low-alloy steels: The proportion of alloying elements by mass is less than 5%
  • High-alloy steels: the proportion by mass of the alloying elements is over 5%

Classification of ferrous materials according to usage requirements / quality classes (steel)

A further classification can be made under the steels according to the usage requirements, whereby the steel types are divided into different quality classes. A division can be made into:

  • Basic steels,
  • Quality steels and
  • Stainless steels.

Basic steels

The characteristic of basic steels is that they are unalloyed and the requirements for their structure do not require any measures during manufacture. They are not required to have any special properties. In addition, base steels are not intended for heat treatment (other than annealing).

Quality steels

As the name suggests, higher demands are made on quality steels. They can be unalloyed but also alloyed. Since higher demands are placed on quality steels than on base steels, special care is also required in their manufacture. Additional requirements can be, for example, a certain sensitivity to brittle fracture, grain size, formability or weldability with high strength, cold formability, etc.

Stainless steels

Stainless steels are (alloyed but also unalloyed) steel cakes that have a high degree of purity and whose chemical composition is particularly tightly tolerated. This includes, for example, steels whose sulfur and phosphorus content (so-called iron companions) does not exceed 0.025%.
Contrary to what is often assumed, a stainless steel is not necessarily a stainless steel (just as a stainless steel is not necessarily a stainless steel).
Further details on stainless steels can be found in a separate script.

The different types of steel are described in detail in the following materials technology scripts.