Cast iron and steel are more than 80 % weights of all metals used. Most carbon steels contain up to 1,5 % carbon, whereas cast irons usually contain up to 2 to 4 % carbon.
Iron crystal lattices
Known, what iron exhibits allotropic properties and changes its crystal structure when heated or cooled:
- When pure iron solidifies upon cooling from its melting point 1540 ºС it takes on a body-centered cubic structure (OCK), which is designated δ-Fe (delta ferrite).
- With further cooling between temperatures 1395 ºS i 912 ºС it has a face-centered cubic crystal structure (Fcc), which is called austenite and denoted γ-Fe (gamma iron).
- Below temperature 912 ºС iron again becomes bcc-structure and is designated α-Fe (ferrite).
State diagram "iron-cementite"
The phase diagram "iron-cementite" is shown in the figure 1
The carbon content in this diagram reaches only 6,70 %. At this carbon content, an intermediate compound iron carbide or cementite is formed (Fe3C), which is shown by the vertical line on the right in the phase diagram. Since all steels and cast irons have a carbon content below 6,7 %, the rest of the phase diagram with a carbon content from 6,7 % to 100 % not of engineering interest. Cementite is a hard and brittle carbide compound with an orthorhombic crystal structure.
Picture 1 - State diagram "iron-carbon"
Solid solutions of carbon in iron
Carbon forms with iron a series of solid solutions, which are denoted by the fields δ, γ and α. However, carbon has very limited solubility in iron.:
- The maximum solubility of carbon in ferrite is only approximately 0,022 % at a temperature 727 ºС and increases to 2,14 % 1147 ºS.
- At room temperature, the solubility of carbon in iron is even less - only 0,005 %.
In this way, the solubility of carbon in the fcc structure of iron is approximately 100 times higher, than in its bcc structure. This is the result of, that the inter-node spaces in the fcc structure are larger and, respectively, deformations, which arise in the surrounding iron lattice much lower. Carbon, even in very small quantities, significantly affects the mechanical properties of ferrite.
Cementite and Graphite
Steels contain carbon in the form of cementite, whereas cast irons can or cementite, or carbon in the form of graphite. Graphite is more stable, carbonaceous phase, than cementite. The formation of graphite is facilitated by the high carbon content and the presence of a large amount of certain alloying additions, in particular, silicon. Therefore, graphite is an important phase in cast irons., but rarely found in steels. Cementite is metastable and will remain at room temperature as a compound indefinitely. but, if cementite is heated at a temperature of 650 to 705 ºС for several years, it will gradually turn into iron and graphite. During the formation of graphite, the limits of solubility and temperature ranges of phase stability change somewhat., as shown by dashed lines in the phase diagram (cm. picture).
In iron-based alloys, carbon is most often the main alloying element., as well as other alloying elements. There are three types of iron-based alloys, depending on the carbon content.:
- cast iron.
Industrial grade iron contains less 0,008 % carbon and when cooled to room temperature consists almost exclusively of ferrite.
In steels, which have a carbon content of 0,008 to 2 % and when they slowly cool down to room temperature, their microstructure consists of ferrite and cementite. Although steel can contain up to 2 % carbon, usually its content is limited 1,5 % or less, to avoid excessive fragility. Usually the carbon content is kept low in steels, which require high ductility, high viscosity and good weldability. High carbon content is used in steels, which require high levels of strength, hardness, fatigue strength and wear resistance.
Iron alloys are called cast irons., which contain carbon ranging from 2 to 6,7 %. However, most cast irons have a carbon content of 2 to 4 %.
Source: Elements of Metallurgy and Engineering Alloys /ed. F.C. Campbell – ASM International, 2008