Construction
Pure iron
Pure iron (Fe) is made in the micrograph composed of individual grains. These arise from the fact that the freely mobile in the melt during solidification of iron atoms arranged in a spatial lattice structure. Iron crystallizes in a cubic system at room temperature in the body-centered cubic (bcc) lattice. This is located at the middle of a cubic unit cell of another atom.
Centered cubic lattice
With increasing temperature, the lattice structure transforms to the face-centered cubic (fcc) lattice, is here in the middle of each side surface of another atom.
Face-centered cubic lattice
Alloys
For the practical application of the iron is alloyed with various elements, so as to obtain the desired properties. The various materials are usually prepared by melting together. These are formed depending on the alloy element out different lattice structures.
Storage mixed crystals / add mixed crystals
Mixed crystal deposition
The atoms of the alloying element are stored in the gaps of the iron lattice (interstitial). For this, the atoms of the alloying element can be much smaller than the iron atoms, eg Carbon, nitrogen, hydrogen and oxygen.
Substitution solid solution
The atoms of the alloying element replace individual iron atoms within the iron lattice, the atoms must be similar to the iron atom, eg Chrome.
Connection mixed crystals
The iron atoms with the atoms form new crystals of the alloy element, often with a different lattice structure. This requires an integer ratio. Carbon and nitrogen can form solid solutions with iron, both storage and connection mixed crystals.
Structure of iron-carbon alloys
The largest share of steel production, the so-called 'non-alloy structural steels'. These are iron-carbon alloy with a carbon content of from 0.06 to 0.3%. The quenched and tempered steels are a carbon content of 0.3-0.5%, tool steels with a carbon content of 0.5-1.5%.
Ferritic steels (ferrite - C content <0.02%)
Homogeneous solid solution, when cooled below 723 ° excretion of Fe3C
Hypoeutectoid steels (ferrite and perlite - C content of 0.02 - <0.8%)
On cooling, the γ-solid solution in α-solid solution is formed with a low C content, the remaining γ-solid solutions to increase their carbon content. When they reach 723 ° C and 0.8% perlite to disintegrate.
Eutectoid steels (pearlitic - C content = 0.8%)
On cooling, the eutectoid reaction takes place, is divided into α-iron and Fe3C. The mixture is lamellar pearlite.
Hypereutectoid steel (pearlite and cementite - C content of> 0.8-2%)
On cooling, formed initially Fe3C (cementite), the γ-solid solution depleted of carbon, until the eutectic reaction begins.
Austenitic steels (austenite)
Austenitic steels have always further alloying elements, since the γ-phase of pure Fe-C at room temperature is not stable.
Martensitic steels
Above the line ABCD (liquidus) of the material is molten. With increasing carbon content decreases the melting point of 1536 ° C to 1147 ° C (for cast iron at 1153 ° C), from 4.3% by mass, it increases again.
In the area above the line campaign AHIECF the material consists of a mixture of crystals and melt, below the 'solidus' is the solid state. On further cooling to room temperature still here instead of grid changes and formation of cementite.
In the area enclosed by the line NIESGN area consists of the Materials on the austenite in which the carbon is dissolved interstitially.
At point E, represents the maximum solubility of carbon, one with 2.06 mass% at 1147 ° C. Based on the so-called A3 line (GSE) can be the resistance of the austenite read: the temperature up remains to that of the austenite decreases with increasing carbon content up to 723 ° C (Perlitpunkt) and then again with further increasing carbon content to increase .
Below the GS line gradually decreases during the A1 and A3 temperatures by the limited range of the fcc lattice of austenite, the bcc lattice of the ferrite. The excess carbon is released into the austenite, because the ferrite can hold a maximum of 0.02 mass% of carbon interstitial. At 723 ° C, the austenite reaches a carbon concentration of 0.8% by mass and is transformed into the eutectoid pearlite structure. These are within the individual ferrite cementite with 6.7 mass% carbon.
Effect sizes
Alloying elements
The various alloying elements influence the transformation behavior and the regions of existence of the structural components. Here, solving elements that crystallize in the bcc lattice itself, preferably in the α-iron, fcc γ-iron in the crystallized elements (except aluminum). All atoms dissolve interstitially dissolved preferably in γ-iron. By the alloying elements, the temperature range in which there exists the γ-phase (increased Cu, Au, Ni, γ-Mn (fcc) C, N (interstitial) or reduced (Ti, V, Cr, Mo, Nb, W (bcc) Al (fcc) Si (kdp), P).
Cooling
Already a cooling rate of more than 1 ° / s affects the conversion processes, since rapid cooling of austenite required to segregate the carbon diffusion is hindered.
