Below temperature 912 ° C iron is called ferrite or alpha iron. The atoms in the ferrite are organized in a body-centered (OCK) crystal structure. Above 912 ° C iron is called austenite or gamma iron.
The atoms in austenite are organized in a face-centered (Fcc) crystal structure. Heating ferrite to 912 ° C leads to the formation of the smallest grains of austenite at the boundaries of ferrite grains. Further heating leads to the growth of these new austenite grains, with the complete replacement of the old ferrite grains with new austenite ones. – occur iron transformation.
When cooled below temperature 912 ° С in iron, transformations of the same type occur, only in the opposite direction - ferrite grains replace austenite grains.
Conversion of ferrite to austenite
If pure iron heat to temperature 912 ° C, then some “wonderful” transformations: the crystal structure of iron spontaneously transforms from a body-centered cubic (OCK) to face-centered cubic (Fcc). These two iron structures are shown in the figures. 1 and 2. As their names say, in (OCK)-structure, atoms are located at the corners of the cube and in its center, and in (Fcc)-structure - at the corners of the cube and in the center of each of the six faces of the cube. As well as low temperature (OCK)-structure, (OCK)-the structure has two names: austenite and gamma iron (γ-iron). The letter γ is the third letter in the Greek alphabet..
Picture 1 – Body-centered cubic (OCK)
ferrite crystal lattice
There are mysteries in the transformation of iron. Where is the beta iron (β-iron), by the second letter of the Greek alphabet? The answer is: earlier such hardware "was", but now he is gone. The thing is, that when scientists discovered the structure of iron in the late 19th century, due to the magnetic transition of iron at 770 ° C, they decided, that there is one more iron conversion: from alpha iron to beta iron. Then it turned out, that it was a mistake (and it happens with scientists!), and β-iron was simply "canceled".
Conversion of ferrite grains to austenitic
When ferritic iron is heated to a temperature 912 ° C, old composition of ferrite grains changes to new composition of grains, already austenitic – transformation occurs in the gland.
Imagine, that the ferritic grain structure has just reached the transformation temperature. First we see the formation of new, very small austenite grains, which overlap the old ferrite grain boundaries. Then these grains grow, until all the old ferrite grains are gone.
When ferrite is converted to austenite, two important phenomena occur.:
1) Just like turning ice into water, converting iron from ferrite to austenite requires thermal energy. Therefore, when heated, the temperature of iron will remain at a temperature of about 912 ° C, until all ferrite grains are austenitic.
2) When ferrite is converted to austenite, volumetric changes occur. Density of austenite on 2 % higher, than ferrite, which means, that the austenite atom occupies a smaller volume, than ferrite atom.
All transformations in iron, which occur when it heats up, shown schematically in the figure 3.
Experiments with transformations in iron
Two simple experiments clearly demonstrate phase transformations in iron.
Experiment # 1. Heat the iron bar to a temperature above 770 ° C and hang it to cool in air. Bring a magnet to the rod. When the temperature reaches 770 ° C, the hot bar will begin to be attracted to the magnet. As the diagram in the picture shows 4 ferrite (alpha iron) is magnetic only below 770 ° C, austenite (gamma iron) never magnetic.
Experiment # 2. Pull the iron wire horizontally between two electrical insulators at a distance of about 1 m. Hang a small weight in the center of the wire. Pass an electric current through the wire so, to heat the wire higher 912 ° С - up to orange-yellow color. Raise the voltage slowly with a variable power source. When the wire heats up, it will lengthen and the load will lower a little. After that, you need to disconnect the voltage from the wire and observe its cooling in a darkened room.. When the wire reaches temperature 912 ° C, two phenomena can be observed:
1) When the wire cools, its length will decrease, and the weight will start to rise. However, with 912 ° С there will be a temporary lowering of the load: just at that time, when the austenite turns into ferrite with a lower density and from this the wire will slightly lengthen.
2) Warmly, which is released during the transformation of austenite to ferrite, will lead to visible flickering of the color of the heated wire.
Both of these phenomena can be observed in the reverse order and upon heating., but in this case they are not so clearly visible due to the difficulty of quickly heating the wire.
Transformations in the gland
To understand, why does heat release during the transformation of austenite into ferrite, we must remember the transformation of ice into water. Really, water needs to be cooled, to turn her to ice. It means, that heat is removed from the liquid at freezing point. The same effect occurs when the metal "freezes": heat is removed from the metal. therefore, when the liquid metal is cooled to the point of solidification, then at the solidification temperature, heat is released. During the transition of a liquid state into a solid, a phase transformation between liquid phase and solid phase. Phase transformations, which occur when cooling, give off heat.
When upon cooling austenite it turns into ferrite – this is also phase transformation, but already from a solid state to a solid state and during it, heat is also released. The opposite happens when heated.: when ferrite turns into austenite, heat is absorbed.