What's happening, when the steel is cooled from a high temperature?
Referring to the state diagram in the figure 1.
Suppose, that carbon steel 0,50 % heated to 815 ° C. All carbon will be in solid solution – sure, if the exposure time was sufficient. Is it gamma iron or austenite. Under these conditions, all carbon atoms are located in the spaces between the iron atoms in the face-centered cubic (Fcc) lattice (Figure 2a).
If the alloy is cooled slowly, when reaching a temperature of about 790 ° С the transformation of the fcc phase will begin (austenite) to body-centered cubic (OCK) phase or, otherwise, in the alpha phase or otherwise – ferrite (Figure 2b). If the temperature continues to drop, then the transformation basically ends at a temperature 727 ° C.
During this transformation of austenite, carbon atoms leave the atomic lattice of iron, since they almost do not dissolve in its BCC "version" – alpha phase (ferrets). therefore, if the austenitic alloy is cooled slowly, then it returns to the same phase composition, which he had before heating to the austenitic region. A similar process occurs with high-carbon steels. (with a carbon content of more 0,8 %). The only difference is, that in this case there is no transformation of austenite into ferrite in the two-phase region “austenite-ferrite” (picture 1). In addition to the entry and exit of carbon atoms into the gaps between the iron atoms and back, other changes occur in the alloy., which are important from the practice of heat treatment of steel.
First of all, at 770 ° C magnetic changes occur. Secondly, when an iron-carbon alloy is converted to austenite by heating it, at the transformation temperature this requires a lot of heat. On the contrary, when the alloy changes from gamma iron to alpha iron, then the same amount of heat is released back.
What will happen, if the alloy will cool quickly? When the austenite cools sharply, carbon atoms do not have time “fine” get out of the iron atomic lattice. This leads to atomic disorder and lattice distortion.. This results in high hardness and strength of the alloy.. If the cooling of austenite is fast enough, then a new structure is formed known as martensite, although this new structure (mixture of iron and cementite) is formally alpha phase.