Perlite transformation austenite occurs during the phase transformation of gamma iron - austenite to alpha iron - ferrite. However, the transformation of the face-centered lattice of austenite into a body-centered lattice of ferrite cannot occur immediately due to the presence of dissolved carbon in the austenite.. The austenitic atomic lattice has enough room to accept carbon at the center of its atomic cell. The body-centered ferrite lattice does not have such a place - an iron atom already "sits" there. For this reason, for the transition of austenite to ferrite the solubility of carbon in iron is sharply reduced.
Pearlite transformation mechanisms
During the conversion of gamma iron to alpha iron, almost all carbon is released from the austenitic lattice.. According to the metastable iron-carbon phase diagram, almost all carbon is released in the form of iron carbide. (cementite). it pearlite transformation of austenite can be described in the form of three interrelated processes:
- Converting iron gamma lattice to alpha lattice.
- Release of carbon in the form of iron carbide (cementite).
- Coagulation of carbides.
At point A temperature1 the first two processes take place almost simultaneously with the formation of a lamellar mixture of ferrite and cementite.
Pearlite transformation of austenite as diffusion crystallization
Dissolved carbon atoms are randomly placed on the lattice. Therefore, cementite originates in areas rich in carbon, and ferrite - in regions depleted in carbon, which have little or no carbon. This carbon redistribution is realized through diffusion and depends on temperature and time.
Carbon diffusion rate in the gamma lattice- and alpha iron decreases very quickly with decreasing temperature. Therefore, at a sufficiently high cooling rate, such a degree of overcooling of austenite is achieved, that the formation of pearlite is a purely diffusion process – becomes impossible.
Three types of austenite transformation
At relatively low temperatures, the transformation mechanism and the nature of the formed structure depend only on temperature, at which this transformation occurs. Depending on the degree of hypothermia, three temperature ranges are distinguished:
- perlite interval;
- intermediate interval;
- martensitic interval.
The transition from one mechanism of transformation of austenite to another occurs smoothly and continuously. These processes are highly dependent on the content of alloying elements, especially carbon. They can start on the fastest mechanism, but end with the slowest.
For pearlite transformation characterized by the simultaneous formation of a mixture of alternating plates of ferrite and cementite – pearlite structure. In this case, free ferrite and cementite can be precipitated along the boundaries of austenite grains.. The formation and growth of both phases is controlled mainly by the diffusion of carbon - therefore this process is called diffusion crystallization. Diffusion of iron and alloying elements also play a significant role. With a decrease in temperature, the dispersion of the structure increases., and time, required to complete the diffusion crystallization of ferrite and carbide increases.