Pearlitic structure got its name from that, that after being etched, it has a pearlescent luster under the microscope.
IN perlite range of austenite transformation formed pearlitic structure – mechanical mixture of ferrite and cementite plates. Speed, with which nuclei of pearlite crystallization are formed, depends on the overcooling of austenite in relation to the equilibrium temperature of cementite formation. This hypothermia increases with decreasing temperature.. Islet growth pearlite structure depends mainly on the diffusion rate of carbon and iron atoms. Other decisive factors are the degree of supercooling and the free energy gain in ferrite formation..
Mechanisms for the formation of a pearlite structure
Perlite islands grow not only due to the formation of new plates, but also due to the growth of old plates in all directions. Carbide plates grow faster, than ferritic. Process, However, begins with the formation of ferrite nuclei. The mechanism for the formation of the pearlite structure is still not fully understood.. Classic perlite is a multitude of so-called pearlite colonies, which consist of alternating parallel plates of ferrite and cementite (picture 1).
Pearlite nuclei appear mainly in defective areas crystal lattice: at grain boundaries, on insoluble carbides or non-metallic inclusions.
Interlamellar spacing in pearlite
The most important characteristic of pearlite is the distance between its plates - the inter-plate distance (picture 2). With a decrease in this distance, the strength properties of steel increase..
The rate of formation of centers of crystallization of cementite and ferrite in the pearlite temperature range increases with decreasing temperature. In this case, the inter-plate distance decreases, and the dispersion of the structure increases.
In eutectoid steel, pearlite transformation occurs when it is cooled to a temperature from 700 to 600 ˚С. In this case, the inter-plate distance is 0,5-1,0 μm.
Decomposition of austenite in the temperature range from 600 to 500 ˚С provides inter-plate distance from 0,4 to 0,2 μm. In this case, the eutectoid, it is often called quasi-eutectoid, is a more dispersed structure.
When austenite decomposes in the temperature range from 600 to 500 ˚C, an extremely dispersed pearlite structure with an interplate distance of about 0,1 μm.
Sizes of pearlite colonies
An important characteristic of pearlite, which affects the properties of steels, is the size of the pearlite colony (picture 3). The colony – it is a group of cementite and ferrite plates, which jointly, grew cooperatively in austenite before colliding with other colonies.
A decrease in the size of a pearlite colony is accompanied by an increase in the impact strength of steels and a decrease in their brittleness..
Increasing the brittle fracture strength of pearlite is achieved by spheroidizing cementite plates. This spheroidization can be achieved by deformation of pearlite, followed by heating and holding at a temperature near point Aс1. Another method, which provides a relatively high strength and ductility of pearlite, consists in the deformation of pearlite during pearlite transformation. This leads to the formation of a polygonal structure and spheroidization of cementite..
Abnormal and normal structures of pearlite
Eutectoid transformation, which is accompanied by a non-pearlite, and separated by phase formation, called abnormal. In normal eutectoid transformation, ferrite and pearlite grow cooperatively as colonies with regular alternation of two phases. In the case of abnormal transformation, a coarse mixture of ferrite and cementite does not have the properties of a pearlite structure.. In a real eutectoid transformation, the transformation mechanism can change from abnormal to normal. Therefore, with rapid cooling and, accordingly, large supercooling of austenite, the abnormal transformation can be completely suppressed..
Pearlite structure in hypoeutectoid steel
Hypereutectoid steels include steels with a carbon content of less than 0,8 % carbon. Otherwise, they are called low carbon steels.. In hypoeutectoid steels when cooled below temperature A3 excess ferrite is formed first, and then below temperature A1 – pearlitic structure. This ferrite comes in two forms: compact equiaxed grains and oriented widmanset plates (picture 4).
Compact precipitates of hypoeutectoid ferrite occur mainly at the boundaries of austenite grains, whereas the Widmanstat ferrite is formed inside the grains. Vidmanshtetov ferrite is observed only in steels with a carbon content less than 0,4 % and coarse austenite grains. When the size of the austenite grains decreases, the proportion of ferrite in the form of equiaxed grains increases. Widmanstät ferrite is formed in the temperature range from point A3 to temperature 600-550 FROM. With an increase in the carbon content in steel, the proportion of Widmanstätt ferrite decreases.
Pearlite structure in hypereutectoid steel
Hypereutectoid alloys include alloys with a carbon content of 0,8 to 2,0 %. They are often referred to as high carbon steels.. In contrast to hypereutectoid steels, non-excess ferrite is released first in hypereutectoid steels when they are cooled., and excess cementite. Then, as a result of a decrease in the carbon content in austenite close to the eutectoid and a decrease in temperature below point A1 is forming pearlite structure. In this way, the structure of hypereutectoid steel consists of pearlite and secondary cementite (picture 5).