Pearlite


Crystal mixture of α solid solution (ferrite) and iron carbide Fe3C (cementite).
Pearlite is an eutectoid which fully transforms back into austenite above the A1 temperature of 723°C. In the eutectoid concentration of 0.8 % C the ferrite proportion is 86% and the cementite proportion 14 %.

In the pearlitic structure the cementite is embedded within the ferrite in a lamellarpattern (lamellarpearlite, Figures 1 to 3). The lamellarcementite can be molded into a spherical shape (granular pearlite, Figure 4) by means of spheroidizing annealing, if the cementite of the pearlite is stabilized such that it does not break down into graphite and ferrite. However, this requires manganese contents of more than 1%.

It is possible to check Pearlite with the acoustic resonance analysis (RTE Akustik + Prüftechnik GmbH). Acoustic resonance analysis is a volume-oriented test method, which evaluates the workpiece as a whole. From the resonance frequencies, characteristic workpiece values can be calculated and quality criteria assigned, such as OK, cracks, structure properties (Ferrite, Pearlite), hardness and mass variations. From the resonance frequencies, characteristic workpiece values can be calculated and quality criteria assigned, such as OK, cracks, structure properties, hardness and mass variations.

Additional references:
Metal matrix of cast iron
Structure formation of cast iron
Expansion

  • Fig. 1: Basic structure of nodular pearlitic cast iron, 200:1, etched
  • Fig. 2:  Pearlite formation in unalloyed GJS, 5000:1, etched
  • Fig. 3: Distance between the carbide lamella in the pearlite of Figure 2, 20000:1, etched
  • Fig. 4: Granular pearlite in the structure of GJS, heat-treated, 5000:1, etched
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