A study of the mechanisms of pearlite spheroidization under static annealing conditions was carried out in two materials — AISI 1080 steel and pure Fe-C alloy. By electrolytically etching away the ferrite matrix, the morphology of the cementite phase was directly investigatedvia SEM and TEM techniques. It was clearly observed that the initiation and development of spheroidization are associated with morphological faults such as terminations, holes, and fissures in the cementite plates. During spheroidization the recession of terminations and the expansion of holes and fissures led to the break-up of large cementite plates into small segments. The migration of these faults is anisotropic. The preferred orientations, [010] and [210] directions in the (001) cementite plane, were determined by TEM analysis. This anisotropic morphological change is attributed to the anisotropy of the α/Fe3C interfacial energy. All the experimental evidence strongly supports the idea that the fault migration theory is the main mechanism governing spheroidization.

Mechanisms of Pearlite Spheroidization

The recrystallization of ferrite and austenite formation during intercritical annealing were studied in a 0.08C-1.45Mn-0.21Si steel by light and transmission electron microscopy. Normalized specimens were cold rolled 25 and 50 pct and annealed between 650 °C and 760 °C. Recrystallization of the 50 pct deformed ferrite was complete within 30 seconds at 760 °C. Austenite formation initiated concurrently with the ferrite recrystallization and continued beyond complete recrystallization of the ferrite matrix. The recrystallization of the deformed ferrite and the spheroidization of the cementite in the deformed pearlite strongly influence the formation and distribution of austenite produced by intercritical annealing. Austenite forms first at the grain boundaries of unrecrystallized and elongated ferrite grains and the spheroidized cementite colonies associated with ferrite grain boundaries. Spheroidized cementite particles dispersed within recrystallized ferrite grains by deformation and annealing phenomena were the sites for later austenite formation.

Ferrite recrystallization and austenite formation in cold-rolled intercritically annealed steel

Kinetics of pearlite spheroidisation during static annealing and during hot deformation

The cementite spheroidization process is investigated in hypereutectoid steels with different chromium (Cr) contents. A spheroidized structure in high-carbon steel is usually obtained by a divorced eutectoid transformation (DET) reaction, which occurs during slow cooling of austenite with fine cementite particles. A bimodal distribution of spheroidized cementite particles is experimentally observed. It is shown that Cr addition allows one to obtain the spheroidized structure after austenitization at a higher temperature and a longer annealing time as compared with low-Cr steel. It is found that the DET reaction takes place at low undercoolings compared with the pearlitic reaction. The occurrence of the DET reaction also depends strongly on the spacing between cementite particles during the austenitization process, i.e., on austenitization temperature and time.

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The Cementite Spheroidization Process in High-Carbon Steels with Different Chromium Contents

Passing one electric current pulse through deformed pearlitic steel wires at room temperature causes the formation of cementite particles around 30 nm in size. This is found not only in some particular locations but throughout the cementite area, which reveals a different mechanism from traditional spheroidization of cementite plates because the latter leads to the formation of particles with much large size. Transmission electron microscopy images show electropulse-induced strain relief and formation of fine precipitations. Differential scanning calorimetry analysis demonstrates the additional stored free energy by electropulsing treatment. The raised free energy accounts the increased interface area in finer microstructure of materials. The experiment evidences that the passing electric current in metal has alternated the free energy sequence of various microstructures in comparison with that of current-free system.

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S. Chattopadhyay

Papers

Quantitative measurements of pearlite spheroidization