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Phase transformation in steel
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Invariant Reactions in the Fe-Fe3C Phase Diagram
Peritectic Reaction
Eutectic Reaction
Eutectoid Reaction
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Schematic representations of the microstructures for an iron–carbon alloy of eutectoid composition (0.76 wt% C) above and below the eutectoid temperature.
Pearlite - A two-phase lamellar microconstituent, containing ferrite and cementite, that forms in steels cooled in a normal fashion or isothermally transformed at relatively high temperatures.
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The transformation rate increases with decreasing temperature such that at 540°C, only about 3s is required for the reaction to go to 50% completion.
Generation of Isothermal Transformation Diagrams
540°C
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Diffusion of Carbon in Pearlite
Micrograph of eutectoid
steel, showing pearlite
microstructure.
Schematic representation of the formation of pearlite from austenite.
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Photomicrographs of (a) coarse pearlite and (b) fine pearlite. 3000X.�
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Hypoeutectoid and Hypereutectoid Alloys
The evolution of microstructure of hypoeutectoid and hypoeutectoid steels during cooling, in relationship to Fe3C phase diagram
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The TTT diagrams for (a) a 1050 and (b) a 10110 steel.
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Photomicrograph of a 0.38 wt% C steel having a microstructure consisting of pearlite and proeutectoid ferrite.
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Photomicrograph of a 1.4 wt% C steel having a microstructure consisting of a white proeutectoid cementite network surrounding the pearlite colonies.
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Full TTT Diagram
The complete TTT diagram for an iron-carbon alloy of eutectoid composition.
A: austenite, B: bainite, M: martensite, P: pearlite
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Relationship between TTT diagram and location of different microstructures.
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Moderately rapid and slow cooling curves superimposed on a continuous cooling transformation diagram for a eutectoid iron–carbon alloy.
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Relationship between TTT diagram and different cooling rates and resulting constituents.
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