Strain-Induced Dissolution of Cementite in Cold-Drawn Pearlitic Steel Wires
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INTRODUCTION
COLD-DRAWN pearlitic steel wires, well known for their very high strength, currently exceeding 6 GPa,[1] offer many engineering opportunities, such as tire reinforcement or suspension bridges. Pearlite is the two-phase product, composed of alternating ferrite (a) and cementite (h) lamellae, which forms at the eutectoid point from the austenite parent phase upon cooling. The most salient microstructural changes during drawing are curling of cementite lamellae in transverse section of the wire, their reorientation parallel to the wire axis in longitudinal section, reduction in interlamellar spacing, and dissolution of cementite. Strain-induced dissolution of cementite, and the subsequent carbon enrichment of ferrite, may thus potentially affect mechanical strengthening. It can be explained either by a binding enthalpy between carbon atoms and dislocations in ferrite higher than the enthalpy of dissolution of cementite in ferrite,[2] or by the Gibbs–Thomson effect, when the strain-dependent thickness of the cementite lamellae becomes subcritical.[3] Strain-induced dissolution of cementite has been studied extensively for several decades now, (see Reference 4, 5, and references therein) by various local or global, direct or indirect characterization methods. More precisely, the consequence of the dissolution, i.e., the carbon depletion of cementite and the concomitant carbon enrichment of ferrite, has been evidenced repeatedly, whereas the dissolution as such, i.e., the strain-dependent decrease in cementite fraction, has
received less attention. Unfortunately, residual cementite fraction cannot be easily deduced from carbon measurements because, first, the initially sharp a/h interface is rapidly lost, second, carbon is not homogeneously distributed neither perpendicular, nor parallel to the lamellae,[6] third, cementite dissolves by losing its carbon, and hence can deviate significantly from stoichiometry,[7] and finally, carbon partitioning between ferrite and cementite depends on the initial orientation of the colony with respect to wire axis.[8] It results that an accurate kinetics of dissolution of cementite during drawing is still lacking. With its ferritic matrix reinforced by cementite lamellae, pearlite can be seen as a composite material to which the rule of mixture applies. Since volume fraction of reinforcement is a key parameter of this rule, measuring its dissolution is of primary importance. While it is the easiest and most direct global method for second phase quantification in bulk materials, the basic electrolytic phase extraction (EPE) technique has not yet been used to determine the kinetics of dissolution of cementite in pearlitic wires during drawing. This study presents the application of this technique to drawn pearlite, complements the EPE characterization with transmission electron microscopy (TEM) observations, compares the results to those of the literature, and finally gives a simple explanation to cementite dissolution.
II.
MATERIAL AND METHODS
A. The Wire and Its Drawing NICOL
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