Cementite Decomposition in High Carbon Steel Wires

Abstract

Decomposition behavior of cementite in high carbon steel wires during drawing and aging was investigated by atom probe field ion microscope, transmission electron microscope and tensile testing. Cementite decomposition proceeds with the drawing strain. At the same drawing strain, the carbon concentration of the dry drawn wire is higher than that of the wet drawn. In addition, cementite decomposition proceeds in low temperature aging after drawing. It may be concluded that the carbon atoms segregate to dislocations or cell boundaries and dislocation locking by carbon atoms plays an important role in the work hardening and strain age hardening. The mechanisms of cementite decomposition can be considered as follows. When the wire is drawn, carbon atoms in ferrite move within the dislocation strain fields, and the carbon concentration around dislocations is lowered locally. On the other hand, the matrix carbon concentration should be in equilibrium with the cementite. Accordingly, the diffusion of carbon atoms into the depleted region around dislocations leads to a general decrease of the carbon concentration, and cementite then dissolves tending to maintain the local equilibrium in the matrix carbon content. The highly dense dislocations which act as trapping sites of carbon atoms, and the heat generation of the drawing process which increases the diffusion rate of carbon are prerequisite for cementite decomposition. The rate-determining step of cementite decomposition may be carbon diffusion under the condition of constant dislocation density.