Atom probe and transmission electron microscopy investigations of heavily drawn pearlitic steel wire

Abstract

Transmission electron microscopy (TEM) and atom probe field ion microscopy (APFIM) observations of pearlitic steel wire show that drawing to a true strain of 4.22 causes fragmentation of cementite lamellae into nanoscale grains. The drawing strain amorphizes some portions of the cementite lamellae in regions where the interlamellar spacing is very small, but most of the cementite lamellae are polycrystalline with nanoscale grains. The carbon concentration in the ferrite is inhomogeneous and varies from 0.2 to 3 at. pct; the carbon concentration in nanocrystalline cementite is less than 18 at. pct, significantly lower than that in stoichiometric Fe3C. Silicon is segregated to ferrite/cementite boundaries, but, in regions with a small interlamellar spacing, the silicon concentration is uniform across the lamellae. Annealing at 200 °C for 1 hour does not cause noticeable changes in the microstructure. Annealing at 400 °C or above for 1 hour causes spherodization of the cementite lamellae, and the carbon concentrations in ferrite and in cementite return to the predeformation values.