Abstract

Major processes affecting microstructure of a drawn pearlitic wire including lamellae thinning, changes in inter-lamellar interface and metallographic and crystallographic texture, plastic flow localization, and dynamic strain aging were characterized. Heavily drawn pearlite represents a nano-composite with thickness of ferrite and cementite lamellae decreasing during wire drawing. Volume fraction of inter-phase interfaces is comparable with that of pearlitic cementite and they are associated with high elastic stresses. Stretching and rotation of pearlite colonies result in their alignment with the wire axis. This is accompanied by increase in a local ductility at the true strain below from 1.5 to 2 and then decrease at higher strain levels. Development of a strong crystallographic texture causes anisotropy in mechanical properties. Targeted observations of plastic flow at the same region showed two systems of localized shear bands and provided information on their development. A dramatic decrease in elongation to failure in wires after drawing is linked to the existence of the localized shear bands. Dynamic strain aging increases strength and degrades ductility of drawn wires.

Abstract

Strengthening mechanisms and strength–structure relationships have been analyzed in a cold-drawn pearlitic steel with a structural scale in the nanometer range and a flow stress of about 3.5 GPa. The wires have been drawn up to a strain of 3.7 and the structures analyzed and quantified by transmission electron microscopy and high resolution electron microscopy. The mechanical properties have been determined by tensile testing. It is found that the interlamellar spacing and the thickness of the cementite lamellae are reduced in accordance with the changes in wire diameter up to a strain of 2.5. At a higher strain enhanced thinning of the cementite lamellae points to decomposition of the cementite and carbon enrichment of the ferrite lamellae. Dislocations are stored in the interior of the ferrite lamellae and their density increases to about 2 × 10¹⁶ m⁻². A high dislocation density is also observed at the ferrite/cementite interface. Three strengthening mechanisms have been analyzed: (i) boundary strengthening, (ii) dislocation strengthening and (iii) solid solution hardening. The individual and combined contributions, based on an assumption of linear additivity, of these mechanisms to the wire strength have been estimated. Good agreement has been found between the estimated and the measured flow stresses, which is followed up by a discussion of structure and strengthening mechanisms with a view to extrapolation to larger strains, finer structures and larger stresses.

Highlights

? Interlamellar spacing and thickness of cementite lamellae decrease in accordance with change in diameter of drawn pearlitic steel wire up to strain of 2.5. ? At higher strains enhanced thinning of cementite lamellae points to decomposition of cementite and carbon enrichment of ferrite. ? Dislocations are stored in ferrite lamellae and their density increases to about 2 × 10¹⁶ m⁻² as determined by HRTEM. ? Contributions of boundary strengthening, dislocation strengthening and solid solution hardening are estimated and by addition. Good agreement is found between estimated and measured flow stress.

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Abstract

The evolution of the cementite phase during wet wire drawing of a pearlitic steel wire has been followed as a function of strain. Particular attention has been given to a quantitative characterization of changes in the alignment and in the dimensions of the cementite phase. Scanning electron microscope observations show that cementite plates become increasingly aligned with the wire axis as the drawing strain is increased. Measurements in the transmission electron microscope show that the cementite deforms plastically during wire drawing , with the average thickness of the cementite plates decreasing from 19 nm (¦Å = 0) to 2 nm (¦Å = 3.7) in correspondence with the reduction in wire diameter. The deformation of the cementite is strongly related to plastic deformation in the ferrite, with coarse slip steps, shear bands and cracks in the cementite plates/particles observed parallel to either {110}_¦Á or {112}_¦Á slip plane traces in the ferrite.

The structural and mechanical factors that control the torsion fracture behavior of cold-drawn eutectoid steel wires are examined. Two types of the fracture are identified; namely, flat- and cleavage-type. Torsion cracks are found to initiate in ferrite and propagate along the ferrite/cementite interface. The shear stress distribution within the wires is affected not only by the applied torque, but also by the residual stress. The maximum shear stress occurs halfway from center to the surface, where the cracks initiated. The growth of torsion cracks is sensitive to the orientation of cementite lamellas in pearlite grains. The influence of thermal history on the occurrence of cleavage fracture is ascertained, with the assistance of atom probe. It shows that the cleavage fracture results from a decrease in dislocation mobility, caused by thermally activated diffusion of carbon atoms into ferrite.

Abstract

To make clear the strengthening mechanism of heavily drawn steel wires exhibiting ultra-high strength, in situ neutron diffraction during tensile loading was performed. A ferrite steel (FK) subjected to a true strain of 6.6 and a pearlite steel (PS) subjected to 4.0 were extended on a tensile tester and (1 1 0) diffraction profiles were measured at various holding stresses. Tensile strengths of steel FK and PS are 1.7 and 3.7 GPa, respectively. The change in (1 1 0) spacing with tensile stress is reversible, i.e., elastic, close to the relevant tensile strength. A stress versus (1 1 0) lattice plane strain is linear for steel FK while evidently nonlinear at higher stresses for steel PS. In steel PS in which cementite peaks were hardly observed, the strengthening mechanism is postulated to be different from that for as-patented pearlite steels.