Strengthening iron wires through gradient grain structure

Abstract

Pure iron wires were obtained by a two-step process involving torsion and cold drawing. A steep change in grain size through the cross-section of wires was produced during the torsion stage. The Vickers hardness increased from 193 HV to 252 HV when measured from the inner to the outer layer. The microstructure of the outer region underwent more severe plastic deformation than the interior. The grain structure in the central region did not show significant change, while a large number of sub-grains were formed exhibiting an approximately equiaxed shape toward the wire surface. Under subsequent cold drawing, the grain gradient remained. The Vickers hardness increased from 220 HV to 274 HV when determined from the inner region to the surface. The grains in the interior formed a fibrous structure, while fine equiaxed grains were developed toward the surface with an average grain size of about 0.5 μm. The formation of small equiaxed grains at the near surface region is believed to originate from continuous dynamic recrystallization (CDRX). Moreover, a gradual change from low angle into high angle grain boundaries (HAGBs) was observed, facilitated by the gliding and clustering of geometrically necessary dislocations (GNDs). Consequently, the application of the two-step technique enhanced the tensile strength and elongation to failure of pure iron wires from 607 MPa to 808 MPa and 6.7% to 11.5%, respectively. In doing so, the pure iron wires achieved a desirable combination of high strength and ductility.