Study On Deformation Mechanisms And Recovery Of Cold Drawn Iron Wires

Owing to a desirable combination of ductility and strength,high strength pearlitic steel wires have been widely used in many reliability-critical engineering applications,such as suspension bridge cables,automotive tyre cords and cutting wires.After heavily cold drawn of pearlitic steel wire,the cementite decomposition and the work hardening rate is significantly decrease,which limits the strength of the steel wire to further increase.Compared with pearlitic steel wires,pure iron has much simpler composition and microstructure,and only comprising ferrite phase.A deep understanding of the deformation and thermal stability of the ferrite structure of heavily cold drawn iron wires can therefore shed light on the deformation and strengthening mechanisms in pearlitic wires.In this work,the microstructure evolution and mechanical properties of cold drawn pure iron wires were investigated,and the deformation and strengthening mechanism was analyzed and discussed.In addition,the effect of ferrite lamellar boundary migration on limits of refinement and mechanical properties were explored.Based on the above analysis,the thermal stability of cold drawn iron wires was studied,and the recovery kinetics and recrystallization kinetics of drawing strains were discussed and analyzed.The results show that the tensile strength rises from 380 MPa up to 1812 MPa when the drawing strain increases toε=10.35,which represents the highest strain and the highest tensile strength reported so far for pure iron wires.According to the microstructure evolution of cold drawn iron wires,the cold drawing process can be divided into three stage.（i）stage I withε≤1.3,the ferrite grains are elongated along the drawing direction,the dense dislocation walls form via dislocation activities and form a<110>texture.The work hardening rate is about 120.（ii）stage II with 1.3≤ε≤5.5,the ferrite grains continue to be refined,and the dislocation density increases with drawing strain increase.Therefore,the work hardening rate is significantly increased to 190.（iii）stage III withε≥5.5,grain refinement gradually slows down.The work hardening rate decreases gradually due to boundary migration of ferrite lamellae,and the work hardening approaches zero at drawing strainε=10.35.The dislocation movement is the main deformation mechanism.However,when the dranwing strain exceed 8.9,due to strain-induced ferrite lamellae coalescence by triple junction motion or h junction motion,which hinders the further refinement of ferrite lamellae,leading to the limits of ferrite grain refinement（～80 nm）,is the main factor that significantly reduces of the work hardening rate and the strength tends to be saturated.The yield stress of cold drawn iron wire is a linear function with the reciprocal of the square root of ferrite lamellar spacing,which conforms to the Hall-Petch relationship:σ=σ₀+k×d^-1/2.The K value is between 0.31～0.49 MPa·m^0.5,which is related to the drawing strain.The drawing strain has an important influence on the thermal stability of cold drawn iron wires.In the recovery process,the larger drawing strain,the faster the recovery rate and the more work hardening is eliminated.After the cold draw iron wires was annealed at 500℃for30 min underε=2.04,the work hardening was eliminated by 37.4%,and the elongation in increased from 3.9%to 13.9%.When the drawing strain increases toε=4.28,the work hardening was eliminated by 54.2%,and the elongation in increased from 2.1%to 12.1%.When the drawing strain further increased toε=8.89,the work hardening is further eliminated by 75.6%%,however,the elongation only increases from 1.2%to 1.4%.When the drawing strainε≤4.3,the nucleation of recrystallization preferentially in the region with higher storage energy,and the annealed microstructure is composed of recrystallized regions and coarsening deformed regions.Therefore,most of the work hardening is retained and good plasticity is restrored.The recovery mechanisns is dislocation climbing and dislocation rearrangement to form polygonal structures,then,the polygonal structure gradually from recrystallized nuclei.When the drawing strainε≥6.0,the ferrite lamellae uniform coarsening during heat treatment,and the annealed microstructure is composed of coarsening ferrite lamellae.Therefore,most of the work hardening is eliminated,but the plasticity is no obvious improved.The recovery mechanisns is the deformed lamellar structure gradually coarsens by triple junction motion,and the coarsening ferrite lamellae bouws out to nucleate,then,the the lamellar structure is gradually transition into a more equiaxed structure,in which recrystallization is initiated.