An Investigation Of Nano-structured Pearlitic Steel Wire And The Three-body Abrasive-wear Mechanisms Of It's Composites

Microstnicture characteristics and evolutions of pearlitic steel wire were investigated under the conditions of Heavily Cold Drawn (HCD) loads by TEM and X-ray analysis approaches. The investigation involves: (1) the characteristics and formation mechanicals of nano-structure in pearlitic steel under HCD loads; (2) the evolutions of microstructure and capability of post-deformation steel wire caused by annealing; (3) strengthen mechanism of steel during HCD and post-deformation annealing. Besides, we also introduced the manufacturing and preparation technique of the nano-structured steel wire reinforced Polyurethane (PU) composite, and tested its wear-resistance property on our own-made three-body abrasive wear trial machine, and analyzed its wear features by scanning electron microscopy (SEM) approach, as well as its wear mechanism.Under HCD deformation, ferrite lamellae in pearlitic steel (leads patenting of 0.72%C steel) changes into ultra-fine and nano-sized grains. During the deformation, most of the slipping systems in ferrite crystals have been unlocked, and most slipping result in ultra-fine grains. With plasticity deformation increase, ferrite crystals' deformation and aberration lead to a lot of submicron grains' forming; and diffraction pattern of these unlocked slip planes changes into rings gradually, which shows that a majority of small angle boundaries of ferrite crystals change into big angle boundaries. The slippery or shear-susceptible dislocations whose Burgers vector are parallel to crystal boundary increase with plastic deformation and result in the rotation of crystals to form boundaries with large angles.It can be observed that a majority of cemmentite lamellae in pearlite have been transformed into nano-crystals under the HCD deformation. During the drawing, plastic deformation(such as bending), thinning, necking, and fracture of cemmentite plates occur by tensile strain. The shearing and slipping of cemmentite is formed by the tensile and shear stress, therefore small slipping stages of cemmentite surface appear. The cemmentite thinning and formation of small slipping stages will increase its inter-surface area per volume, which lead to and enhance cemmentite decomposition due to thinning of cemmentite and its unstabilization through the Gibbs-Thommson effect, or because of the increase of dislocation density and the contact area between cemmentite and dislocations which are located mainly at the interface. Another interpretation of dissolution of cemmentite is that the combining enthalpy of carbon and dislocation in ferrite exceed the solution heat of cemmentite in ferrite phase.After a 200° C annealing, it shows that the size of ferrite lamellae , as well as themicroscope internal strain inside these lamellae, is approximately constant. Numerousnano-sized cemmentite globules are precipitated between adjacent ferrite lamellae. Upto a temperature of 300° C annealing, the number of cemmentite globules increaserapidly. It's observed that, at low temperature annealing, the equilibrium of cemmentitephase, as well as many defects are restored. After an annealing at 400 ° C, therecrystallisation of ferrite occurs, and the cemmentite globules become coarser . whenthe annealing temperature is up to 500° C, there are all equal-axed grains in thestructure. When annealing temperature is higher than 550° C, cemmentite globules arecombined and integrated into cemmentite lamellae. According to the tensile test, lowtemperature annealing leads to the increase of tensile stress and the decrease ofelongation of as-drawn steel wire, and the interactions between cemmentiteprecipitates and dislocations are thought to be responsible for the observed increase instrength. When the annealing temperature is up to 700° C, the ductility of steel wire isimproved remarkably, while the tensile strength decrease a lot.Heavily cold drawn pearlitic steel wire has very high strength and hardness, together with an acceptable level of plastic toughness. When the scale of microlite changes into nanosized grain,...