Study On The Microstructure Evolution Of Ferrite And Cementite During Heavily Cold Drawing Of Pearlite Steel Wire

This paper systematically investigate the evolution of microstructure and mic-texture during the cold-drawing of pearlite steel wire by means of hardness testing, scanning electron microscope(SEM) transmission electron microscopy(TEM)and three-dimensional atom probe(3DAP). Moreovre the dissolution mechanism of cementite during heavily cold-drawing have been discussed. The insvetigation has yielded following results :The tensile strength increased of three different carbon content of steel(65 #,72 A and 82B) with the increase of drawing strain, the microstructure turns to the drawing axis; At the same strain, the higher carbon content of the steel, the greater tensile strength; The pearlitic steel(65 #) gradually formed crack due to the uneven distribution of the ferrite around pearlite colonies during the cold-drawing.The evolution of microstructure during the cold-drawing of the carbon content of0.768% of eutectoid pearlite steel shows the following results:(1) The tensile strength of pearlite steel increased with the increase of drawing strain, the microstructure turns to the drawing axis and lamellar spacing decreases during the cold drawing and the pearlitic lamellar was observed refined. The dislocation density in ferrite increases with drawing strain and cementite lamellar was observed plastic deformation;Finally,the formation of lamellar curled in a cross section and parallel to the drawing direction of the lamellar tissue in longitudinal section(2) The bending and torsion of cementite during drawing, the interface steps could be induced by the shearing of tensile stress and shear stress along the inclined directions, the dislocations which nucleated at the cementite/ferrite interfaces penetrate from the ferrite into the cementite. Moreover, the orientation of cementite changed. 3DAP testing show that partial dissolution of cementite can be observed during cold deformation, the dissolution of carbon atoms mainly nucleated at the cementite/ferrite interfaces. It is interesting that there exists a microstructural threshold of cementite dissolution,cementite lamellae thickness is a critical parameters of cementite decompostion.(3)The ferrite produced parallel to the drawing direction(110) fiber texture, and ferrite(110) fiber texture enhancement with the increased strain of cold drawn steel wire,other texture components weakened; close to the ferrite core wire portion(110)fiber texture enhancement magnitude greater than other regions during deformation,the ferrite grain boundaries mainly small-angle grain boundaries and the distribution ratio based first increases and then decreases.The dissolution mechanism of cementite: cementite lamellae was observed refined during cold-drawing,which enhances the energy of the particles through the Gibbs–Thomson effect so that they finally dissolve. And dislocation penetrate from ferrite into cementite and transport of carbon from cementite into ferrite. Dislocation mechanisms and interfacial energy mechanisms were play an important role in cementite dissolution.