Abstract
Relevance: This article examines an important problem of the surface layer structure in fine cold-drawn wire and of how it forms as the strain changes with the changing diameter of the wire. The study was performed using electron microscopy. Objectives: This research aims to look at the surface layer microstructure in 5.5mm wire rod and 4.2-0.933 mm wire and define the patterns of the forming microstructure. The authors also aim to determine how the structure forming process can be controlled across the wire section to ensure the best combination of mechanical and performance properties. Methods Applied: The following materials were used for the purposes of the study: 5.5mm wire rod, 4.2mm wire and fine brass plated wire of the following diameters: 1.75mm; 1.73mm; 1.574mm; 1.325mm; 1.113mm; 0.933mm. All the materials are of the commercial steel grade 70. The study was conducted with the help of the transmission and scanning electron microscopy (TEM and SEM) equipment and a microhardness tester made in Japan. Findings: As a result of the study, a number of sub-surface areas were identified with a presumably turbulent structure, which can be attributed to the shear stresses present in those areas. Such shear stresses, together with the principal shear stresses, form an additional rotational deformation mode. The authors were able to determine the occurrence depth and the radius of such abnormal areas. The authors demonstrate that as the cold strain increases, both the material and the surface areas become harder. This trend is confirmed by how the microhardness changes depending on the degree of strain during a drawing operation. Thus, at lower strains an inconsistent microhardness distribution can be observed in the radial direction. However, as the strain increases, there is clearly observed a microhardness pattern with microhardness being the highest in the surface areas with the abnormal structure. This can be explained by the fact that when in fine wire drawing the strain rate tends to be higher at the surface than in the wire. As the strain rate rises, the hardness growth rate becomes more intense. The authors looked at how the dislocation structure changes under strain. The structural patterns of the pearlite colonies in high-carbon steel were identified. The knowledge gained can be used to determine maximum deformability of wire rod and wire in a drawing operation while also defining a combination of structural and qualitative characteristics of the fine wire.
Keywords
Cold strain, fine wire, shear strain, perturbed surface areas, strength, microhardness, dislocation structure, SEM, TEM.
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