The Interdependence of Plastic Behaviour and Final Properties of Steel Wire, Analysed by the Finite Element Method

University dissertation from Örebro : Örebro universitetsbibliotek

Abstract: The finite element method (FEM) has been used in order to study the interdependence of plastic behaviour and final properties in steel wire. Three processes metal forming have been studied: billet forging, wire rod rolling, and cold drawing. The forging study focuses on the possibilities to eliminate voids, pores and pipes by altering the bite ratio in open die forging. Forging is used in wire manufacturing when dealing with materials that are difficult or impossible to roll, e.g. tool steels like high-speed steels, and powder metallurgically manufactured materials like tungsten-molybdenum filament wire. The elimination of inner defects by forging were successfully simulated by the FEM. The rolling study focuses on the problems due of the extreme finish rolling velocities in modern wire rod mills and modern technologies such as thermo-mechanical rolling and sizing. A design for a high-speed laboratory mill for studies of material behaviour at extreme strain rates was made. With a combination of two-high stands and three-roll units is it possible to obtain a smoother temperature distribution in the bar cross section. Two studies of the cold drawing process were made. The first describes the distribution of the plastic deformation due to the die geometry. It was found that the influence of the die angle on the amount of inhomogeneous deformation decreases with increasing area reduction. The frictional behaviour was also studied. The second study describes how the bearing geometry affects the residual stress-state in the wire. It was found that a small taper could change the residual stress state in the surface immensely. This was shown with FE simulations and verified by X-ray diffraction measurements. The results in this paper gives a tool for control of the residual stress-state in the wire. This gives a possibility to increase e.g. the fatigue strength in the final product.

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