Improved cold-drawn eutectoid steel wires based on residual stress measurement and simulation: Part 2. optimization of mechanical properties

Abstract

Cold-drawn eutectoid steel wires are used in prestressed concrete structures to provide compressive stresses to the concrete. For that purpose, they are loaded up to 60 to 70% of their tensile strength. Although the loading stress is lower than the elastic limit (around 85% of the tensile strength), failure may occur in service conditions due to stress corrosion. Wire failure reduces the load bearing capacity and may lead to catastrophic collapse of the prestressed structure. The risk of these failures can be increased by the presence of tensile residual stresses on the surface of the wires. Cold-drawing generates considerable residual stresses which, added to the service stresses, may seriously affect the mechanical properties and durability of the wires [1,2]. Until now, the measuring of residual stresses was considered more a scientific problem and even revealing their presence was a challenging task [3]. However, wire manufacturers were aware of the deleterious effect produced by the presence of tensile residual stresses at the surface of the wires after drawing. That is the reason why they consider residual stresses both dangerous and damaging, and hence attempt to reduce their influence by stress relieving treatments. But residual stresses may also have beneficial effects if we are able to obtain the desired profile. With the advent of powerful experimental techniques for measurement of residual stresses — such as neutron and X-ray diffractometers — and of faster computers to simulate numerically the wire drawing processes, this phenomenon is seen now in a new light. A significant research effort has been undertaken in recent years in order to understand, measure and control the residual stresses in cold-drawn wires [3-5]. In the first part of this paper [3], the advances on the measurement and simulation of residual stresses have been discussed. In this second part, the influence of residual stresses on the mechanical properties required by standards to this kind of wire (tensile, stress relaxation and stress corrosion tests) is reviewed. The control of residual stresses may play an important role in the design of new post-drawing treatments and the optimization of the performance of the wires.