Optimisation of post-drawing treatments by means of neutron diffraction

Abstract

The mechanical properties and the durability of cold-drawn eutectoid wires (especially in aggressive environments) are influenced by the residual stresses generated during the drawing process. Steelmakers have devised procedures (thermomechanical treatments after drawing) attempting to relieve them in order to improve wire performance. In this work neutron diffraction measurements have been used to ascertain the role of temperature and applied force -during post-drawing treatments -on the residual stresses of five rod batches with different treatments. The results show that conventional thermomechanical treatments are successful in relieving the residual stresses created by cold-drawing, although these procedures can be improved by changing the temperature or the stretching force. Knowledge of the residual stress profiles after these changes is a useful tool to improve the thermomechanical treatments instead of the empirical procedures used currently.

The Role of Residual Stresses in the Performance and Durability of Prestressing Steel Wires

“…It was found that the lowest relaxation values corresponded to samples treated at the highest temperatures (around 400°C) and loaded to the highest stresses (around half the tensile strength) [47]. In fact, these figures agreed quite remarkably with those provided by the authors in [32] for the best stress relieving postdrawing treatments. Although it seems clear that existing residual stresses will modify the actual stress state in the wire when it is loaded in service, there was no clue about their effect on stress relaxation when the authors started this investigation [48].…”

“…stretching at less than half the rupture load) or lower temperature (below 400°C) were not particularly effective in reducing the residual stresses. However, when the temperature (up to 460°C), or especially the force (up to 0.64 times the rupture load) were increased, all residual stress components became almost constant and slightly compressive in the ferrite phase [32],…”

“…Once the experimental and numerical techniques were demonstrated to work, the following step was to apply these techniques to commercial steel wires. Residual stress measurements were carried out by neutron diffraction in prestressing wires with differing post-drawing treatments [26,32]. In such a way, the influence of temperature and tensile stress-during post-drawing treatments-on the resulting residual stress state was ascertained.…”

“…Several cuts were carried out by electro-discharge machining which left isolated parallelepipeds in the wire sample. The unstressed lattice spacing employed was an average of the results corresponding to the different parallelepipeds [32]. Again, this is not a crucial issue if what is sought is comparison of the effectiveness of different treatments of the same material.…”

“…By measuring residual stress it was possible to assess the effect of the different post-drawing treatments, and hence optimize the process parameters [26,32], Although neutron and synchrotron sources are highly powerful (because they can provide information on the triaxial residual stress state in the bulk of materials), the access to them is limited. However, commercial X-ray diffractometers are available in many laboratories.…”