13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Role of Surface Defects in the Initiation of Fatigue Cracks in Pearlitic Steel Jesús Toribio 1,*, Juan-Carlos Matos2, Beatriz González1 1 Department of Materials Engineering, University of Salamanca, E.P.S. Zamora, Spain 2 Department of Computing Engineering, University of Salamanca, E.P.S. Zamora, Spain * Corresponding author: toribio@usal.es Abstract In this paper, tensile fatigue tests were performed under load control, with constant stress amplitude, on pearlitic steel wires with different cold drawing degree, from the hot rolled bar (not cold drawn at all) to the commercial prestressing steel wire (which has undergone seven cold drawing steps and a stress relieving treatment). Results show that fatigue cracks in pearlitic steels initiate at the wire surface starting from small defects, whose size decreases with the drawing process, as the cross sectional area of the wire does. Some of these defects appear during the drawing process itself. Fatigue cracks created from defects (initiation phase) exhibit a fractographic appearance consisting of ductile microtearing events which can be classified as tearing topography surface or TTS. Such microtearings are more planar in the initiation period than in the propagation phase and exhibit a spacing remarkably lower in the pretressing steel wire than in the hot rolled bar, so that their size decreases with cold drawing as the steel microstructure does. Keywords Pearlitic steel, Cold drawn steel wire, Initiation of fatigue cracks, Surface defects 1. Introduction Fatigue life of steel wires depends on their surface state and the existence of defects (such as microcracks, inclusions, etc.) present in the material. In wires made of eutectoid cold drawn steel, the fatigue process initiates in surface defects [1-3], broken martensite layers (due to a overheating during the wire drawing process), longitudinal groves and holes mainly caused by surface inclusions [1]. Many times surface defects are caused by the drawing process itself [4]. Surface defects present in the material before wire drawing can be eliminated by such a mechanical treatment. A scratch on the wire surface can be removed by repeated drawing; however, the flaw remains inside the wire because of the development of an overlap, and it is thus difficult to completely remove the flaw [5]. In corrosion-fatigue the aggressive environment can blunt the surface defects due to material dissolution, increasing the number of cycles required to initiate cracking [6]. The main cause of failure in steel wires is the presence of non-metallic inclusions during wire drawing or service [7]. The existence of non-metallic inclusions in cold drawn pearlitic steel affects its fatigue properties [8] by modifying the local stress state surrounding the inclusions, depending on the size, localization, composition and geometry of the inclusion [9]. This paper studies the defects able to initiate the fatigue phenomenon in pearlitic steel applied in two forms —i.e., as a hot rolled bar and as a commercial prestressing steel wire— by analyzing the effects of cold drawing on such defects and the microstructural arrangement and how these changes affect the fatigue performance of prestressing steel. 2. Experimental Procedure 2.1. Material The material used was eutectoid pearlitic steel (chemical composition 0.789% C, 0.681% Mn, 0.210% Si, 0.010% P, 0.218% Cr, 0.061% V). It was studied in two forms: firstly, as a hot rolled bar
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