Issue 30

J. Toribio et alii, Frattura ed Integrità Strutturale, 30 (2014) 424-430; DOI: 10.3221/IGF-ESIS.30.51 424 Focussed on: Fracture and Structural Integrity related Issues Role of the microstructure on the mechanical properties of fully pearlitic eutectoid steels J. Toribio, B. González, J.C. Matos, F.J. Ayaso University of Salamanca, Spain toribio@usal.es A BSTRACT . This paper analyses how the microstructure of an eutectoid pearlitic steel affects its conventional mechanical properties (obtained by means of a standard tension test) and the associated micromechanisms of fracture. Results show how the yield strength, the ultimate tensile strength (UTS) and the ductility, increase as the continuous cooling rate rises, whereas on the other hand the strain at UTS decreases. The fracture surface exhibits more brittle features when the continuous cooling rate decreases, in such a manner that in the fracture process zone an increasing area appears where the pearlite lamellae can be detected and less regions of microvoids are observed. K EYWORDS . Pearlitic steel; Heat treatment; Ductility; Fracture surface; Micromechanisms of fracture. I NTRODUCTION n pearlitic steel, hardness and yield strength increase with decreasing interlamellar spacing following a Hall-Petch type relationship [1-11]. Resistance in pearlite is controlled by processes that occur in the ferrite, the main role of the cementite lamellae (at low stresses) being to limit the slip distance in the ferrite [11]. In contrast, the ductility depends on the prior austenite grain, so that it increases with the decreasing size of such characteristic parameter of the material [12-14]. The high work-hardening rate of pearlite is mainly attributed to the load transferred from the ferrite to the cementite, with a significant contribution due to the effect of the constraint of the hard phase on the ferrite [3]. Steels with thin pearlite and coarse pearlite show similar strain hardening coefficients [15]. The coarse pearlite is deformed in a non-homogeneous way, presenting localized plastic strain in narrow slip bands, while thin pearlite shows a much more uniform strain distribution [13, 15, 16]. In pearlitic steel, plastic strain induces compressive residual stresses in the ferrite and tensile stresses in the cementite, so that the residual stress level is higher in steel with greater interlamellar spacing [10]. Fracture in standard tensile test is determined by processes in the colonies having pearlite lamellae parallel to the tensile axis, where the deformation occurs in narrow bands of locally intense shear stress [11]. This paper studies the influence of pearlitic steel’s microstructure on conventional mechanical properties obtained with the standard tensile test. Moreover, an analysis of the different micro-mechanisms involved in fracture by simple tension of the steels has been carried out. I