PSI - Issue 28

Jesús Toribio et al. / Procedia Structural Integrity 28 (2020) 2396–2403 Jesús Toribio et al. / Procedia Structural Integrity 00 (2020) 000–000

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In the case of ferritic-pearlitic steels in which the local areas of pearlite are uniformly distributed in the global ferrite, the fatigue crack path is more tortuous than in those with isolated pearlitic areas surrounded by ferrite, with larger angle deflections appearing during crack advance (Korda et al., 2006a). With regard to banded ferritic-pearlitic steels, the bands of pearlite (oriented in preferential directions) promote a decrease of the fatigue crack growth rate, since they produce a more tortuous crack path, with more frequent and more angled deflections and branchings during fatigue crack growth (Korda et al., 2006b). Eutectoid steels with fully pearlitic microstructures show evidence that the crack advance tends to break the ferrite/cementite lamellae (Toribio et al., 2009, 2014, 2015) In this case the kind of fatigue fracture surface can be classified as transcollonial. Such a tortuous propagation path frequently produces crack interlocking and the crack branching reduces the local crack tip driving forces for fatigue propagation. With regard to fully pearlitic steels with oriented pearlite microstructure as a consequence of heavy cold drawing during the manufacturing process, the orientation of ferrite/cementite lamellae slows down the fatigue crack growth rate (Toribio and Toledano, 2000; Toribio et al., 2009, 2014, 2015) The reason for this particular behavior is the fact that cementite lamellae behave as serious obstacles for dislocation movement and therefore for crack advance. In the present paper a materials science relationship is proposed between material microstructural arrangement (specially ferrite/cementite lamellar orientation) in cold-drawn pearlitic steel after cold drawing, tortuosity of the fatigue crack path, real fatigue crack increment and macroscopic cyclic crack growth rate on the basis of a Paris law approach to fatigue crack growth. 2. Materials and microstructure The materials used were two pearlitic steels with identical eutectoid chemical composition shown in Table 1. Table 1. Chemical composition of the two eutectoid pearlitic steels. % C % Mn % Si % P % S % Al % Cr % V 0.789 0.681 0.210 0.010 0.008 0.003 0.218 0.061 It was studied in two forms: firstly, as a hot rolled bar (non cold drawn at all) and, secondly, as a commercial prestressing steel wire which has undergone seven cold drawing steps up to reaching a cumulative plastic strain ε P =1.57 and a posterior stress-relieving treatment to eliminate, or at least diminish, residual stresses. Steel was supplied in form of wires with circular section, the diameter ranging respectively between 11 and 5 mm for the hot rolled bar and the prestressing steel wire. With regard to the influence of the manufacturing process on the mechanica properties of the pearlitic steel under consideration, the cold drawing process activates in the material a strain hardening mechanism, so that it produces a clear improvement of conventional mechanical properties (Table 2) obtained from a standard tension test: both the yield strength ( σ Y ) and the ultimate tensile strength (UTS, σ R ) increase with cold drawing, while the Young’s modulus ( E ) remains constant and the strain at UTS ( ε R ) decreases with it.

Table 2. Mechanical properties of the material in both conditions, i.e. as a hot rolled bar and as a prestressing steel (cold drawn) wire Steel E (GPa) σ Y (MPa) σ R (MPa)

ε R

Hot rolled bar

202 209

700 1480

1220 1820

0.078 0.060

Prestressing steel wire

The afore-said improvement of classical mechanical properties is the final aim of manufacturing by cold drawing. However, the process could affect the fatigue behavior at the macro- and micro-levels, the analysis of this phenomenon being the final aim of the present paper.