PSI - Issue 28

Jesús Toribio et al. / Procedia Structural Integrity 28 (2020) 2378–2381 Jesús Toribio et al. / Procedia Structural Integrity 00 (2020) 000–000

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Karlsson and Lindén (1975) obtained that yield stress depending on the average free sliding distance in the ferrite, according to the Hall-Petch equation, cf. Hall (1951) and Petch (1953). Rawal and Gurland (1977) showed that the fracture toughness decreases with the cementite volume fraction and the cementite particles size, while Kim and Kim (2000) concluded that the Paris curve does not change with spheroidization. In the spheroidized microstructure the fatigue crack is confined in the ferritic matrix except for high carbon content or high deformation ranges, where carbides cracking appears, as described by Milligan (1974). In relation to the nucleation of ductile cracks, Rosenfield et al. (1972) and Park and Thompson (1988) have obtained that microvoids begin in the larger cementite particles close to the grain boundaries by decohesion with the matrix or by breakage. Park and Bernstein (1979) have obtained that the mechanism of brittle fracture occurs through certain crystallographic planes in the form of cleavage facets whose size is linked to that of the prior austenitic grain (PAG). 2. Equations The material studied was a steel of slightly hypereutectoid composition (0.789% C, 0.681% Mn, 0.210% Si, 0.010% P, 0.218% Cr, 0.061% V) and pearlitic microstructure, subjected to an isothermal spheroidization treatments consisting of keeping the steel at 700 ºC during a time of 50 h followed by cooling inside the furnace. Spheroidization produces fragmentation of the cementite lamellae, so the microstructure is formed by cementite globules inside the ferritic matrix and larger globules of carbides precipitated in the thin layer of proeutectoid cementite. The mechanical properties were obtained through the standard tension test: Young modulus E = 203 GPa, yield strength σ Y = 0.43 GPa, ultimate tensile strength (UTS) σ R = 0.70 GPa and strain at maximum load ε R = 0.14. The specimens were bars of 250 mm in length and 11 mm in diameter. The fatigue tests consisted of applying a sinusoidal axial tensile load with a constant stress range Δ σ , frequency of 10 Hz and factor R ~ 0. The fracture tests were carried out in samples precracked by fatigue subjected at a displacement rate of 2 mm/min. A fractographic and fractometalographic study was carried out. To this end, the fracture surface and the crack profile were observed by scanning electron microscopy (SEM). In the micrographs, crack propagation always occurred from left to right. 3. Experimental results 3.1. Fatigue cracking Macroscopically, the fatigue fracture surface is flat and is contained in the cross section of the wire that is perpendicular to the direction of application of the load (mode I). However, at the microscopic level it shows micro tearing events (Fig. 1a), produced by accumulation of very localized plastic deformation, which cause superficial micro-roughness. In some regions it was observed a kind of steps that allow saving small heights (Fig. 1b).

(a) (b) Fig. 1. Fatigue surface: (a) microplastic tearing events; (b) region with small changes in height.