Issue 30

J. Toribio et alii, Frattura ed Integrità Strutturale, 30 (2014) 424-430; DOI: 10.3221/IGF-ESIS.30.51 425 E XPERIMENTAL PROCEDURE he material used was steel with eutectoid composition and some alloying elements: 0.789% C, 0.681% Mn, 0.210% Si, 0.010% P, 0.218% Cr and 0.061% V. The steel, previously produced by hot rolling, was subjected to various heat treatments, consisting of heating the material in a furnace at 900°C (above the eutectoid temperature) for 1 hour and then cooling it in the closed furnace (PF); with the furnace partially open (PS); or outside the furnace, in the air, at room temperature (PA). The microstructure of the material was analyzed by metallographic microscopy and by scanning electron microscopy (SEM), after the metallographic preparation of small samples of steel by mounting, grinding and polishing (up to a mirror finish) and attack with Nital 4% (a mixture of 4 ml of nitric acid with 96 ml of ethanol) for several seconds to reveal the microstructure. Standard tensile tests (three for each material) were performed by using cylindrical specimens of 11 mm in diameter and 300 mm in length. The crosshead speed was 2 mm/min and an extensometer with a gage length of 50 mm was placed in the central part of the specimens. Finally, the fracture surfaces and longitudinal sections of the fractured specimens (after metallographic preparation and attack with Nital 4%) was observed through SEM. E XPERIMENTAL RESULTS Microstructural analysis sing optical microscopy and scanning electron microscopy, the microstructure of pearlitic steels obtained by various heat treatments (Figs. 1 and 2) was observed. The diameter of the pearlite colony d C was measured by the intercept method [17], which consists of counting the number of grains intercepted by a straight line drawn at random along the micrograph, considering the line’s randomness and the randomness of the metallographic cut by means of statistical methods. The average interlamellar spacing of pearlite s 0 was calculated by the method of the circular line [18], a variant to the linear intercept method, in which a circle of known length is drawn on the photograph, and the number of intersecting pearlite lamellae is counted (also taking into account the randomness). The transformation temperature, which in continuous cooling is determined by the cooling rate, controls interlamellar spacing. Decreasing the cooling rate (or increasing the cooling time) increases interlamellar spacing and the size of the pearlite colony (Tab. 1). Interlamellar spacing is independent of the prior austenite grain size and the size of the pearlite colony [4]. Mechanical properties From the load-displacement records, resulting from standard tension tests, the true stress-strain curves σ - ε characteristic of each material have been calculated (Fig. 3). In them the existence of a small plateau is observed, following the linear elastic region (with the same slope for all the curves), whose size increases with the cooling rate used in the heat treatment. The elasto-plastic region shows a quasi-parallel curve for the steels analyzed. (a) (b) Figure 1 : Optical micrographs: (a) air-cooled steel, (b) steel cooled inside the closed furnace. T U