Issue 51

D. Fernandino et alii, Frattura ed Integrità Strutturale, 51 (2020) 477-485; DOI: 10.3221/IGF-ESIS.51.36 479 Once the advance of the austenite precipitation within the intercritical temperature range was characterized, this information was used to design the heat treatment cycle aimed at producing a microstructure consisting of 60% ferrite and 40% ausferrite. The heat treatment consisted of a partial austenitization of the ferritic samples, by holding them into the furnace within the intercritical interval at the selected temperature for one hour, followed by an austempering step in a salt bath at 350°C for 90 min. Tensile test sample A very small-size specimens were machined from the calibrated zone of one-inch Y block. Then, samples were ground to 1 mm in thickness and polished using standard procedures. The dimensions of the small tensile specimen are shown in Fig. 1. Figure 1: Dimensions of the small-size tensile test specimen. Step by step damage evolution The small-size tensile samples were tested by using a tensile testing device specially designed to perform a step by step analysis (Fig. 2). The tensile device includes a Linear Variable Differential Transformer (LVDT) and two miniature load cells (10 kN each). The device is prepared to perform a step by step loading, keeping the load constant within steps by means of the tensile holder as is shown in the magnification of Fig. 2. The samples are then loaded to some specific value and then examined at microstructural level using both Light Optical Microscopy and SEM. The microscopic observations were conducted “in-situ” by using a digital microscope Leica VZ105C equipped with a digital microscope camera Leica DVM2000 and a SEM Philips 505. Figure 2 : Schematization of the tensile test device designed to perform step by step tensile tests. The magnification area shows tensile holder details. R ESULTS Material characterization and intercritical temperature interval amples of the same melt used in this study were also used by Fernandino and Boeri [14]. The chemical composition is listed in Tab. 1. After the pre-conditioning annealing, a fully ferritic metallic matrix was achieved. The microstructures obtained after heating the samples at three different temperatures within the intercritical interval and quenched in water are shown in Fig. 3. The metallographies show ferrite (white), graphite (black), and martensite (gray). The martensite is formed after quenching of the austenite present at the austenitization temperature; therefore, its amount S