Issue 42

A. Strafella et alii, Frattura ed Integrità Strutturale, 42 (2017) 352-365; DOI: 10.3221/IGF-ESIS.42.36 354 Figure 1 : Creep specimen dimensions. Creep tests The creep tests were carried out in air and in stagnant lead, at 550°C that is typical working temperature of LFR. The loads were applied in uniaxial tensile mode; stress values were in the range of 300-560 MPa, which is lower than yield stress value (theoretical 620 MPa ≤ Rp0.002 ≤ 840 MPa, after 20% CW). All tests were performed by MAYES machine with a maximum loading capacity of 20kN and maximum temperature of 1000°C. Because of the measurement uncertainty in the determination of the specimen cross section, the initial tensile stress was measured with an accuracy of ±3%. The specimen elongation during the tests was measured by monitoring in continuous the grip movement; Super Linear Variable Capacitor (SLVC) transducers were used for measuring and recording elongations smaller than 0.5 μm. The acquired data were plotted as conventional strain versus time curves. The matrix of main tests is shown in Tab. 2. Table 2: Matrix of the main tests. where “  ” represents the creep test performed in air and “  ” that in lead. A particular cell, showed in Fig. 2, was specifically designed and manufactured for tests in lead. For the tests in lead, the creep machine is equipped with a specifically designed and manufactured cell, for the containment of the molten liquid metal, as shown in Fig. 2 and 3. The cell consists of a lead container to fully immerse the specimen during test, low and upper grips where the specimen is mounted to be subjected to tensile stress and rod grips for the SLVC to measure the sample elongation during the test. The whole testing cell apparatus is then put into the furnace. The cell is mounted on a MAYES machine; specimen was fixed to the grips of cell, filled with solid lead, heated up to the lead melting temperature and then the test temperature and load were applied. Data points were acquired every 1 h. Morphological and microstructural characterization The steel, both raw and after rupture, was analyzed by Optical Microscope and Scanning Electron Microscope (SEM). The specimen surfaces were observed both before and after boiling glyceregia etching (30 mL HCI, 20 mL glycerol, 10 mL HNO 3 ). A Reichert-Jung MeF 3 optical microscope was used to observe the microstructure, while a SEM Leo 438 VP to characterize in high vacuum the morphology and microstructure of the specimens surfaces. Material Temp. [°C] Stress [MPa] 15-15Ti(Si) 550°C 300 400 500 560 575    ,   ,  