Research On Low Temperature Properties Of Heavy Section Ductile Iron

Heavy section ductile iron casting used widely in manufacturing of spent fuel tank and use of wind power as a result of its excellent mechanical properties and low cost. The low temperature mechanical properties of heavy section ductile iron is particularly important because of its special use. The tensile properties, low temperature impact toughness and three point bending fracture toughness of heavy section fan-shaped block were studied. Fianlly draw some rules of heavy section ductile iron's low temperature mechanical properties.The matrix of heavy section ductile iron was almost all ferritic, just a few pearlite distributed in the center of the fan-shaped block. The average diameter and the shape factor peak of the graphite nodule on the edge of the sector were about 34.6μm and 0.9; the average diameter and the shape factor peak of the graphite nodule in the center were about 42.1μm and 0.8. The fracture surface formed from the crack at room temperature in basic body fluctuated strongly; the matrix between the graphite nodules formed the channel of the crack propagation. The impact ductility on the edge of the sector dropped from 17.7 J/cm2 at room temperature to 5.3 J/cm2 under -50℃at low temperature. The fracture way of the impact rupture on the edge of the sector was ductile fracture at room temperature, the ductile fracture torn edges could be found around the graphite nodule, with the temperature reduced gradually, the fracture way was changed from ductile fracture to ductile fracture +cleavage fracture. The center of sectors was affected by the big volume of the graphite nodule and some pearlite in the matrix, the impact rupture surface almost no changed from the room temperature to the low temperature, which was always cleavage fracture.Microscopic mechanism of the crack fracture was formed with the nucleation of micro voids,the growing up of voids,the aggregation of voids three phases at room temperature; Microscopic mechanism of the crack fracture was formed with the nucleation of micro cracks,micro cracks linking to a main crack,the expansion of a main crack three phases at low temperature. The secondary crack was found in the plane which was vertical to the rupture amount the impact ruptures at low temperature, these secondary cracks were initial topography of the original cracks, and they extended around the graphite nodule.The three-point bending fracture surface of the fan-shaped block for different positions show, the three-point bending fracture of the edge position at the low temperature of -40℃, -60℃, and -80℃, there are still tear edges of ductile fracture on the matrix aronud the graphite nodules; At the temperature of -60℃and -80℃, the three-point bending fracture of the center position is cleavage fracture completely, and the fractography is significantly different with the temperature of -40℃.The fracture toughness of edge position increase, with the decrease of experimental temperature and the same trend of tensile strength change of the fan-shaped block edge position;By the influence of factors, such as the volume of graphite nodules are too large, the fracture toughness of the fan-shaped block for center position increase, with the decrease of experimental temperature.