Mechanical and Fatigue Properties of Heavy Section Solution Strengthened Ferritic Ductile Iron Castings

Abstract

Recently, standardized solution strengthened ferritic ductile irons have met the interest of the industrial world since their improved mechanical properties and workability compared to standard ferritic-pearlitic ductile irons. For this reason, mechanical and fatigue properties of heavy section solution strengthened ferritic ductile irons castings are examined. Mechanical tests are carried out on specimens taken from the center of castings with long solidification times. Metallographic analyses are performed by using optical microscopy. Fracture surfaces of fatigue specimens are investigated in order to evaluate crack initiation and propagation zones. Finally, fatigue curves of the analyzed alloy are compared with those obtained from pearlitic ductile iron with the same casting geometry.

Novel method for the fatigue strength assessment of heavy sections made by ductile cast iron in presence of solidification defects

“…In particular, the microstructure of the GJS 400-18 LT (casting F-F) is characterized by spheroidal graphite nodules within a ferritic matrix, with areas of pearlite due to the segregation of undesired carbide promoting element present in the alloy. 32,34 Tensile test results confirmed, as reported in Borsato et al, 12 that ferritic ductile irons are less affected by the wall thickness and cooling rate, compared with pearlitic ductile irons. Finally, the solution strengthened ductile iron (S-I) shows a fully ferritic matrix with limited traces of pearlite.…”

“…It is also important to note that, due to the presence of chunky graphite, the ultimate tensile strength and more markedly, the elongation at failure, showed a considerable reduction compared with chunky-free castings. 32,34 Tensile test results confirmed, as reported in Borsato et al, 12 that ferritic ductile irons are less affected by the wall thickness and cooling rate, compared with pearlitic ductile irons.…”

“…Heavy section castings with various solidification times have been analysed. In particular, experimental data have been taken from both authors' works in literature 7,12,32,[34][35][36] and a new set of experimental measurements. Castings were Y-shape cast samples with thickness of 50 and 75 mm (geometry according to UNI EN 1563), blocks 300 × 250 × 300 mm 3 with feeders on the top surfaces, cylinders (diameter, 300 mm; height, 520 mm), a cube 600 × 600 × 600 mm 3 and a cast cylinder with diameter equal to 650 mm.…”

“…Similar conclusions have been achieved when solution strengthened ferritic ductile cast irons have been investigated. 34 In other works, 35,36 it was observed that microshrinkage cavities, near the surface of the specimens, were the cause of the fatigue failure of heavy section pearlitic ductile iron castings. It was also shown that a combined effect between microshrinkage and degenerated graphite (spiky) could exist when they are both present in the alloy.…”

“…In many works found in literature, [8][9][10][11][12] various types of solidification defects have been studied, in order to evaluate their influence on the static and fatigue resistance of the components. The ultimate tensile strength and the elongation at failure are lowered by solidification defects, while hardness and yield stress are less affected.…”