Effects of strain rate and matrix structure on transition of toughness of spheroidal graphite cast irons

Abstract

ensile, three point bend J c, and Charpy V-notch impact tests were carried out at various loading speeds and temperatures on ferritic, pearlitic–ferritic, and pearlitic spheroidal graphite cast irons. The 0·2% proof stress increased monotonically with decreasing temperature. Similar behaviour was observed for tensile strength, but the temperature range over which it occurred was limited. Both 0·2% proof stress and tensile strength were increased slightly by increasing the crosshead speed. The energy for fracture per unit volume in tensile tests was evaluated as the mean of the tensile strength plus proof stress multiplied by elongation. This energy reveals the ductile–brittle transition behaviour in the same manner as J c value or Charpy impact energy. An increase of loading speed shifted the transition curve of the energy for fracture to higher temperature, leaving the upper shelf value unchanged. An increase of pearlite volume fraction in the matrix also shifted the curves to higher temperature, decreased the upper shelf value, and reduced the slope of the transition curve. The transition temperatures of the energy for fractureJ c value, and Charpy impact energy were found to be linearly related to the logarithm of strain rate.MST/1343