The influence of graphite nodule diameter on Charpy impact test characteristics was studied for some spheroidal graphite cast irons with ferritic and pearlitic matrix. The graphite nodule diameter was varied by changing the cooling rate of the same molten metal.
The Charpy impact value at the ductile fracture region decreased with decreasing graphite nodule diameter in the case of ferritic matrix, but increased in the case of pearlitic matrix. However, the Charpy impact value at the brittle fracture region increased with decreasing graphite nodule diameter in both ferritic and pearlitic matrix, because graphite nodules and grain boundaries have the ability to halt the crack propagation. The transition temperature of Charpy impact test value in the case of ferritic matrix was considerably lower than that of the pearlitic matrix case, and in both cases the transition temperature became lower with decreasing graphite nodule diameter. These results are considered to be due to the multiple effects of crystal grain size and graphite nodule diameter.
