The formation of voids and the ductile fracture behavior in tension of low carbon steels and decarburized steels were studied using light and electron microscopy. Many microcracks which were associated with inclusions or carbides were observed on the sections of specimens prior to ductile fracture, but these changed into voids by repetition of etching and polishing. Sections of specimens containing central fissure seem to show the ductile fracture preceded by the propagation of “microcracks”. But after the repeated etching and polishing, these “microcracks” changed into void coalescence. When the substance between microcracks in decarburized specimen was thrusted with a pin, it fell into a void which had been concealed under it.
It is reasonable to consider from the above results that “microcracks” observed on the section of a specimen in the process of ductile fracture are not real cracks but gaps between the matrix and the lid which covered void during polishing. Then we conclude that the mechanism of ductile fracture of a low carbon steel is a process of void coalescence.
In fractograph, the dimple pattern of coarse grained decarburized steel was coarser than that of the fine grained specimen. It appears that the grain boundaries play an important role in expanding small voids during the process of void coalescence.