On the Ductile Fracture Surface of Low Alloy Steels

Metallurgical structures and ductile fracture surfaces of spheroidized carbon steels and quenched and tempered steels were investigated and following results were obtained.1) The fibrous rupture surfaces of tensile specimens of steels used were, micro-fractographically, consisted of dimples. Among them, in the center of rupture surface, they were “equi-axed” dimple.2) The dimple size was found to distribute as “logarithmically normal distribution” and in the large dimple, second phase particle or its trace was observed. On the other hand, in the small dimple any particle or its trace was not observed. So, in this paper, we call temporarily the former “primary dimple” and the latter “secondary dimple”3) The depth of dimple was observed to be about 1.0 or 1.5 times the diameter of dimple, but it is more reasonable to suggest dimple to be funnel shape, so the ratio of the depth of dimple to the diameter will be larger than the value mentioned above.4) A close correlation between the dimple size and the mean free ferrite path of spheroidized carbon steel was found, but any relation between some kinds of metallurgical factors of quenched and tempered steel and dimple size was not observed.5) On the basis of the existence of primary dimple and secondary dimple, a ductile rupture model was presented that primary dimple corresponds to the void nucleated around a second particle before rupture and secondary dimple will be formed when unstable fracture occurs between neighboring primary dimples at final rupture.