Anisotropy of the charpy impact value of carbon steels and corrective heat treatment

Abstract

The anisotropy of ductility, toughness, etc., of rolled steels has a great effect on the forming limit and on formability in forging or press forming. In this paper, how the anisotropy can be reduced by different heat treatments has been explored, making use of an instrumented charpy impact machine. It has been found that the presence of elongated manganese sulphide (MnS) in spherodised steels with more than 0.017% sulphur has a great effect on the anisotropy. Though the elongated MnS was modified to globular by homogenising, the array of inclusions remained and the anisotropy of the steels could not be completely removed: however, by homogenising, the anisotropy of steels with less than 0.009% sulphur could be almost completely eliminated. In the case of a specimen orientation of 22.5° (with respect to the rolling direction) for as-received and spherodising steels with more than 0.017% sulphur, the absorbed energy became remarkably high — since ductile cracks are easily initiated along the original longitudinal direction — but this did not appear for steels with large amounts of pearlite because of the effect of the latter in reducing anisotropy. It was established that the anisotropy arises chiefly from inclusions and is also influenced by pearlite, and it was found that heat treatments are available to eliminate the anisotropy.