The effect of graphite shape on the tensile deformation behaviour in hot-swaged and hot-rolled ferritic spheroidal graphite cast iron

The shape of the graphite nodules in ferritic spheroidal graphite (SG) cast iron is changed by hot-swaging and hot-rolling, when symmetric ellipsoidal graphite and asymmetric ellipsoidal graphite can be obtained, respectively. For the symmetric ellipsoidal graphite, the graphite shape ratio tends to decrease with increasing reduction ratio. The increase of flow stress with increasing reduction ratio can be explained by the decrease in the stress concentration induced by the graphite with increasing reduction ratio. On the other hand, the graphite shape ratio is more complex for the asymmetric ellipsoidal graphite than the symmetric ellipsoidal graphite specimens. The graphite shape ratio tends to increase and decrease with increasing reduction ratio in two transverse directions. As a result, a balance exists in the specimen between increasing and decreasing stress concentration, showing that the tensile flow stress is independent of reduction ratio.

To understand the effect of the graphite shape ratio on the tensile flow stress during tensile deformation, specimens with different carbon contents and graphite shape ratios were used in this study. Furthermore, the triaxial parameter (k₁) has been defined such that the larger stress concentration effect will have the larger k₁ value. Based on the present experimental results, the graphite shape ratio tends to decrease with increasing reduction ratio or tensile strain, so that the stress concentration effect and triaxial parameter (K₁) tend to decrease with increasing reduction ratio and tensile strain. Therefore, the effect of the graphite nodules in ferritic SG cast iron on the tensile flow stress is not only reduction of the effective area fraction of the matrix but also development of an uneven triaxial stress field.