Fatigue crack initiation in hardened medium carbon steel due to manganese sulphide inclusion clusters

Forged steel generally suffers from strong fatigue anisotropy. This anisotropy is more pronounced in standard sulphur (SS) steels (typically 0.04 wt percent) with a dense manganese sulphide (MnS) population as compared with low sulphur (LS) steels (down to 0.004 wt percent or less). Anisotropy due to MnS arises because of the flattened shape which sulphides obtain upon a forging operation. The higher grade of anisotropy in SS materials is attributed to the occurrence of sulphide clusters that are exclusive to these materials. It is shown that the prediction of fatigue limit with respect to conventionally recorded inclusion size is highly non-conservative when it concerns cluster-affected material, but only when the clusters are oriented perpendicular to the loading axis. Fatigue failures occurred at stresses much lower than expected. An interaction of the single sulphide within one cluster and an interaction of the clusters within the test volume are believed to cause the inferior fatigue performance.