Formation Mechanism of Near‐Surface Defects in Crankshaft Induced by Chain‐Like Al ₂ O ₃ Inclusions

This study identifies and traces defects in the crankshaft that failed magnetic particle testing, calculates cavity bridge force and van der Waals force between Al 2 O 3 and MgAl 2 O 4 inclusions in molten steel, and ultimately illuminates the formation mechanism of chain‐like inclusions near the crankshaft surface. Magnetic powder defects are concentrated in the near‐surface region of the crankshaft, with industrial computed tomography imaging confirming that these chain‐like inclusions are the direct cause of surface defects. Upon dissection of the sample, energy‐dispersive X‐ray spectroscopy analysis reveals that the large inclusions, measuring in millimeters, are oxides formed by the aggregation and extension of numerous smaller inclusions, primarily Al 2 O 3 inclusions, with some MgAl 2 O 4 inclusions present. The original rod samples also exhibit a significant presence of large Al 2 O 3 inclusions near the edges. By integrating calculations and simulations, the formation mechanism of the inclusions at the defect site is elucidated. During the continuous casting process, large clusters of inclusions primarily form through cavity bridge force between particles, which are subsequently captured by the billet. During the rolling process, these clustered Al 2 O 3 inclusions transform into chain‐like formations aligned with the rolling direction. Following forging, they are further fragmented and exposed on the crankshaft surface during final processing.