Failure analysis of a hardened and tempered structural steel (42CrMo4) bar for automotive applications

A round ∅ 78 mm structural steel bar of 42CrMo4 grade with an internal crack was brought with the request to pursue a failure analysis investigation. The crack was detected after ultrasonic inspection at the end-user plant. Visual inspection, optical and scanning electron microscopy were used as the principal analytical techniques in the present study. Further to fractographic interpretation (dendritic morphology found in failure area) and the inclusions detected in the interior of the crack surfaces, it can be concluded that the major crack in the steel bar was probably originated from casting. Plastic deformation processes during rolling resulted probably in the rotation of the crack while overload after subsequent heat treatment (quenching and tempering) may have caused secondary cleavage morphology cracks.