Microstructural evaluation and failure analysis of flow forming mandrels: Case studies

• Metallography reveals defects and structural flaws in cold flow forming mandrels. • Subsurface cracks in failed mandrels identified by SEM, XRD, and stress analysis. • Proper EDM reduces white layer formation and surface tensile stress in mandrels. • Inter-critical annealing improves fracture toughness of DC53 tool steel. This study presents comprehensive metallographic observations on defects and other structural insufficiencies, such as micro-crack development and their causes during cold flow forming of different steel parts. Flow forming is an ideal process for manufacturing parts made of high-strength materials. The process’s high forming forces can produce these parts within tight dimensional and thickness tolerances. This requires the highest standards in surface consistency of hardened flow forming tools to perform serial production of parts (within the range of thousands). This paper shows the importance of structural insufficiencies, particularly under surface area and their role in the failure of flow forming tooling. Tool life is presented for different mandrels made of DC53 cold work steel. All of them were used in the flow forming process. Failure case studies were based on the failed versus well-performing mandrel taken from the production line. SEM, XRD analyses, metallographic studies and residual stresses evaluation were performed. Importance of properly performed EDM to minimize the white (re-cast) layer formation and surface stress is outlined. The present work also points out the role of inter-critical annealing on the fracture toughness of steel DC53.