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Forging die failures for automotive components are caused by inadequacy of variables such as die materials, die design, die manufacturing and forging operations [1]. The forging die frequently fails from the corner where stress concentrate. Fig.1 shows a typical failure of hot forging die for a knuckle. From the macroscopic fracture surface observation it looks to be easily judged that a brittle failure initiated from the corner. These fractures are frequently observed on hot forging dies for knuckle, connecting rod, crank shaft and flange yoke for automotive components. However it can be clearly identified by low magnification observation that the most of the brittle fractures initiated from short cracks initiated from the corner due to fatigue and thermal fatigue. Fig. 2 shows the fracture surface of a flange yoke die made in SKT4 steel after 2000 forging operations [2]. It is clear that the brittle crack initiated at 3.5mm from the surface where nonmetallic inclusion MnS located. The stretched zone is most frequently observed at the transition zone from fatigue to impact fracture. In this case a stretched zone with 8 to 10μm width was observed between fatigue and impact fracture surface as shown in Fig. 3 [2]. The reason of this failure was determined to be an insufficient preheating temperature for the hot forging die by use of the relationship among dynamic fracture toughness, temperature and stretched zone width. The stretched zone can also be observed in failed cold forging die [3].It is possible to evaluate the fracture toughness of the failed cold forging die for automotive component by measuring the stretched zone width.

Fatigue and Fracture Behavior of Forging Die Steels