Brittle Fracture of Hot-Forging Cast Dies

The site applications of cast hot-forging dies made from a new type cast steel were performed and several of the dies experienced brittle fracture. Analysis of the operating modes and evaluation of mechanical properties were used to establish relations among failure patterns and lives of dies. The results show that the dies experience elevated-temperature wear, plastic deformation and thermal fatigue and that brittle fracture was the predominant failure pattern. Brittle fracture of cast dies has two patterns: early brittle fracture and impact fatigue fracture. The early brittle fractures seldom occur but, when this failure mode occurs, the failure severely shortens the life of the cast dies. The early brittle fracture is attributed to abnormal heat-treatment and operation. Impact fatigue fracture is the dominant failure pattern in the cast dies. This failure mode usually initiates from sand inclusions, micro-cracks and/or thermal fatigue cracks. The specifications of the die material should include a requirement for impact toughness to assure that the dies are resistant to multiple impact loads. This means that strength-dominant toughness must be controlled and optimization of the heat-treatment processes must focus on this requirement.