Influence of aging on microstructure and toughness of die casting die steels

This study investigates the modification of the carbide type, size, shape and the associated changes in the toughness properties after prolonged aging at temperatures close to that of maximum hardening during tempering treatment. The aging effects on the microstructure of both steels still consist of a tempered martensite matrix, with an increased amount of precipitates and more market prior austenitic grain boundaries. For H13 steel, the aging effects on fracture toughness show an appreciable increase in up to 50 hours’ treatment; raising the aging time to 250 hours causes considerable embrittlement. For H11 steel however, the fracture toughness decreases early, from the first stages of aging; this behaviour has been associated to an increase in the carbide size. At increasing aging time, also for H11 steel, the embrittlement of prior austenite grain boundaries prevails in controlling fracture initiation and in decreasing fracture toughness.