Delayed Fracture Behavior Of Ultrafine Grained High Strength Steel
超细晶粒高强度钢的延迟断裂行为

42CrMoVNb steel with ultrafine prior austenite grain, its size as fine as 2 m, was obtained through rapidly cycle heat treatment. The effect of ultrafine grain size on delayed fracture behavior is studied using sustained load tensile test with notched specimen. It is shown that both strength and delayed fracture resistance of 42CrMoVNb steel increase when its grain size is refined from 20 m to 4 m, but they do not increase when refined to 2 m. When the steel was tempered at high temperature, its fracture characteristic at crack initiation area changes from intergranular to transgranular when its grain size is refined from 20 m to 8---4 m, while the fracture characteristic is intergranular when its grain size is refined to 2 m. Its fracture characteristics are all intergranular fracture for all the grain sizes investigated when tempered at low temperature. The reason for this kind of delayed fracture behavior is discussed mainly from the stress concentration and segregation at grain boundary.