The key to the ductility of seamless steel tube lies in steel grades such as alloy tool steel and alloy structural steel with extremely high compressive strength. The fracture surface of brittle precision steel pipes is either an intergranular fracture surface or a mixed fracture surface of intergranular and quasi crystalline materials. The reason for brittle fracture caused by ultra-low temperature quenching is widely believed to be:
(1) It is closely related to the precipitation of pearlite in the form of lamellar dislocation in the original martensite during ultra low temperature quenching, which leads to dislocation aging.
(2) The dislocation segregation of residual element phosphorus in the original martensite is also one of the reasons for the ductility of ultra-low temperature quenching. High purity seamless steel tube with phosphorus content less than 0.005% do not cause ultra-low temperature quenching ductility. When phosphorus is heated by fire, martensite dislocation segregation occurs, which is preserved after heat treatment. Phosphorus precipitates in the original martensite dislocation segregation and pearlite quenching, which lead to intergranular brittle fracture and promote the formation of ultra-low temperature quenching brittle fracture.
Above is the reason for the brittle fracture caused by ultra-low temperature quenching of seamless steel tube.