Effect of Sulphur Content on Delayed Fracture Susceptibility of High Strength Steel for Bolts

A study has been made on the eject of sulphur content on the tempering behaviour and the delayed fracture susceptibility of high strength steels for bolts at a strength level of 130kgf/mm2. By means of the bending type testing, it was confirmed that the delayed fracture resistance improves with decrease of sulphur content. Fracture modes of delayed fracture specimens of 0.015 and 0.024% S steels exhibit intergranular ones, and sulphur segregation to prior austenite grain boundaries are detected by Auger electron spectroscopy, while 0.003% S steel shows transgranular fracture. On the other hand, Charpy impact fracture modes are all transgranular, regardless of sulphur contents. Accordingly, it is concluded that the intergranular fracture in delayed fracture is induced by both the sulphur segregation to prior austenite grain boundaries and the hydrogen introduction. Further study indicates that the increased resistance to delayed fracture of low sulphur steel is caused mainly by the prolonged incubation period to slow crack initiation.