More than 30 high strength bolts made of 20MnTiB had fractured during the assembling process.Through macro and micro fracture analysis

metallurgical analysis

and micrographic hardness test

as well as fastening axial force and wedge load tests of the bolts carried out according to related standards

it is believed that the non-uniformity of microstructure and performance was resul- ted from imperfect manufacture technology

and overhigh fastening axial force of some bolts may induce delayed fracture of the bolts. The causes leading to delayed fracture have been also analysed and discussed.