Effects of Ni and Si on delayed fracture properties of high strength steels with 1420MPa grade were investigated. 0.25% to 3.5% of Ni or 0.25% to 1.7% of Si was added respectively to 0.3%C-boron-containing steel. Delayed fracture test and measurement of diffusible hydrogen were carried out using bar steels for prestressed concrete with a diameter of 7.1 mm. As each addition of Ni or Si was increased, diffusible hydrogen decreased and delayed fracture properties improved in condition as heat treated. Diffusible hydrogen in Ni-containing steels increased after polishing the surface in comparison with as heat treated and no diffusible hydrogen in Si-containing steels was varied. In Ni-containing steels, Ni concentrating and Ni-sulphide forming at the surface retarded the absorption of hydrogen and delayed fracture properties were improved, while in Si-containing steels, variation of microstructure decreased trapping sites of hydrogen and consequently the absorption of hydrogen was prevented and delayed fracture properties improved.
