Effects of Ni and Si on delayed fracture properties of high strength steels with 1420 MPa grade

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.