Hydrogen entry and its effect on delayed fracture susceptibility of high strength steel bolts under atmospheric corrosion

Abstract

Research on hydrogen embrittlement (delayed fracture) susceptibility of high strength steel bolts was reviewed from the viewpoint of metallurgical factors enhancing the resistance to delayed fracture, assessment of delayed fracture susceptibility based on the hydrogen absorption from atmospheric environments, and the mechanism of hydrogen entry into steel under atmospheric exposure. Resistance to delayed fracture was quantified based on the threshold hydrogen concentration C th by means of laboratory constant load tests under cathodic hydrogen charging. Vanadium containing antidelayed fracture steel bolts had higher Cth values than conventional steel bolts, and showed good resistance to delayed fracture at tensile strength levels of 1 400 MPa under actual atmospheric exposure. Susceptibility to delayed fracture was evaluated by comparing Cth to absorbed hydrogen concentration C0 into steel bolts under atmospheric exposure. The good resistance to delayed fracture of Vanadium steel high strength bolts was attributable to sufficiently higher Cth than C 0. Hydrogen permeation experiments under atmospheric exposure enabled more exact measurements of sub-surface hydrogen concentration into steel. Hydrogen permeability under atmospheric exposure showed a strong dependence on the time of the day, seasons and exposure locations. Mechanism of hydrogen entry was investigated by hydrogen permeation measurements under laboratory wet-dry cyclic conditions. Dominant factors that control hydrogen entry were temperature, relative humidity, and the amount of sea salt on the steel surface. Dependence of hydrogen permeability under actual atmospheric environments on daily temperature-humidity cycle, season and locations were explainable based on the effect of temperature, humidity and sea salts. © 2012 ISIJ.