Delayed Fracture of High Strength Steels under Atmospheric Corrosion

Abstract

Notch tensile strengths (NTSs) of circumferentially notched round bar specimens of high strength steels were measured after hydrogen precharging using SSRT, and power law relationship between NTS and hydrogen concentration was found. The occurrence of delayed fracture during constant load test of atmospherically corroded specimens corresponded to the balance between critical hydrogen concentration for delayed fracture (H_C) and hydrogen concentration from the environment (H_E). This fact suggests that the delayed fracture of actual high strength bolt also occurs when H_E exceeds H_C. Delayed fracture properties of high strength steels were successfully evaluated using SSRT after atmospheric corrosion to take into consideration the hydrogen entry caused by atmospheric corrosion. The ultrahigh strength steel containing hydrogen traps possesses relatively high H_C. However, the hydrogen traps resulted in an increase of H_E, so that the resistance to delayed fracture was not improved. Hydrogen entry under cyclic corrosion test monitored by electrochemical hydrogen permeation test indicated an increase of hydrogen entry with time probably because of enhancement of hydrogen entry caused by the changes of pH of inner rust layer and potential. It is suggested that the “delay” of delayed fracture is the time required for the enhancement of the hydrogen entry efficiency.