Delayed Fracture Mechanism of 1700 MPa-Class Quenched and Tempered Bolt under Atmospheric Corrosion Environment

Overcoming delayed fracture under atmospheric corrosion environment is a critical challenge for the practical application of ultra-high-strength bolts. This study investigated the delayed fracture behavior of 1700 MPa-class quenched and tempered bolts fabricated from a 0.4%C-2%Si-1%Cr-1%Mo steel (mass%) by means of outdoor exposure tests, accelerated tests and thermal desorption spectrometry. In a 10-year outdoor exposure test, the delayed fracture behavior of the ultra-high-strength bolts was categorized into two types: rupture at the thread runout and the first thread root in nut, and fracture at the thread root in nut and the under-head fillet. Outdoor exposure tests using notched bolts could reproduce the delayed fracture at the thread runout. By measuring the stain change for the shank of notched bolt during the outdoor exposure test, we successfully observed the delayed fracture process: the initiation of intergranular fracture along the boundaries of prior-austenite grains, followed by unstable fracture, and culminating in the formation of a shear lip. The delayed fracture of the bolts was attributed to hydrogen embrittlement. Slow-strain-rate testing for hydrogen-pre-charged notched specimens showed a linear relationship between the fraction of intergranular fracture (IGF) area and the peak value of the maximum principal stress σmax* at the notch root. The delayed fracture mechanism of the ultra-high-strength bolts was discussed based on the σmax* and the IGF area fraction at the fracture locations.