Evaluation of Fatigue Life of Ultra-High-Strength Steel under Stress Corrosion Environment

Abstract

Ultra-high-strength steel (UHSS) structures are exposed to corrosive environments during service, and hydrogen-assisted cracking (HAC) may occur owing to stress corrosion cracking and hydrogen embrittlement. In this study, the HAC threshold stress intensity factor and fatigue life of UHSS steel were evaluated by applying stress in a corrosive environment to prevent structural fracture. For specimen with semicircular slits by electric discharge machining, fatigue limit was obtained by static fatigue test under corrosive environment. The fatigue limit of the crack specimen was evaluated by the fatigue limit of the experiment and HAC threshold stress intensity factor, and comparative evaluation was performed. On the surface of cracks, grain boundaries were embrittled by corrosion, and grains were clearly observed. Meanwhile, cracks in the surface direction propagated slightly, unlike cracks in the depth direction. The static fatigue limit of UHSS (SKD11:HV670) was determined to be 400 MPa, and the fatigue limit of the crack specimen could be evaluated. The experimental results agreed well with the evaluation results.