| Summary: | The relationship between microstructure, stress ratio, and fatigue fracture behavior of high-strength steel for flexible marine risers was investigated by microstructure characterization and fatigue test. The microstructure characteristic, S-N curve, fatigue fracture morphology, and fatigue crack morphology after quenching and tempering treatment and cyclic heat treatment, respectively, were evaluated. The results revealed that after cyclic heat treatment, the microstructure was refined considerably, the grain boundary density and the content of high-angle grain boundary increased, which inhibited the fatigue crack propagation and improved the fatigue strength. The average stress increased by increasing the stress ratio, which promoted the crack initiation and propagation; consequently, the fatigue strength and fatigue life of the tested steel decreased.
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