Very High Cycle Fatigue Behavior of the Steel with Carbide-free Bainite/Martensite Complex Microstructure

摘要

The fatigue behavior of a novel high strength steel with microstructure of carbide-free bainite/martensite (CFB/M) multiphase and outstanding combination of strength and toughness was studied. Fatigue tests were carried out by ultrasonic fatigue testing equipment at a frequency of 20 kHz. The fatigue crack propagation rate, da/d_N, and fatigue threshold value, △Kth, were measured using Compact-Tensile specimens. Results show that there is no horizontal part in the S-N curve. Fatigue fracture still occurs when the life of specimen exceeds 10~7 cycles. Based on the observation of fracture surface, it is found that the originations of the fatigue crack tend to locate at the interior of the specimen as fatigue cycle exceeds 10~7. For many specimens of CFB/M steel, the originations of fatigue crack are not induced by inclusions, but by some kind of "soft structure". The fatigue limit of some specimens whose life is over 10~8 cycles is more than 700MPa, which is about 50% of its tensile strength. Comparing with the steel with full martensite structure, the steel also has higher fatigue threshold value, △Kth, and lower fatigue crack propagation rate, da/d_N, which probably originates from special CFB/M microstructure and advanced closure resistance to the fatigue crack tip.