Ultrasonic fatigue life of 40CrNiMoA (AISI4340) steel and its influencing factors

摘要

The fatigue life of 40CrNiMoA (AISI4340) steel with different microstructures under ultrasonic fatigue loading (f = 20 KHz, R = -1) and its influencing factors were studied by the ultrasonic fatigue test. The results showed that the ultrasonic fatigue life of the specimen was markedly increased by surface treatment, and the ultrasonic fatigue life with the microstructures characterised by both strength and toughness was longer. The specimens' fatigue fractures were analysed by a scanning electron microscope. The results showed that crack initiation took place at the defect of the material surface at the ultrasonic fatigue loading and the crack propagated towards the inside. The fracture had both the features of cleavage and that of quasi-cleavage. The different microstructures of 40CrNiMoA steel have two different types of S-N curve: a continuously decreasing curve and a stepwise curve. The specimens continued to fail over 10~7 stress cycles.