Formation Mechanism of Fold Defects and Their Effect on Fatigue Properties in Ultrasonic Impact Treatment Q355 Steel

Q355 steels with different surface roughness values were subjected to an ultrasonic impact treatment(UIT)for different times(one, three, and five times)to study fold defects’ formation under the UIT and the defect’s influence on the fatigue property of Q355 steel. First, the ultrasound impact on the modality and roughness of the first sample was tested, and the result revealed that the ultrasound impact left pits. Consequently, the jagged outlook tended to smoothen out, resulting in consistent surface roughness of approximately 9.3 μm. Second, residual stress tests were respectively conducted on all three sets of samples after the impact. The three sets of samples manifested the same result after several numbers of UIT: the residual stress increased as the ultrasound impacts increased and then finally stabilized. Third, fold defects were observed by a scanning electron microscope(SEM)after the impact. The result showed overlapped valleys on the surface, crushing each other and resulting in metal flowing. Therefore, fold defects resulted due to the narrowed and overlapped gaps among the valleys. The initial modality, number, and amplitude of UIT influenced the type, number, and depth of the fold defects. First, the number of the fold defects increased and the depth augmented after having respectively one, three, and five times of ultrasound impact on the Q355 steel of the same initial roughness; second, the rougher the surface at first, the more the number and the greater the depth of the fold defects under the same number of ultrasound impacts at the end; and third, fold defects differed with increased amplitude. Fold defects had no noticeable change under one UIT. However, the greater the amplitude, the faster the crack propagation appeared under three times of UIT. Under the fifth UIT, a large number of fold defects and crack propagation occurred, each with a different amplitude. Finally, ultrasonic impact specimens were subjected to a three-point bending fatigue test. The result showed that the residual stress gain outweighed the negative effect of fold defects, resulting in the enhancement of fatigue life of 1.8 and 3.1 times when the Q355 steel was subjected to UIT one and three times, respectively. However, the fold defects increased, and the negative effect of the residual stress outweighed the gain for the five times of UIT, resulting in a significant drop in the fatigue life to a level below the untreated sample. © 2022, Editorial Board of Journal of Tianjin University(Science and Technology). All right reserved.