超声冲击Q355钢叠形缺陷的形成及对疲劳性能的影响

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

为了研究超声冲击产生叠形缺陷的过程与缺陷对材料疲劳性能的影响,针对不同初始表面粗糙度的Q355钢进行不同次数(1次、3次、5次)的超声冲击处理(UIT).首先,对冲击后试样形貌及表面粗糙度进行了测试.结果表明,超声冲击会在样品表面引入凹坑,使表面形貌由锯齿状趋于平滑,导致表面粗糙度最终稳定于一个数值,约为9.3μm.其次,对冲击后试样进行残余应力的测试.结果表明,多次UIT处理后,3组样品最表层残余压应力随冲击次数增加而增加,最表面残余压应力值趋于稳定.然后,对冲击后叠形缺陷进行SEM表征.冲击后试样表面的峰谷互相重叠挤压,产生金属流动,峰谷间隙缩小直至闭合,产生叠形缺陷.叠形缺陷的种类、数量及距表面距离受初始形貌UIT次数及UIT振幅的影响:相同初始粗糙度试样,冲击1次、3次和5次,叠形缺陷数量增加,距表面深度也增加;相同冲击次数下,初始粗糙度越大,叠形缺陷数量与深度都将增加;冲击振幅增加,叠形缺陷在1次UIT时无明显变化,3次UIT时,大振幅下产生的叠形缺陷更容易先开始类裂纹扩展,5次UIT时,都会产生多条叠形缺陷并发生类裂纹扩展.最后,对处理后的超声冲击试样进行三点弯疲劳试验.结果表明,在冲击1次、3次时,残余压应力的增益效果大于叠形缺陷的负效应,疲劳寿命分别提高了1.8倍和3.1倍.在冲击5次时,叠形缺陷分布更密集,负效应大于残余压应力产生的增益值,疲劳寿命大幅度下降,甚至达到低于未处理样品的水平.

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 9.3μm.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.