Influence of ultrasonic stress relief on stainless steel 316 specimens: A comparison with thermal stress relief

In conventional stress relieving by vibration, the residual stress reduces by induction of low frequency, high amplitude dynamic stress into an object. This method is confined to large pieces since the high amplitude of vibration deforms the thin or small objects. In this paper, a new stress relieving method based on ultrasonic vibrations is introduced which is applicable on the small or thin parts. The effectiveness of the method was verified by comparing it with thermal stress reliving. The stainless steel 316 was selected for residual stress reduction. The effects of ultrasonic vibration amplitude, relief time and preload parameters are evaluated. Acoustic softening of the metal is also evaluated since this phenomenon and dislocation activation by means of the acoustic waves are the main mechanisms behind this process. Experiments show that the residual stress of the small Almen strips can be reduced about 40% by thermal stress relief, while the ultrasonic stress relief is about 36%. It means the amount of stress reduction by ultrasonic method is comparable with thermal stress relieving. Statistical analyses of the experimental results show that the amount of stress reduction is directly proportional to the vibration amplitude and the stress relief time. The vibration frequency and the amplitude of the experiments were 24,500 Hz and 23–46 μm, respectively. Acoustic softening results showed that the tensile strength of the metal strongly decreases by superimposing of ultrasonic vibration in universal tensile test. The amount of this reduction is proportional to the ultrasonic intensity. Metallographic tests showed that there are no changes in grain size during the ultrasonic stress relieving process, and the only effect may be the dislocations movement.