Study On Residual Stress Modification Of Welded 16MnR Parts By Means Of Ultrasonic Impact Treatment

The existence of welding residual stress will result in destruction for bearing and various mechanical properties of welded components, so people have groped many methods about the stress relieved for a long time and ensure the normal service of welded structures. Compared with traditional stress relieved methods, ultrasonic impact treatment technology is a new technology developed in recent years and applied successfully in some engineering at present.This paper select 16MnR low alloy steel widely used pressure vessel and vehicle manufacturing industry for research object, the specimens of plate butt-welded is prepared by MAG welding, ultrasonic impact treatment and low temperature heated treatment was performed respectively, and measuring the residual stress in original welded states, after ultrasonic impact treatment and low temperature heated treatment, the basic low and characteristics of residual stress eliminated by ultrasonic impact was systematically studied, and the effect of weld tissue performance by ultrasonic impact treatment was discussed.In this experiment,the complete coverage impact is used to weld surface using domestic ultrasonic impact machine the result verifies that ultrasonic impact can significantly decreased the welding residual stress. The specimens which are processed by ultrasonic impact treatment have been formed plastic deformation layer on the surface, residual tensile stress in surface layer of weld has been changed into advantaged residual compressive stress entirely, longitudinal and horizontal bidirectional residual stress field in the high stress area has been also adjusted to single stress state and homogeneous distribution, have more obvious effect of stress elimination comparing with the heat treatment. At the same time noted, the elimination effect of compressive stress area far away from weld present decreased trend from fusion zone to heat affected zone then to base metal by ultrasonic impact treatment, because impacting is difficult to compress and flatten grain here for the existence of low stress itself, the effect of ultrasonic impact is very weak. In the distribution of residual stress field, high middle and low end longitudinal residual stress of weld in the welded states was changed to single compressive stress by ultrasonic impact treatment, and same to horizontal residual stress, this is positive significance for bearing and service life of welded components. In addition, the effective depth test of ultrasonic impact treatment indicates, this method is strong effective for only a certain depth of the weld surface, and the effect of stress elimination has become increasingly weak with added depth. The depth of impact influence zone is about 3 mm for 12mm test plate, low depth elimination effect of ultrasonic impact treatment are corresponding to the mechanism of stress eliminated.According to the mechanism of stress relief about ultrasonic impact treatment, the author holds that it has matter with dislocation movement caused by macroscopic plastic deformation, dislocation may change to stabilization from the unstable high energy state (corresponding to tensile stress) to the low energy state (corresponding to compressive stress)after local yielding, resulting the peak residual stress can be released by micro-plastic deformation, namely decreased the welding residual stress, two important conditions of external factors that are naturally high-frequency energy and impact stress of vibration caused by ultrasonic impact .External cause actions the effect through internal cause,and induce the reduction of residual stress jointly.Acquired the effect of stress elimination, meanwhile, ultrasonic impact treatment also makes the surface weld metal grain refined obviously, and the distribution of all the tissue homogenized. The mechanism of grain refinement is still that existing the high-frequency energy provided by ultrasonic impact treatment, leading to have all the conditions of lattice distortion, dislocation motion and grain boundary migration, sub-grains formed by the multilateral process subsequently is the main reason for grain refinement.The results of subject test fully demonstrated that possessing welding residual stress elimination, adjusting longitudinal and horizontal bidirectional residual stress field to single and homogeneous stress state, and refining the grain of surface weld metal by ultrasonic impact treatment have wide significance. Meaning, its low power consumption, high efficiency and flexibility also have broad development potential in the future.