Effect Of Ultrasonic Impact Treatment On Residual Stress Of Butt Joint Of S355J2W+N Steel

Welded structure is one of the most widely used structural forms in rail transit equipment.Welded bogie frame is the core component of railway vehicle running part.Non-uniform heating and cooling during welding process,as well as ever increasing restraint,aggravate the nonuniformity of joint microstructure,mechanical properties and residual stress.Moreover,the high stress concentration at the weld toe might lead to produce cracks and propagate.As a result,the fatigue strength and service life of the welded frame could be reduced.Therefore,heat treatment,sandblasting and grinding are usually needed after welding process to reduce the welding residual stress and the stress concentration,and then improve structural fatigue performance and so on.Aiming to improve residual stress state of welded bogie frame and the welding quality,this paper carries out the research on the reduction of welding residual stress by employing ultrasonic impact treatment(UIT).The butt joint of bogie frame material,namely S355J2W+N low alloy structural steel,is designed as experiment object.The butt joint was treated once,twice,four times,six times and eight times using ultrasonic impact method.The residual stress of the treated joints and as-welded joints at different characteristic positions and at different depths are measured by employing X-ray diffraction method.By comparing the results of transverse and longitudinal residual stresses,the reasonable number of ultrasonic impact treatment in engineering is determined.In order to evaluate the effect of ultrasonic shock treatment on reducing residual stress at different depths,electrolytic polishing was used to polish the joints,and ultrasonic thickness measurement was used to measure the polishing depth.At the same time,heat treatment and sand-blasting test were carried out on the specimens after ultrasonic impact treatment to study the evolution of welding residual stress in different states.In addition,in order to study the effect of ultrasonic impact on reducing welding residual stress,the traditional heat treatment and sand-blasting methods were used to treat as-welded butt joint,The residual stress was tested and compared respectively.The results show that in a certain range,the more impact times,the more obvious the effect of compressive stress and the greater the impact depth of ultrasonic impact.Considering the effect of residual stress reduction,engineering application efficiency and benefit,it is considered that four impact times is reasonable,The corresponding depth of compressive stress is not less than 1.5mm,and the residual compressive stress of 274.89 MPa can be obtained at least on the surface.The residual compressive stress of 160 MPa-315 MPa can be obtained at 1.5 mm below the surface of the material.From the evolution of residual stress of the ultrasonic impact joints with different states,it can be seen that if the structure needs ultrasonic impact treatment,heat treatment and sandblasting treatment respectively,the ultrasonic impact treatment should be carried out after h eat treatment and before sandblasting treatment to guarantee UIT effect.In this way compressive stress is not released by heat treatment,but also can obtain the uniform distribution of residual compressive stress.Compared with the other two treatment methods,the compressive stress produced by UIT is larger than that produced by heat treatment,but not as uniform as that produced by sand-blasting.T he advantages of ultrasonic impact in the treatment of local high stress welds are obvious,while sand-blasting and heat treatment are suitable for the overall treatment of residual stress of welds.