In order to improve the very high cycle fatigue performance of welded joint of welded bogie frame,the ultrasonic impact treatment (UIT) and mechanical polishing treatment on the welded joints of SMA490BW steel for EMU bogie were carried out under different impact parameters and modes,respectively.Fatigue tests were performed on the very high cycle fatigue performance of welded joint both in welded and treated conditions by ultrasonic fatigue test system.Through residual stress tests,metallographic structure and fracture surface observation,the mechanism of UIT and mechanical polishing treatment to improve the very high cycle fatigue performance of SMA490BW steel welded joint was analyzed.Results show that under 330 MPa stress condition,the average fatigue life of welded joint specimen before and after mechanical polishing treatment is about 0.252 × 107 cycles and 1.232 X 107 cycles,respectively.The fatigue life is improved about 5 times.When the impact current is 2.0 A and the impact time is 20 min,the fatigue life of welded joint can be up to 4.978 × 107 cycles after the local UIT on weld toe and root,which is about 20 times higher than that without treatment.The introduction of residual compressive stress,the refinement of surface grain and the modification of weld toe shape are the main reasons for the improvement of fatigue performance of welded joint.The initiation position of fatigue crack may be transferred from the weld toe or root to other defects on material surface after welded joint being treated by UIT.%为提高转向架焊接构架焊接接头的超高周疲劳性能,分别采用不同冲击参数和冲击方式下的超声冲击和机械打磨,处理动车组转向架用SMA490BW钢的焊接接头,并利用超声疲劳试验系统测试处理前后焊接接头的超高周疲劳性能.通过残余应力测试、金相组织和断口形貌观察,分析用超声冲击和机械打磨提高SMA490BW钢焊接接头超高周疲劳性能的机理.结果表明:在330 MPa应力条件下原始焊态接头的平均疲劳寿命约为0.252×107周次,经机械打磨后疲劳寿命可提高至1.232×107周次,提高约5倍;在2.0A冲击电流下对焊趾及焊根处进行20 min的局部超声冲击后,其疲劳寿命可达4.978×107周次,提高约20倍;残余压应力的引入、表层晶粒的细化和焊趾外形的修整是焊接接头疲劳性能得到提高的主要原因;焊接接头经过超声冲击强化后,疲劳裂纹的萌生位置可能从焊趾或焊根处转移到材料表面其他缺陷处.
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