| The designed fatigue life of welded bogie frame has reached 109 cycles, in order to ensure bogie’s safety during service period, it is necessary to undertake deep research into the very high cycle fatigue behavior and fracture mechanism of SMA490 BW steel welded joints for high-speed train bogie at the numbers of cycles from 107 to 109. Ultrasonic impact treatment technology has been widely applied to improve the fatigue properties of metal welded joints. In this paper, the effect of ultrasonic impact treatment on the very high cycle fatigue properties and failure mechanisms of SMA490 BW steel butt welded joints had been studied; the mechanism of improving fatigue properties of welded joints treated by ultrasonic impact treatment had been discussed from three aspects as follows: stress concentration at weld toe of joints, residual stress distribution and surface grain size. Surface nano crystallization mechanism of welded joint treated by ultrasonic impact treatment had been investigated emphatically. The experimental results show that:(1) In the case of 20 kHz loading frequency and stress ratio R=-1, i.e. specimens were subjected to symmetric tension and compression loads, the S-N curve equation of SMA490 BW base metal was5 8lg N ?60.386 lg ?? ?159.226 10 ?N ?4.85?10; the S-N curve equation of butt welded joints was5 6lg N ?45.025 lg ?? ?104.729 10 ?N ?6.45?10;6 8lg N ?45.025 lg ?? ?104.729 6.45?10 ?N ?7.86?10; the S-N curve equation of welded joints treated by ultrasonic impacting was5 6lg N ?7.081 lg ?? ?23.034 10 ?N ?4.86?10;6 8lg N ?57.110 lg ?? ?138.822 4.86?10 ?N ?3.69?10.(2) In the cycle of65?10, fatigue strength of butt welded joints was only 45.5% of base metal; high cycle fatigue strength of welded joints treated by ultrasonic impacting was206 MPa, which increased by 34.6% compared with 153 MPa of welded joints. Similarly, in the cycle of81?10, very high cycle fatigue strength of welded joints treated by ultrasonic impacting was 195 MPa, which increased by 38.3% compared with 141 MPa of welded joints.(3) Under the condition of same stress level, fatigue life of joints treated by ultrasonic impacting was significantly longer than welded joints. Under 240 MPa and 197 MPa stress level conditions, fatigue life increased 7 times and 74 times respectively. Ultrasonic impact treatment could significantly prolong the fatigue life of SMA490 BW steel welded joints.(4) Fatigue fracture mechanism of SMA490 BW steel base metal was quasi-cleavage fracture. For welded and impact-treated joint specimens, high cycle and very high cyclefatigue crack sources mostly located at surface defects of weld toe, crack of individual welded specimen initiates at weld defect of slag; while crack of impact-treated joint specimen initiated at internal inclusions. Crack propagation regions presented clear cleavage river pattern, and a large number of irregular tear ridge lines, also accompanied by cleavage steps and secondary cracks. Different sizes of equiaxed dimples were presented at fatigue final rupture region, which indicated that quasi-cleavage fracture still was fracture mechanism of SMA490 BW steel butt welded joints. Ultrasonic impact treatment could not change the failure mechanism of welded joints in the high cycle and very high cycle fatigue regime.(5) Crack propagation path of welded joint was relavitely flat, while impact-treated joint was much more tortuous. In the crack propagation process, the phenomenon of crack tip blunting and bifurcation(secondary cracks) appeared, which showed typical transgranular fracture, also accompanied by traces of obvious plastic deformation. Ultrasonic impact treatment could not change fatigue crack growth mechanism of welded joints.(6) Stress concentration factor of SMA490 BW steel welded joint was 1.686, while impact-treated joint was 1.364 with a decrease of 19.1%. Horizontal and vertical residual stresses of welded joint were 11.13 MPa and 20.32 MPa respectively; while impact-treated joint were-40.46 MPa and-23.59 MPa respectively, which indicated that tensile residual stress at weld toe were eliminated and transformed into benefical residual compressive stress.(7) Significant plastic deformation occurred at weld toe surface of SMA490 BW steel welded joint treated by ultrasonic impact treatment. When the impact processing parameters were 1.5A/10 min, 2.0A/5min, 2.0A/10 min and 2.0A/20 min, the corresponding thickness of plastic deformation layer were 305μm, 270μm, 360μm, 435μm respectively. Through TEM observation, grains of outermost layer were significantly refined, and randomly oriented nanocrystalline microstructure were obtained, which average grain size were 80~100nm,120~150nm, 50~70nm and 20~30nm respectively.(8) Surface grain nano-crystallizing procedure of SMA490 BW welded joint treated by ultrasonic impact treatment included four parts: 1. A large number of dislocations were formed in the original coarse grain, and internation between dislocations(slip, proliferation,annihilation and rearrangement) continued to strengthen. High-density dislocation walls and dislocation tangles were formed; 2. With the deformation strain increased, dislocation tangles or dislocation walls were evolved into small-angle subgrain boundaries; 3. High-density dislocation continued to proliferate and annihilate at sub-grain boundaries, and grain misorientation increased constantly. Small-angle subgrain boundaries were evolved into high-angle subgrain boundaries, and subgrain structure with fine-grain size was formed; 4.Subgrain structure was evolved into equiaxed nano-crystal with uniform size, clear grain-boundary and random orientation through the evolution of dislocation tangles,dislocation walls and subgrain boundaries. |
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