| The main reason of the structure (components) and mechanical (spare parts) failure isfracture, especially fatigue fracture, which causes the disastrous accident. According torelated statistics, the fatigue fracture of welded joints accounted for the entire structuralfailure70%~90%, it often originates from the weakest area of the weld joint surface. In ourcountry, most locomotive bogie frames are steel welded structure, the railway is developing inthe high speed and heavy load direction, so it puts forward higher requirements on the fatigueperformance of welded bogie. Therefore, the research on the fatigue property and improvingthe surface property of the bogie welded joint have important theoretical significance andpractical value.This paper mainly includes two aspects: one is compared the fatigue property ofP355NL1butt joint before and after ultrasonic impact treatment. The welded butt joints S-Ncurve equation is lg N=26.818.709lgΔσ; The ultrasonic impact butt joints S-N curveequation is lg N=35.0911.671lgΔσ.In the cycle ofN=2×106, the fatigue limit value of P355NL1welded butt joints is227MPa, the fatigue limit value of ultrasonic impact butt joints is293MPa, compared with thenon-treatment, its fatigue limit increases by30%.The fatigue of P355NL1butt joints treated by ultrasonic impact was12~15times thanwelded joints under the same level condition. The reason is that after ultrasonic impacttreatment process, the grains at the weld toe became refined, the welding defects and surfacemicro crack at weld toe disappeared, which can retard the fatigue crack; the stressesconcentration was decreased by increasing the weld toe radius; the welding residual stressfield was adjusted.The fatigue fracture morphology was observed by SEM, there exist continuous, clearfatigue striations, a large number of high density, short and curved tear ridge, dot crack sourceand the river pattern diffused by quasi cleavage from center to the around in P355NL1steelbutt joint fracture after UIT treatment. Some different sizes of secondary cracks could be seenin the different region of the fracture, a great deal of tearing dimples which connected byplastic tearing mode at the transient fault zone. It indicated that the fatigue fracture ofP355NL1steel butt joint is a kind of quasi cleavage semi plastic fracture model.The other is surface grain refinement mechanism research of P355NL1steel butt weldtoe treated by ultrasonic impact. A certain thickness of fine grain strengthen layer wasobtained by ultrasonic impact treatment of P355NL1steel butt weld toe surface. In the same impact current, the impacting time were5min,10min and20min respectively, correspondingto the deformation layer thickness were60μm,100μm and150μm respectively. Theexperimental results showed that the thickness of deformation layer was increased with theimpacting time.After the observe of weld toe joints cross-sectional nano-surface by TEM, the threedifferent process parameters (5min/1.5A,10min/1.5A and20min/1.5A) all obtain nano grainwhich can be refined to100~150nm，60nm and30nm,respectively was discovered.The grain refinement procedure of P355NL1weld toe by ultrasonic impact treatmentincludes three parts:1. The high dislocation density of dislocation tangles and dislocationwalls was appeared in original grains and in the refined grains;2. Dislocation walls ordislocation tangles were evolved into small misorientation subgrains following with thedeformation strain gradually increased;3.With the deformation strain further intensified, thesubgrains change into large angle grain boundary and then the uniform, equiaxed and randomorientation nano-crystal was formed.The high strains, high strain rate and temperature are necessary for the formation ofnanocrystallite during surface nanocrystallization of weld toe by ultrasonic impact treatment. |
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