Effect Of Vibration On Microstructures And Properties Of Welded Joint And Engineering Application

Vibratory welding (VW) is the new technology of vibrating workpieces with periodical force to improve the welded joint properties. The effects of vibration on welding temperature, residual stress, distortion and mechanical properties were studied by the means of experiments, mathematical modeling and theoretical analysis. The influence of vibration on the microstructures and initial dynamic recrystallization process of the welded joints was discussed. Difficulties in welding distortion control of H-type steel welding, coarse grains and low side bend properties in special steel thick plate electro-slag welding (ESW) and welding of big diameter full welded body ball valve without post-weld heat treatment (PWHT) were solved by VW. These practices would provide a good foundation for the application of VW in important steel structures with big diameter.First, the cross vibratory submerged-arc welding (SAW) test scheme was designed with three kinds of heat input parameters and four kinds of vibratory accelerations. The experimental platform was set up with temperature testing system. Welding temperature was real-time monitored effectively with this temperature testing system based on UT60E.Second, the effects of vibratory acceleration and welding heat input on the properties of SAW welded joint were investigated thoroughly. It was found that less heat input with larger acceleration or larger heat input with less acceleration was propitious to weld formation. The residual stress was reduced and uniformed after vibration. When the welding heat input was 22.8 kJ/cm, residual stress was reduced evidently as the acceleration increased. Residual stress was lowest when the welding heat input was 26.6 kJ/cm and acceleration was 10 m/s~2. While heat input was increased to 31.6 kJ/cm, the transverse stressσ_x and maximum main stressσmax distinctivly decreased under the acceleration of 15 m/s2. Transverse contraction distortion was added with the increase of heat input. Longitudinal distortion was reduced significantly compared with that without vibratory acceleration. Less heat input with larger vibratory acceleration and larger heat input with less acceleration favored the welding distortion reduction. The mechanism of residual stress and distortion decrease after vibration is that vibration speeds heat elimination of the weld pool during solidification of the molten pool. As a result, the temperature gradient of the pool and non-uniform plastic distortion are decreased. And the eventual residual stress related to the temperature gradient is reduced.The coarse microstructure and the highest microhardness both appeared in the coarse grain zone, which represented the relationship of structure and property. The influence of vibration on impact toughness and impact fractograph was discussed. The relationship of impact toughness and vibratory parameters was analyzed by regression method.The amplitude-dependent internal friction (ADIF) was tested with dynamic mechanical analyzer (DMA). The influences of the shape and quality of the dislocation on the dynamic mechanical property under different VW conditions had been analyzed. The results indicated that the internal friction increased the heat input reduction without the vibratory acceleration. This situation revealed the effect of grain size on ADIF with different heat inputs. When the vibratory acceleration was 6 m/s~2, the internal friction at 31.6 kJ/cm increased obviously with strain amplitude increasing, which was related to character of the pinned dislocation and strain-strengthening. When the vibratory acceleration was 10 m/s~2, the internal friction at 26.6 kJ/cm reached the peak value first because of the length and density of movable dislocations. When the vibratory acceleration was 15 m/s~2, the internal friction at 22.8 kJ/cm increased rapidly due to the multiplication and high density of dislocations.The development microstructures and dislocation configuration were studied systemically using metallurgical microscope and TEM. The mechanism of the effect of vibration on the welded joint was discussed. The metal macro-property is dependent on the microstructures. The relationship between metallurgical structure and vibratory welding parameters was investigated. The results indicated that vibration improved the metallurgical structures of weld and heat-affected zone (HAZ). The acicular ferrite in the weld was fine and uniform. The width of HAZ was decreased. The Widmanstaten structure was reduced. The degree of microstructure refinement was different from the vibratory acceleration applied. The microstructure was better with the vibratory acceleration varied in the range of 6～10 m/s~2. The weld grain size could not be reduced by excessive vibratory acceleration.It was concluded that dynamic recrystallization had taken place in the welding pool. Strain capacity, strain rate and dislocation density were raised rapidly under the special welding heat and stress condition. Vibration promoted the dislocation slipping and climbing, which was necessary for dynamic recrystallization. The testing results indicated that the dislocation shape and distribution in the weld changed greatly with the increasing vibratory acceleration. The quantity of dislocations was improved. The dislocations were collected to form a dislocation wall. It was also found that dislocation rings of different sizes were scattered in the grains. The grain boundaries were distorted seriously. The welding pool temperature was raised, the capacity of atoms diffusion was built up and movement of grain boundaries was quickened with heat input increasing. The equation of thermal distortion in initial dynamic recrystallization was set up.The mathematical models of the highest temperature close to the weld, residual maximum main stress and impact toughness were established through multi-variant nonlinear regression analysis. These mathematical models had high accuracy after verification and could help to select vibratory welding parameters in the engineering projects.At last, difficulties in steel structures with big diameter were solved by VW.The welding wave distortion of the H-section Q235B steel was reduced by adoption of the submerged-arc VW, which guaranteed the production quality.VW was applied in the blast furnace steel 14MnNbTi with thick plate electro-slag welding. The results indicated that in the 6 m/s2 vibratory condition, the pass of side bend performance was up to 100 percent. The ESW microstructures of weld and coarse grain zone were refined evidently. But vibration had less influence on the microstructures far away from the fusion line. The manufacture of big diameter full welded ball valve is the homemade project in China Petroleum West-East Gas Transfer Pipeline project. For the first time, vibratory welding was used successfully in valve cylinder welding to solve the principal technical difficulties of residual stress control, distortion control and welding procedure without PWHT in big diameter full welded body ball valve welding. The results showed that the residual maximum main stress was decreased below yield strength. Welding distortion was reduced by 50 %. The stability and safety of the welding structure had been improved greatly. The average impact toughness of the weld in VW was elevated by 21 % compared with that in normal welding condition. The metallurgical structures of weld and HAZ were improved after vibration. The weld microstructure was refined. The fusion area and coarse grain zone were narrowed. The Widmanstaten structure was decreased. At the same time, the fracture toughness of the welded joints was increased by vibration. The differences of fracture toughness between each part of the joint were weakened. The fracture toughness test showed that the welded joint under VW condition met the international requirements and the welded valve could be used safely without PWHT.