Nano-regeneration Reinforcement Method And Acceleration Mechanism Of Nanostructured Bainite Welded Joint

To solve the problem of poor weldability of a new generation of nanostructured bainite steel, regeneration welding of the nanostructured bainite steel was presented. The nanostructured bainite regeneration welding method was performed by controlling the cooling process of nanostructured bainite steel welded joints. When the fusion and austenitised zones cooled down from the high temperature to bainite transformation temperature, they were treated isothermally for certain time, promoting microstructure in welded joint to transform into nanostructured bainite. The nanostructured bainite regeneration characteristic in welded joint and heat affected zone was illustrated. Also, it was studied for tempered zone. Considering the long time for bainite transformation during regeneration process, two methods including make plastic deformation and grain refinement of austenite in welded joint were presented for acceleration.In this paper, the weldability of nanostructured bainite steel was systematically focused on. Results indicated that by lowering the welding heat input and mechanical restriction, hot cracking can be eliminated. In conventional welding, cold cracking were unavoidable due to the formation of high-carbon twinned martensite. By trailing welding isothermal heat treatment, cold cracks can be avoided. On the other hand, the cementite precipitation in tempered zone can degrade the performance in this zone.Microstructures and properties of each region of welded joints after nano-bainite regeneration were characterized. Results show that the microstructures in fusion and austenitised zones are mainly composed of nano-bainite. The corresponding mechanical performance is comparable to base metal. Coarse residual austenite appeared between dendrite in welded seam, which is resulted from solute segregation. Such residual austenite leads to lower elongation of welded joint than that of base metal. The effects of regeneration time and temperature on microstructure and mechanical performance were experimentally studied. When the regeneration time is short, martensite is formed partially in the welded joint. When the regeneration temperature varies, different matches between strength and elongation for welded seam and austenite zone are obtained.After bainite regeneration, tempered zone becomes the weakest location in the welded joint, which is due to amount of cementite precipitation. Microstructures evolution in tempered zone during welding tempering and regeneration process were investigated. The evolution kinetics of blocky austenite, plate austenite, plate bainite and whisker austenite were discovered. Results show that carbides precipitate from massive austenite during tempering. Bainite is formed again in the nano-bainite regeneration process. Lamellar austenite decomposes and gets narrowed until it becomes discrete intermittent austenite, while bainite lamellae get widened eventually and joins together. Besides, whisker austenite decomposes and becomes spherized. Effect of welding heat input on microstructure evolution was analyzed. Lowering the welding heat input can reduce the content and size of carbides. Dislocation density reduction is avoided, and thus the mechanical property degradation of tempered zone is controlled.To accelerate the regeneration, the method by which large plastic deformation occurs in welded joints is put forward. The influences of deformation temperature, deformation amount and strain rate on austenite to nano-bainite transformation are addressed. Results show that when the deformation temperature is above750°C, the plastic deformation inhibits the bainite transformation. When the deformation temperature is below500°C, plastic deformation promotes the bainite transformation. When plastic deformation at low temperature is introduced, larger deformation will cause faster bainite transformation. Besides, the effect of strain rate on nano-bainite regeneration is subtle.The rotating extrusion welding device is designed, of which there is a curved symmetrical concave extrusion head. When the curved head is adjusted that the trailing impacting and rotating force are implemented, a large amount of plastic deformation is produced in the welded joint of nano-bainite steel. By this method, the strength of welded joint obtained is as high as2131MPa, which is one time higher than that of nano-bainite regeneration welded joint for the same time but without deformation.To meet the demand of low temperature for nano-bainite regeneration by plastic deformation, the method that accelerates bainite transformation by austenite grain refinement through dynamic recrystallization is presented. New technique which can refine the grains in heat affected zone by static recrystallization is designed. It is possible to refine the grains compared with that of base material. Temperature field of semi-infinite double moving point heat source is solved. Static recrystallization kinetics in the nanostructured bainite steel welded joints is derived, and the influences of welding process parameters on static recrystallization in CGHAZ are delivered. It is shown that shorter distance of heat source, shorter distance between the welding bead, smaller welding speed and larger welding heat input are preferable conditions for the occurrence of static recrystallization. Using the impacting rotating extrusion device with curved surface, austenite grains are refined in the coarse grain zone through the static recrystallization, thus the nano-bainite regeneration process is accelerated. When the regeneration time is2h, the welded joint after regeneration process is2100MPa, which is90%as high as that of base metal. The strength of welded joint is improved by50%compared with that which is regenerated for the same time but acceleration technique is precluded.