Study On The Process Optimization And Performance Of Electro-superplastic Welding Of Cr12MoV/40Cr

Electro-superplastic welding is advanced welding method which combines the advantages of superplastic welding and electro-plastic deformation. In this study, the steels containing large quantities of carbides were welded by electro-superplastic welding, in order to optimize processing–property relationship so as to realize sound welding of the steels and to enrich and develop the solid-state welding processes.The materials used in this study were Cr12MoV steel and 40Cr steel. The Cr12MoV steel, as hot-rolled and annealed, contains large quantities of carbides. The 40Cr steel has been heat treated through cyclic salt-bath quenching. The electro-superplastic welding of these two steels has been studied. Firstly, the electro-superplastic compression behavior of Cr12MoV steel was researched. The influences of electro-superplastic compression process parameters on carbides evolution and dislocation configuration in matrix were investigated. Orthogonal test of main welding process parameters for Cr12MoV/40Cr welding were done, then these parameters were optimized. The influences of electro-superplastic welding process parameters, especially the electric field intensity, on the microstructure and properties of the Cr12MoV/ 40Cr joint were discussed. The electro-superplastic welding mechanism model was clarified.The true stress-strain curves of electro-superplastic compressive deformation for Cr12MoV and 40Cr steel had superplastic characteristics. The steady flow stresses were lower than that obtained under conventional superplastic compression conditions. In the case that the Cr12MoV steel was compressed under electro-superplastic compressive deformation when welding temperature was 800℃, initial strain rate was 1.5×10-4s-1, and electric field intensity was -2kV/cm, the strain rate sensitivity was larger and the superplastic flow activation energy was lower compared with that under conventional superplastic compressive deformation conditions. Namely, the superplasticity of the Cr12MoV and 40Cr steel were improved under compression deformation with electric field. An optimum welding parameters level, which electric field intensity +3kV/cm, welding temperature 800℃, welding time 10min and initial strain rate 1.5×10-4s-1, was found through the orthogonal test of Cr12MoV/40Cr welding parameters. The tensile strength of the joint, which was welded under the optimum parameters level with a compression strain of 5%, was up to 95.4% of that (700MPa) of 40Cr steel undergone same thermal process, and was 8% bigger than that of the joint welded under normal superplastic welding condition. It is considered that the additional electric field improves the Cr12MoV steel's superplasticity.The electro-superplastic welding joint of Cr12MoV/40Cr could be divided into interface zone, transitional zone and the base zone. In the interface zone, some discontinuous nano Fe-Cr particles and local dynamic recrystallization were found. Moreover, this zone included random distribution ferrite and ferrite (α/α), ferrite and carbide (K/α) and carbide and carbide (K/K). In the electro-superplastic transitional zone, the grains in the Cr12MoV steel side were equiaxial; the dislocations occured mainly at grain boundaries and around carbide particles; and the grain boundaries were evidently wide, which indicated a superplastic deformation. It is considered that the additional electric field improves the dislocations'movability, grain boundary sliding and grain rotation. The additional electric field accelerates the atom diffusion at welding interface, which conducive to form metallurgical combination zone and results in a wider poor-carbon zone in 40Cr side near the interface. the fracture morphology of electro-superplastic welding joint could be divided into similar original interface zone and metallurgical bond zone. Fracture of the joint occurred mainly between carbide in Cr12MoV and base ferrite (K/α) in the 40Cr. Defects in the electro-superplastic welding joints, which included mainly mechanical bonding zone, interface microscopic voids and facial impurity, were observed.Results show that the electric field can increase element diffusion and dislocation movement, can encourages grain boundary sliding and grain turning both sides of grain boundary, and can accelerates the formation of metallurgic region, which is beneficial to obtain sound joints.