| Base on an experimental fact that big volume carbide steel have superplasticity and electric field could promote metal superplasticity and solid state welding performance, a new conception of electro-superplastic solid state welding is put forward. It will enrich and develop solid state welding theory and will break a new path for plastic working and solid state welding for difficult welding metals.In this paper, Cr12MoV steel and 40Cr steel were objected. The influence of electric field on Cr12MoV steel mechanical properties and microstructure before and after superplastic deformation were researched, which offered experimental parameters for superplastic solid state welding of Cr12MoV steel under electric field. On the base of the ultra-fining treatment of 40Cr steel, the solid state welding experiments with electric field steel of Cr12MoV and 40Cr steel was carried. The influence of process parameters, electric field intensity and polarity and microstructure on welding qulity of Cr12MoV and 40Cr steel were studied. The joints microstructure, phase structure, welded surface fracture and microhardness of the joint have been observed and analyzed in detail. The welding mechanism of the greate quality joint with electric field was preliminarily studied.The results show that, the true stress-strain curves of Cr12MoV steel indicate obvious compression superplastic flow characteristic at the temperature of (760-780)℃and the initial strain rate of (1.5×10-5-1.5×10-3)s-1 with electric field intensity of (1-5)kV/cm. The no steady plastic deformation stage is shorten by electric field. The yield strength and steady flow stress are reduced by the field. At the electric field intensity of 2kV/cm, the steady flow stress of the Cr12MoV steel decreases more than 7%, the strain rate sensitivity values increases from 0.21 without electric field to 0.24 and the superplastic deformation activation energy decrease from 241kJ/mol without electric field to 224 kJ/mol in the manner of the specimen connected to negative terminal and the ring electrode connected to positive terminal. The effect of changing terminal is not significantly. The grain can keep equiaxed grain after specimens superplastic compressing at 800℃and the initial strain rate of 1.5×10-4s-1. The electric field retard grain grow and improve grain equiaxial, but the new phase structure do not emergence. The electro-superplastic welding of Cr12MoV steel and 40Cr steel after cyclic quenching can be carried out at the conditions of pre-pressed stress of 62.2 MPa, holding time of 20 minutes, welding temperature of (760-800)℃, initial strain rate of (0.75×10-4-3×10-4)s-1 with electric field intensity of (0-4)kV/cm, electric field improve the joint strength and inflate tatio of Cr12MoV steel. The electric field intensity of 2kV/cm have the most influential, and the joint strength is up to 690MPa when welding 10 minits, which has reached 95% of base metal of 40Cr steel. The carbon diffusion in interface zone of superplastic solid state welding is from 40Cr steel to Cr12MoV steel, which is the up-hill diffusion. There is decarbuized cases on the interface of 40Cr and recarbuized cases of Cr12MoV steel. Electric field promote atomic diffusion and broad decarbuized cases width. The higher of welding temperature or the longer of welding time, the broader of the width of decarburized case or recarburized cases. Tensile fracture is composed of metallurgy bonding zone and mechanical bonding zone, and electric field increase the number of metallurgy bonding zone. 40Cr steel after pretreating of cyclic quenching could be welded more easily compared with the hot rolling treatment.
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