The Activation Energy of Superplastic Flow above the Critical Temperature for Steel CrWMn

By means of superplastic tensile test above the Ac1 temperature (γ→α transformation temperature), superplastic deformation activation energy of ultrafine-grained commercial die steel CrWMn is investigated on the basis of the Arrhenius theory equation, ) / exp( 1 RT Q A m − = σ ε& , which indicates the resistance of the superplastic deformation. According to the Arrhenius equation, the activation energy is estimated from the log σt vs 1/T relationship at a constant of sensitivity index of strain rate. The results show that the strain rate sensitivity index is a constant and rather high at the conditions of superplastic deformation for the CrWMn steel, the activation energy for superplastic deformation of steel CrWMn above the critical temperature is 187KJ/mol, and the superplastic deformation activation energy is approached to the grain boundary diffusion activation energy of γ-Fe. This indicates that the grain boundary sliding (GBS) in superplastic deformation of CrWMn steel is controlled by grain boundary diffusion. The characters of superplastic deformation of the steel above the critical temperature, on the other hand, are also analyzed in this paper. Introduction The superplasticity is a good property of steels and superplastic working is an advanced processing technique for steels. Many researches showed that [1-3], the superplasticity of steels can be found above the critical temperature. Based on the superplasticity of steels above the critical temperature several superplastic working can be accomplished, such as superplastic forming, superplastic thermo-mechanical treatment, superplastic boronizing, superplastic welding and so on[4-6]. The accomplishment of the superpiastic working is determined by the superplastic deformation of above the critical temperature. The superplastic deformation of steels is a thermal activation process and the research on the activation energy of superplastic deformation is very useful for analyzing the deformation. In present work, based on the study of superplasticity of commercial die steel above the Ac1 temperature γ→α transformation temperature , the activation energy of superplastic deformation above the Ac1 temperature was determined by superplastic tensile test and the superplastic deformation mechanisms of the steel was analyzed by using activation energy.