The superplastic deformation characteristics, of commercial 40Cr (i.e., 5140) steel that was water-quenched only 1 times and subsequent high-temperature tempered, were investigated.The results showed that the 40Cr steel has a fine grain of 10-15μm at room temperature,and exhibits a tensile elongation of 304, a true flow stress of 89.3MPa and a strain rate sensitivity m-value of 0.227 at the initial strain rate of 1.0×10^-3s^-1 and at the temperature of 750℃. The final fracture is caused by the development of neck. The experimental result of elongation is in good agreement with the theoretically predicated value according to the analytical expression ef =(1/f )^m exp(nv +me )-1 (where ef, m, f, nv and ε is respectively elongation, average strain rate sensitivity, initial geometric defect, average strain hardening sensitivity at constant deformation velocity and average true strain). The fracture surface is intergraular, and superplastic deformation induces an equiaxed and grown grain. Decreasing strain rate increases tensile elongation and strain rate sensitivity m-value. The primary superplastic deformation mechanism is thought to be atom-diffusion-controlled grain boundary sliding.
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