1996 Volume 37 Issue 12 Pages 1758-1762
An ultra high carbon steel containing 8.0 mass%Al-2.0 mass%Cr-1.3 mass%C was processed to obtain a fine grained microstructure and superplastic behavior. Thermomechanical processing consists of warm rolling from 1373 to 1023 K (η≈90%) and then continuous rolling (η≈25%) around the Ac1 transformation temperature. The final microstructure consists of a bimodal distribution of large proeutectoid and small κ-carbides in a ferritic matrix with a grain size of about 7 μm. Two different stress exponent regimes in the temperature range from 1023 to 1193 K as a function of the strain rate have been found. At strain rates up to ≈5×10−3s−1, the alloy was superplastic with stress exponents, n, of about 2 and tensile elongations as high as 865% at 1153 K. The plastic flow data in this regimen are in agreement with grain boundary sliding creep model. As the applied stress is increased, the stress exponent increased to about 4–5. The creep behavior is explained in this regimen by a single dislocation climb mechanism. The activation energy for creep is in both regimens comparable with that for lattice diffusion of Fe in ferrite.
