The effect of the microstructure on the superplasticity of a duplex stainless steel

The aim of this work is to study the effect of the microstructure on the superplasticity of 22% Cr-5% Ni-3% Mo-0.18% N ferrite (alpha)-austenite (gamma) duplex stainless steel. To obtain different microstructures several thermo-mechanical treatments were employed. Superplasticity was evaluated at 1253 K by m-value measurements, elongation to failure, deformation homogeneity, and changes in microstructure after tensile tests. A maximum elongation of 1510% was reached with an initial strain rate of 3 x 10(-4)/s. For this condition, the m-value attained 0.64 and necking was not verified. Higher degrees of superplasticity are obtained when the initial microstructure of the material is fine and homogeneous. Such microstructure can be obtained by a solution treatment in the a-single phase field, followed by a heavy cold rolling operation and a new solution treatment in the alpha/gamma dual phase field. The precipitation of sigma-phase during superplastic deformation promotes cavitation and decreases the deformation homogeneity and the elongation to failure. (C) 1998 Acta Metallurgica Inc.