Superplasticity of .DELTA.-ferrite/austenite duplex stainless steels.

Superplastic behavior of δ-ferrite/austenite duplex stainless steels has been studied by means of isothermal hot tensile test at temperatures (T)from 700 to 1100°C at initial strain rates (ε) from 10-4 to 10-1 s-1 in relation to microstructural aspects prior to and during deformation. Superpiastic elongations were observed in wide ranges of T and ε. The maximum elongation greater than 2500% was obtained under the condition where c phase precipitation occurred during deformation. The elongation depended on the precipitation rate of c phase especially in the lower temperature deformation as well as on the prior microstructure. The most suitable microstructure obtained for superplasticity was fine grained δ-ferrite matrix with fine dispersion of γ particles. In the deformation of the specimens with such optimun microstructure, elongations greater than 200% were obtained at T≅1000°C even at ε_??_10-1s-1. During superplastic flow, the large microstructural changes were observed. At above 1000°C γ phase was separated and was refined into spherical particles within the δ-ferrite matrix and at temperature below 1000°C, a γ/σ mixed structure was formed by the eutectoid decomposition of δ-ferrite, resulting finally in the stable equiaxed micro-duplex structures with δ/γ and γ/σ, respectively. Such microstructural changes can play an important role for the superplastic behavior in addition to the effect of m-value.