SUPERPLASTICITY OF delta -FERRITE/AUSTENITE DUPLEX STAINLESS STEELS.

Abstract

Superplastic behavior has been studied by means of isothermal hot tensile tests at temperatures from 700 to 1,100 degree C at initial strain rates from 10** minus **4 to 10** minus **1s** minus **1 with emphasis on microstructural aspects prior to and during deformation. Large elongations are observed; the maximum value, above 2,500%, is obtained under the condition that sigma phase precipitation occurs. The most suitable prior microstructure can be obtained by refining the delta -ferrite matrix. Above 1,000 degree C gamma phase is separated and refined into spheroidized particles within the delta -ferrite matrix. Below 1,000 degree C, gamma / sigma mixed structure is formed by eutectoid decomposition from delta -ferrite, resulting in stable equiaxied micro-duplex structures with delta - gamma and gamma / sigma , respectively.