The hot deformation behaviour of two 23% Cr 4% Ni austenitic-ferritic stainless steels was analysed as a function of temperature by hot tensile tests at constant strain rate. The analysis of the stress-strain curves, as well as the microstructural investigation of fractured specimens allowed to assess the influence of each constituent phase in the deformation behaviour of duplex stainless steels.

At high deformation temperature the occurrence of dynamic recrystallisation of austenite and dynamic recovery of ferrite favours the achievement of large strains before rupture. Dynamic recrystallisation is favoured by the strain transfer from the easily deformed ferrite to austenite. The grain boundary sliding, which limits the strain transfer to austenite, is less effective when the austenite/ferrite interface area per unit volume is low, that is at low austenite volume fraction.

At low deformation temperature, ferrite easily recovered while austenite work hardened but not recrystallised; a consequent flow stress gradient between the two phases caused the void formation at the a /g interface. Rupture occurred by coalescence of such voids at low strain, resulting in a low ductility. Under the latter deformation condition, the impurity content, mainly the sulphur content, was detrimental to the hot ductility of the stainless steel because S segregation facilitated the decohesion at a /g interface.