Influence of microstructure on the flow behavior of duplex stainless steels at high temperatures

Three kinds of duplex stainless steel, with different ferrite-to-austenite ratios, were deformed in torsion over the temperature range 900 °C to 1200 °C; the corresponding microstructural evolution was observed and correlated with the deformation conditions. The shapes of the high-temperature flow curves depend strongly on the volume fractions of the phases, the characteristics of the ferrite-austenite interface, and the active softening mechanism. At low volume fractions of austenite, the mechanical behavior is determined by the ferrite matrix and the flow curves are typical of materials that soften by continuous dynamic recrystallization. When the volume fraction of austenite is increased, coherent γ particles distributed within the grains and at the grain boundaries hinder the deformation of the softer α matrix, increasing both the yield and the peak stress. These peaked flow curves are characterized by rapid work hardening followed by extensive flow softening; under these conditions, the hard austenite particles become aligned with the deformation direction after large strains. At high volume fractions of austenite (∼50 pct), the material tends to form a duplex structure, with the flow curves displaying extended work-hardening and work-softening regions; however, a drastic decrease is observed in ductility because of the dissimilar plastic behaviors of the two phases.