Comparative study on the forming and reversion of strain-induced martensite in two duplex stainless steels

Abstract

An original feature of this work is the proposal of two equations to fit the volume fraction of ferromagnetic (α+α′) phases that can be applied in the measurement of magnetic saturation of comminuted duplex and superduplex stainless steels (for example, powders or filed chips). Duplex stainless steels contain similar volume fractions of austenite (γ) and ferrite (α) in their microstructure. Two steels exemplify this class, namely the most widely used duplex UNS S31803 and superduplex UNS S32520 stainless steels. The phenomena of work hardening, formation, and reversion of strain-induced martensite (α′) in austenite were compared in both stainless steels. Samples were work-hardened and annealed under identical conditions, and their behavior was evaluated mainly through X-ray diffraction and magnetic measurements. Notably, the volume fraction of strain-induced α′ in duplex stainless steel was as high as 32%, which indicated that this steel had a greater tendency to form α′ than superduplex stainless steel, for which the corresponding value equaled 15%. Annealing at 650 °C for 2 h promoted the reversion of strain-induced α′ into γ, decreasing the volume fraction of the former phase from 32 to 2% (duplex) and from 15 to 6% (superduplex).