Microstructures and element distributions in an aged hyper duplex stainless steel and corresponding hardness variation

Abstract

The microstructures and the distribution of alloying elements in alloy and intermetallic phases forming in a hyper-duplex stainless steel during aging at temperatures in the range of 750°C–1050°C are characterized by Electron probe micro analysis. Due to long aging times of up to 48 h, close-to-equilibrium configurations are obtained over well recrystallized microstructural regions. This allows for validation of computed equilibrium thermodynamics, including the solvus temperature of the σ-phase. During aging below 1050°C primary ferrite is replaced by a fine-grained aggregate of σ-phase and secondary austenite. The replacement of ferrite occurs by a eutectoidal reaction producing microstructural sub-regions with fine lamellar and coarse blocky morphology. In contrast, aging at 1050°C produced continuous rims of σ-phase surrounding primary austenite. At this temperature, the σ-phase, which is compositionally different from the σ-phase from the eutectoidal reaction, probably formed by the preferred nucleation and growth at austenite-ferrite and ferrite-ferrite boundaries. At the lower aging temperatures, χ-phase is observed as a transient precursor of the σ-phase. Temperature-dependent element enrichments and depletions across primary matrix phases and secondary intermetallic phases and secondary austenite shed light on the degree of equilibration in the different microstructural domains. Finally, nano- and microhardness was determined for different phases.