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

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.