σ-Phase in lean duplex stainless steel sheets

The precipitation kinetics of secondary phases in two austeno-ferritic lean duplex stainless steels (lean DSS) were examined after aging the materials at 800 °C. Owing to the instability of ferrite, all DSS are known to be sensitive to solid-state phase transformations in the critical temperature range 600-1,000 °C and different secondary phases may form, depending on composition and microstructure. The performed thermodynamic simulations revealed the proneness to the precipitation of such phases also have been done in lean DSS, but only information on the equilibrium microstructures were achieved. Therefore, the materials were aged at various times, in order to verify the simulations and determine the precipitation kinetics. The occurred structural modifications were observed and quantified by scanning electron microscope and X-ray diffraction measurements, determining phase type, composition and volumetric fraction. At 800 °C, grade 2101 was found to be only affected by Cr2N nitrides precipitation, whereas a significant amount of r-phase was found to form in LDX 2404 for treatment longer than 1 h, almost totally replacing ferrite after 50 h. Up to now, the intermetallic r-phase has been observed only in the high alloyed DSS, and the unexpected precipitation in grade 2404 highlighted that the increased content of molybdenum in this steel might be considered as determinant for the formation.