Study on the σ-phase precipitation of SAF2906 duplex stainless steel

Samples of SAF2906 duplex stainless steel were aged at 650 degrees C, 700 degrees C, 750 degrees C, 800 degrees C, 850 degrees C, 900 degrees C and 950 degrees C for 1 h, after high temperature solution heat treatment. The morphology of sigma-phase in aged SAF2906 duplex stainless steel was then studied by means of scanning electron microscopy, X-ray diffraction and transmission electron microscopy, and the chemical composition of the sigma-phase was determined using X-ray energy dispersive spectrometry. The results indicated that the sigma precipitation in SAF2906 duplex stainless steel formed both inside delta-ferrite with high Cr and Mo concentrations, and along delta-ferrite/gamma-austenite interfaces; however, relative to the SAF2205 and SAF2507 duplex stainless steels, the content of Cr in sigma-phase increased significantly up to 70 %, which was much higher than Fe in the same phase and became the major element of sigma-phase.