Effect of Cooling Rate on the Precipitation Behavior of a Fe-Cr-Ni Alloy

Ferrite (delta) in two-phase austenite-ferrite Fe-Cr-Ni alloys decomposes into Mo- and Cr-rich phases like sigma (sigma) and chi (chi), when aged in the temperature range of 873-1273 K (600-1000 degrees C). The precipitation of these phases for a particular Fe-Cr-Ni alloy has an adverse effect on its mechanical properties and corrosion resistance. In the present work, precipitation behavior of UNS S32205 duplex stainless steel (a Fe-Cr-Ni alloy) during controlled cooling and heating (isothermal aging) has been studied in the temperature range of 973-1073 K (700-800 degrees C). Scanning electron microscope (SEM), X-ray diffraction (XRD), electron backscattered diffraction (EBSD) and energy-dispersive spectrometer (EDS) attached to SEM were used to characterize the microstructures. The effect of precipitation of sigma and chi phases on the micro-hardness was also studied. The precipitation sequence for 1023 K (750 degrees C), when cooled from 12,000 to 5 degrees C/min, was delta -> carbides -> chi -> sigma, while for 1073 K (800 degrees C), it was found to be delta -> chi -> sigma. The Mo-enriched metastable chi phase nucleates at the initial stage of aging which then transforms to stable sigma precipitates. The amount of sigma and chi phases increased with temperature and aging time, but temperature was found to have a dominant role than the cooling rate due to higher diffusion of solute atoms at high temperatures. EBSD studies did not show any orientation relationship between parent delta ferrite and sigma phase.