Effect of Cooling Rate on δ -Ferrite Formation and Sigma Precipitation Behavior of 254SMO Super-Austenitic Stainless Steel During Solidification

The formation of δ -ferrite and the precipitation behavior of the sigma phase during the solidification of 254SMO super-austenitic stainless steel (SASS) were investigated at five typical cooling rates (6 °C/min to 1000 °C/min) via high-temperature confocal scanning laser microscopy (HT-CSLM). The results showed that the 254SMO steel featured a significant dendritic solidified structure. With increasing cooling rates from 6 °C/min to 1000 °C/min, the initial solidification temperature and the secondary dendrite arm spacing (SDAS) of 254SMO gradually decreased and could be expressed as a function of cooling rates, the average sizes of SDAS were 113.33, 57.05, 34.98, 32.84 and 14.04 μ m. δ -ferrite was formed through a divorced eutectic reaction in the late solidification stage and existed in the interdendritic region. As the cooling rate increased, the δ -ferrite phase content in the steel sample first decreased from 3.610 (6 °C/min) to 0.051 pct (100 °C/min) and then increased slightly to 0.089 pct (1000 °C/min). The sigma phase was formed from the solid-state phase transition of δ -ferrite. With increasing cooling rates, the variation trend of the sigma phase content was opposite to that of the δ -ferrite phase. The solidification mechanism of 254SMO at the five typical cooling rates was explored. Moreover, the distributions of Cr, Ni, and Mo in the solidified 254SMO SASS were characterized via electron probe microscopy. Cr and Mo were segregated in the interdendritic region, while Ni was clustered in the dendritic region.