Transformation of the δ-Ferrite in SS2343 Austenitic Stainless Steel upon Annealing at 1050 °C, 1150 °C and 1250 °C

The solidification behaviors of laboratory cast austenitic SS2343 stainless steel in terms of the volume fraction of delta-ferrite in the as-cast state and its transformation after subsequent annealing were investigated. Monitoring of morphological transformations of delta-ferrite in the microstructure show the progress of delta-ferrite dissolution. Annealing tests were conducted at 1050 degrees C, 1150 degrees C and 1250 degrees C with soaking times of 5 and 40 min. The thermodynamic prediction and metallographic identification of delta-ferrite are presented. The ferrite fractions were measured using a magnetic method and determined to be in the range between 10.7% and 14.6%. The volume share of delta-ferrite decreased with an increase in temperature and the time of annealing. About 50-55% the delta-ferrite was effectively transformed. The delta-ferrite phase, originally present in a dendritic morphology, tends to break up and spheroidize. The morphology varies from vermicular, lacy and acicular shapes to globular for higher temperatures and for longer exposure times. In the delta-ferrite after annealing, concentrations of Cr and Mo decrease, and conversely the concentration of Ni increase, all by small, but significant, amounts. The observed changes in the solute concentration can be explained in terms of the transformation of ferrite into austenite and sigma phases.