Abstract
The solidification behaviors of laboratory cast austenitic SS2343 stainless steel in terms of the volume fraction of δ-ferrite in the as-cast state and its transformation after subsequent annealing were investigated. Monitoring of morphological transformations of δ-ferrite in the microstructure show the progress of δ-ferrite dissolution. Annealing tests were conducted at 1050 °C, 1150 °C and 1250 °C with soaking times of 5 and 40 min. The thermodynamic prediction and metallographic identification of δ-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 δ-ferrite decreased with an increase in temperature and the time of annealing. About 50–55% the δ-ferrite was effectively transformed. The δ-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 δ-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.
Highlights
Austenitic stainless steels have numerous applications since they have a good combination of properties, such as corrosion and oxidation resistance, toughness, weldability, and mechanical strength at low and high temperatures
The properties and performance of austenitic stainless steels are closely related to their microstructures, especially the amount and distribution of delta-(δ–)ferrite, which, in the case of castings, depend mainly on the chemical composition and the cooling rate during solidification [1,2,3,4,5,6,7]
Expected to be fully austenitic at room temperature, the as-cast microstructure of austenitic stainless steels generally contains a small amount of residual δ–ferrite
Summary
Austenitic stainless steels have numerous applications since they have a good combination of properties, such as corrosion and oxidation resistance, toughness, weldability, and mechanical strength at low and high temperatures. The properties and performance of austenitic stainless steels are closely related to their microstructures, especially the amount and distribution of delta-(δ–)ferrite, which, in the case of castings, depend mainly on the chemical composition and the cooling rate during solidification [1,2,3,4,5,6,7]. Expected to be fully austenitic at room temperature, the as-cast microstructure of austenitic stainless steels generally contains a small amount of residual δ–ferrite. The presence of this δ–ferrite is detrimental to the hot ductility and can lead to cracks appearing either at the edges of the hot-rolled product as open cracks or on the surfaces of the finished product as silver defects containing oxides [4]. In order to predict the solidification sequence for the Fe-Cr-Ni ternary system using chromium and nickel equivalents, the Creq and Nieq creativecommons.org/licenses/by/
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