Correlation of Delta-Ferrite with Tensile and Charpy Impact Properties of Austenitic Fe-23Mn-Al-C Steels

The correlation of delta-ferrite with the tensile and Charpy impact properties of austenitic Fe-23Mn-Al-C steels with different Al and C contents for cryogenic applications was investigated in this study. Microstructural analysis showed that the two steels with duplex phases had banded or layered microstructures with relatively fine-grained austenite and coarse-grained delta-ferrite which increased with Al content and decreased with C content. The geometrically necessary dislocation density increased with the fraction of the delta-ferrite phase. The steel with the highest delta-ferrite fraction of 26.6 pct had the highest yield strength of 576 MPa, while that with the delta-ferrite phase of less than 10.0 pct exhibited a mixed deformation behavior comprising deformation twins and dislocation glide which contributed to the highest tensile strength of 802 MPa and total elongation of 43.6 pct owing to an increase in the work hardening rate. In contrast, the Charpy impact test results revealed that the steels with duplex phases of austenite and delta-ferrite exhibited ductile-to-brittle transition behavior, whereas the steel with single-phase austenite had a higher absorbed energy exceeding 100 J at − 196 °C. The delta-ferrite phase had no significant effect on the absorbed energy at room temperature, but it acted as the main site for the propagation of brittle cracks at cryogenic temperatures and thus decreased the low-temperature toughness.