κ-carbide hardening in a low-density high-Al high-Mn multiphase steel

κ-carbide hardening in a ferrite-austenite duplex low-density FeMnAlC steel has been investigated as a function of annealing processing within the temperature range 500−900 o C. Below 600 o C, a high hardness can be attained by the fine microstructures, e.g. the thin lamellar κ-carbides in austenite and dispersed nanosized particulate κ-carbides in ferrite. After further annealing, the alloy softened rapidly, and its hardness deteriorated drastically due to the formation of boundary κ-carbides composed of discontinuously coarsened κcarbides and spheroidized κ-carbides. Interestingly, all κ-carbides within ferrite were transformed into austenite by the κ+α→γ transformation at 800 o C, while the κ-carbides within austenite were dissolved at 900 o C. The κ-carbides are found to have a close to Nishiyama-Wasserman orientation relationship with the ferrite matrix.