Phase Transformation and Hot Working Studies on High-Al Fe-Al-Mn-C Ferritic Low-Density Steels

Studies on phase transformations in Fe-11Al-5Mn-1C (Alloy 1) and Fe-11Al-10Mn-1C (Alloy 2) ferritic low-density steels were carried out using differential thermal analysis (DTA) and dilatometry (all compositions in wt.%). An appreciable difference was observed between the phase transformation temperatures predicted by the ThermoCalc software and the experimental values. The ferrite + kappa carbide → austenite phase transformation started around 1017 °C for Alloy 1 and 960 °C for Alloy 2. The dissolution of carbides was completed around 1115 °C and 1050 °C for Alloy 1 and Alloy 2, respectively. Studies on water quenched alloys were carried out to confirm the phase transformation temperatures. Both the alloys were hot rolled at 1000 °C. The rolled alloys exhibited high yield and tensile strength due to the presence of a large volume fraction of kappa carbides. After hot rolling, Alloy 2 exhibited higher ductility than Alloy 1. This is attributed to the presence of a higher volume fraction of metastable austenite in Alloy 2. Reasons for the presence of metastable austenite are discussed. Fracture surfaces of the tensile samples revealed crack initiation and propagation along ferrite-kappa carbide interfaces.