Kappa Carbide Precipitation in Duplex Fe-Al-Mn-Ni-C Low-Density Steel

The microstructural evolution of a Fe-Mn-Al-Ni-C low-density steel was studied. The lightweight low-density steels are a promising material for the transportation industry, due to their good mechanical properties and low density. The base microstructure of the investigated steel consists of ferrite and austenite. Thermo-Calc calculations showed the formation of an ordered BCC (body-centred cubic) B2 phase below 1181 °C and kappa carbides below 864 °C. The steel was produced in a vacuum induction furnace, cast into ingots and hot forged into bars. The forged bars were solution annealed and then isothermally annealed at 350, 450, 550, 650, 750, and 850 °C. The microstructure of the as-cast state, the hot forged state, solution annealed, and isothermally annealed were investigated by optical microscopy and scanning electron microscopy. The results showed the formation of kappa carbides and the ordered B2 phase. The kappa carbides appeared in the as-cast sample and at the grain boundaries of the isothermally annealed samples. At 550 °C, the kappa carbides began to form in the austenite phase and coarsened with increasing temperature.

Evolution of Microstructure during Isothermal Treatments of a Duplex-Austenitic 0.66C11.4Mn.9.9Al Low-Density Forging Steel and Effect on the Mechanical Properties

In the last decades, low-density steels for forging have increasing interest in the automotive industry, and good mechanical properties are required for their real application. This paper describes the results obtained for a 0.66C11.4Mn9.9Al duplex austenitic low-density steel after applying a set of isothermal treatments at different combinations of time and temperature, aimed to promote kappa carbide precipitation, and improve the mechanical properties obtained with a water quenching treatment. The effects of the different isothermal treatments on the microstructure and on the mechanical properties have been analyzed and compared to those obtained from a quenching heat treatment. We found that isothermal treatments in the range temperature between 550–750 °C promoted the profuse precipitation of coarse kappa carbides at grain boundaries, which dramatically reduced the ductility of the alloy, whereas a traditional quenching treatment resulted in a better combination of ductility and mechanical strength.

Superplastic behavior of a kappa carbide material (Fe3AlCx)

Fine-grained kappa carbide (Fe3AlCx) materials, containing 12.5 and 14% Al, and 3.5% C, were prepared by powder processing and hipping procedures. The creep behavior of the kappa materials was shown to be identical to that observed in superplastic iron carbide, and was shown to follow a grain boundary–diffusioncontrolled grain boundary sliding relation. The tensile fracture strains in kappa, however, were shown to be considerably less than in iron carbide with a maximum elongation of 92% noted. This difference is attributed to either a low stress intensity factor or to contamination of the powder surface in the kappa material. The compression creep strength, at a given strain rate, was shown to be about two times higher than the tension creep strength.