γ Decomposition in Fe–Mn–Al–C lightweight steels

Abstract

In the last few decades the steelmaking industry has been focusing on lightweight steel development and the Fe–Mn–Al–C system has been put under the spotlight. In fact, this quaternary alloy has been shown to be able to lower common steel density by more than 20 wt.%. In addition, these steels may also be suitable for cryogenic applications, corrosion and high-temperature oxidation resistance and wear resistance. Austenite plays an important role in Fe–Mn–Al–C steels because of its possible evolutions. Each configuration derived from γ reaction has its own specific properties and/or drawbacks, which need to be mastered in order to develop valuable products. The aim of this paper is to study the different austenite decomposition reactions at different temperatures for different chemical compositions: the microstructures obtained have been characterized by means of optical and SEM microscopy with the support of Vickers macro-hardness tests, SEM-EDS and phase volume fraction diagrams. Results have made it possible to characterize four different γ transformations. Limiting conditions for triggering each reaction have been established, in terms of the chemical composition driving force, thermal energy input and thermodynamic stability of austenite. Discontinuous precipitation occurred at 600 °C and in a medium Mn, high Al and high C combination. Cellular transformation developed at 800 °C annealing between 9–12% Al. For 1% C spinodal decomposition was triggered at the expense of cellular transformation, as far as austenite stability is influenced by the κ-carbide driving force as well.