Influence of Aluminum on Microstructure and Mechanical Properties of Air‐Hardened Medium Manganese Steel

The influence of different aluminum concentrations (0 and 0.25 wt%) on the mechanical properties and the microstructure of recently developed air‐hardening ductile forging steels with 4 wt% manganese is presented together with the effects of additional heat treatments after air cooling. Based on the achieved mechanical properties, the necessity of aluminum alloying is evaluated due to the processing challenges associated with the Fe–Mn–Al–Si system. For this matter, tensile properties, hardness, and Charpy impact toughness areevaluated with respect to microstructural features characterized by light optical microscopy and scanning electron microscopy equipped with electron backscatter diffraction (EBSD). It is demonstrated that the intermediate addition is not sufficient to reach the target values for the impact toughness, which demonstrates the importance of the addition to this steel grade. Additionally, insights on the tempering embrittlement are gained, as it can be shown that 0.25 wt% Al is sufficient to shift the tempering embrittlement from 300 to 350 °C. Finally, potential intercritical annealing temperatures can be specified: while high balances of strength and ductility can be reported at 650 and 675 °C, an exceeding of this temperature leads to insufficient austenite stabilization and consecutively martensite formation during cooling , as demonstrated by EBSD and X‐ray diffraction.