Microstructure and mechanical properties of multiaxially forged Fe–30Mn–9Al–0.8C and Fe–20Mn–10Al–1C low-density steels

Abstract

This article investigates the mechanical properties of Fe–30Mn–9Al–0.8C and Fe–20Mn–10Al–1C low-density steels before and after multiaxial forging (MAF). The selected compositions of Fe–Mn–Al–C steel possess numerous industrial applications due to its high strength-to-weight ratio with high ductility. The steel workpieces are multiaxially forged (MAFed) at 250°C until they break and are studied for their microstructural evolutions and mechanical properties. Grain size and dislocation density measurement, tensile test, and hardness test have been carried out for selected MAF pass with an equivalent strain of 0.46 in each forging pass. The results show a comparable decrease in grain size and a significant improvement in tensile strength and hardness. The yield and ultimate strengths increase 4 times and 2 times, respectively, in Fe–30Mn–9Al–0.8C steel while 2.4 times and 1.8 times, respectively, in Fe–20Mn–10Al–1C steel. X-ray diffraction (XRD) results show distinct peak broadening and peak intensity of the MAFed samples due to a decrease in crystallite size and an increase in micro-strain. Both grain boundary strengthening and a rise in dislocation density are found to be responsible for increased hardness and tensile strength. After the tensile tests, the fracture surfaces reveal a decrease in dimple size and exhibit cleavage features with an increase in MAF pass.