Effect of milling speed on an Fe–C–Mn system alloy prepared by mechanical alloying

Mechanical alloying (MA) of elemental iron, graphite and manganese powders consisting of Fe–7 mass% C–6 mass% Mn were carried out by using a planetary ball mill at different milling speed. The structural evolution of the MAed powders was characterized by X-ray diffraction (XRD), differential thermal analysis (DTA) and scanning electron microscopy (SEM). The MAed powders were hot isostatic pressed (HIPed) at 100 MPa for 1 h at lower (853 K) or higher (1053 K) than the eutectoid transformation temperature A1. The processes of amorphization, super-saturated solid solution and particle refinement are influenced by the milling speed. These processes are faster with the increase in the milling speed. The hardness and microstructure of the HIPed compacts are influenced by the morphology of the MAed powders obtained. The powders MAed at high speed has higher hardness at the HIPed compacts than the powders MAed at low speed. Since the sintering progresses well at a temperature higher than A1 and the phase transformation of γ-Fe into fine carbide and α-Fe occurs during cooling process, the hardness of the compacts HIPed at 1053 K higher than A1 is greater than that at 853 K lower than A1.