Microstructures of Fe-C system hard alloys produced from mechanically alloyed powders

Fe-C system hard alloys were produced from mechanically alloyed powders and their properties were examined by means of hardness testing and microstructural examinations. Mixtures of iron and graphite powders(Fe-2-6mass%C) were mechanically alloyed with vibration-ball-mill in Ar atmosphere. MA powders obtained were packed in stainless steel tubes and then sealed in vacuum after degassing at 873K. The stainless steel tubes were hot-rolled in the temperature range of 923K-1273K to consolidate MA powders. Bulk materials were subjected to isothermal annealing at 1273K, followed by water quenching. After the ball-milling of 360ks, extremely large deformation energy was stored within powder particles. This leads to the formation of ultra-fine α-Fe grain and very fine dispersion of graphite. On heating to a consolidation temperature, the reaction forming Fe₃C takes place at around 600K. MA powders were completely densified to full density by hot-rolling at 1123K where the structure is of (Fe₃C+γ Fe) two-phase, and this consolidation process does not accompany significant coarsening of the microstructure. In a case of an Fe-6mass%C MA bulk material, the structure at room temperature is composed of 80vol% Fe₃C and 20vol%α-Fe, so that hardness of a bulk material reaches to Hv900. But, Fe₃C is not so stable at elevated temperatures that it easily decomposes to Fe and graphite after long annealing.