Microstructure Control of Fe-25mass%Cr-(1.5-1.8)mass%C Alloys by Mechanical Alloying

Microstructures and mechanical properties were investigated for Fe-25mass%Cr-(1.5-1.8)mass%C alloys produced by mechanical alloying. Mixtures of iron , chromium and graphite powders were ball-milled in Ar atmosphere and then hot-pressed at 1273K in vacuum. Bulk materials obtained were subjected to annealing in temperature range of 873K-1473K, followed by air-cooling or water quenching. In Fe-25mass%Cr-(1.5-1.8)mass%C powder mixtures, nano-sized ferrite(αFe) grains are formed within powder particles after the ball-milling of 720ks. In powder with 720ks milling, M₂₃C₆ carbide finely precipitates during heating to the consolidation temperature, and this results in the formation of microduplex structure of (αFe+M₂₃C₆). Microstructure of the alloys are very sensitive to heat treatment conditions: Matrix is almost fully austenitic in annealing at around 1473K, while is martensitic in annealing at around 1273K. In the martensite structure formed in fine austenite grains of 1-2μm, it is characteristic that each prior austenite grain consists of only one set of laths with the same variant. The bending strength of MA bulk materials is much higher than that of materials produced by ingot-metallurgy process due to having the microduplex structures.