Production of Rapidly Solidified Fe-C-Si Alloy Powder by the Rotating-Water-Atomization Process and Its Structure

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Abstract
The rotating-water-atomization process (RWAP) developed by the authors was applied to produce rapidly solidified powder of a large number of ternary Fe-C-Si alloys with 0.42 to 4.8 mass% C and 0 to 7.15 mass% Si. The particle shape was not spherical unless Si was free. The particle size was distributed in logarithmic normal and the mean particle diameter was in the range of 100 μm and 150 μm. Total oxygen content of powder was about 1 500 ppm for Si free alloys and it drastically decreased with increasing Si content. It was about 400 ppm for alloys with 2 mass% Si. Rapidly solidified particles consisted of ferrite(δ), cementite, austenite and martensite which were achieved by conventional quenching techniques. In addition to the above, metastable ε (hcp) and new X1 and X2 phases were also formed at high carbon and silicon contents. Crystallographic structures of X1 and X2 were not yet identified. The DSC curves of X1 and X2 were quite similar to those of ε and austenite, respectively inspite of having different crystal structures. The formation of these phases depended on the cooling rate. In a Fe-2.79C-5.06Si alloy, austenite and ε phases were dominant at higher cooling rate, on the other hand at lower cooling rate X2 were preferentially formed. A large exothermic reaction of 84 J/g due to the decomposition of austenite to ferrite and cementite was observed upon heating Fe-1.89C-2.97Si powder.