Production of bulk cementite and its characterization
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Professor, Z.G. LIU, Research Associate, H. TAKAOKA and M. SAWAKAMI, Graduate Students, and K. TSUCHIYA, Associate Professor, are with the Department of Production Systems Engineering, Toyohashi University of Technology, Toyohashi 441-8580, Japan. K. MASUYAMA, Research Associate, is with the Department of Mechanical Engineering, Toyama National College of Technology, Toyama 939, Japan. Manuscript submitted September 19, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
to have more meaningful data to evaluate the properties of steels. Our recent endeavor has led to successful fabrication of bulk cementite by MA with subsequent SPS. Although MA had been applied to the Fe-C system several years ago,[13,14,15] the emphasis was not put on the synthesis of bulk cementite. Bulk cementite with dimensions of up to 15 ⫻ 10 mm has been produced in the present study. To our knowledge, this is the first produced bulk cementite. The basic mechanical and physical properties were measured and compared to the previous results of cementite produced by other methods. More intensive work on the effect of composition and of substitutional additions, such as Cr and Mn for Fe, is currently under way. The starting materials were elemental powders of Fe and graphite with a purity and particle size of Fe: 99.9 pct and ⬍100 m, and C: 99.9 pct and ⬍5 m, respectively. The powders were mixed at compositions of 75 at. pct Fe and 25 at. pct C. Mechanical alloying was performed on a conventional horizontal ball mill with a ball-to-powder weight ratio of 100:1. The maximum milling time was 18,000 ks. All the milling was carried out under an argon atmosphere. The resultant powders were then subjected to x-ray diffraction (XRD) analysis for phase determination. Thermal analysis of the milled powders was monitored in a Rigaku Thermo Plus 2 DSC 8230L, (Rigaku Denki Co., Tokyo, Japan) differential scanning calorimeter (DSC) at a heating rate of 20 K/ min up to 973 K under a flow of argon gas at 30 mL/min. Two continuous runs were carried out for the same sample, in which the first run was blanked by the second one to obtain the true DSC curve. Spark plasma sintering was used to produce compacts of the cementite.[16] The sintering was carried out in vacuum under a pressure of 50 MPa for 300 seconds at various temperatures, in which 1173 K was finally chosen as optimal. The density of the obtained compacts was measured by the Archimedes method. Subsequently, optical microscope (Nikon ME600, Tokyo, Japan), scanning electron microscope (SEM, JEOL* JEM-6300), and trans*JEOL is a trademark of Japan Electron Optics Ltd., Tokyo.
mission electron microscope (Hitachi H-800, Tokyo, Japan) were used for structural characterization. Compression tests were done in air using an Instron universal tester at a crosshead speed of 0.5 mm/min, at room temperature, 573 K, and 773 K. Hardness measurements were performed on an Akashi Co. (Tokyo, Japan) MVK-G1 micro-Vicker’s hardness tester with a 0.98 N load for 15 s and on a Nikon QM high-temperature hardness tester. Therm
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