Synthesis of titanium and tungsten carbides in iron matrices

Abstract

Carbides, TiC and WC, having high melting points and high hardness, are effective reinforcements in steel and iron. In fact, their steel and iron matrix composites have been commercially applied as wear resistant materials. Usually, these composites are produced by a powder metallurgy route, which is complex and costly [1, 2]. Undoubtedly, cost effective production of these composites will extend their commercial application. Stir casting process is attractive in economy and simplicity, and is widely employed in production of metal matrix composites with low melting points such as aluminum and magnesium etc. [3]. But, it is hardly applied to steel matrix composites because of their high melting points. Recently, in situ production of metal matrix composites has received much interest worldwide. Its eminent advantage is that it eliminates interface incompatibility of matrices with reinforcements by creating more thermodynamically stable reinforcements based on their nucleation and growth from the parent matrix phase. The technique was exploited to synthesize TiC reinforcing steel and iron matrix composites [4, 5]. Particulate TiC was successfully synthesized in liquid iron by addition of ferrotitanium or titanium filing to ironcarbon alloy melts. In situ formed TiC reinforcements are spherulitic and distributed uniformly in the matrices. The result is encouraging. However, it should be noted that there is great difference in density between TiC and iron alloy melt. Relatively light TiC particulates may float up during a large-scale production resulting in their uneven distribution in casting. On the other hand, no work was conducted on in situ synthesis of WC, an important reinforcement, in iron melt. Different from TiC, WC is heavier than iron. Therefore, it may sink down in the melt. But, (TiW)C phase, whose density is between those of TiC and WC, may be more prospective candidate for reinforcements synthesized in situ in steel and iron melts. The present work tried to synthesize in situ TiC, WC and (TiW)C in iron matrices, respectively, and examined their microstructural characteristics. It is expected that the preliminary results can be significant in promoting the development of in situ synthesized steel and iron matrix composites. Commercial powders of titanium, tungsten, graphite and iron were used for the preparation of the batches. The batches, Fe-16wt%Ti-4wt%C, Fe-18.8wt%W1.2wt%C and Fe-8wt%Ti-9.4wt%W-2.6wt%C, respectively, were prepared to obtain 20 wt% TiC-Fe, 20 wt%