Fabricating FE-base alloys containing rapidly solidified eutectic cementite and ultra-fine graphite by heat treating laser-melted ductile iron

Abstract

Laser surface remelting has been utilized to improve the abrasion resistance of ductile irons by generating a rapidly solidified ultra-fine ledeburite surface layer. However, this significant improvement in abrasion resistance is at the complete expense of the excellent tribological and damping properties of the graphite phase which are equally as important as the abrasive wear properties in some service conditions. Therefore, it would be attractive if some fine graphite particles could be incorporated into the laser rapidly solidified ledeburite structure, fabricating a novel iron microstructure containing both the as-rapidly solidified eutectic cementite and the fine graphite particles. This iron-base alloy will, no doubt, have the combination of the excellent abrasive wear resistance of the rapidly solidified eutectic Fe{sub 3}C carbide or cementite, and the outstanding self-lubricating, heat conducting and damping properties of the graphite phase. Unfortunately, it is impossible to fabricate the novel material by rapid solidification processing alone, because following rapid solidification the melt solidifies according to the metastable Fe-Fe{sub 3}C system, no longer forming a graphite phase in the rapidly solidified iron structure. However, the laser melted structure of ductile iron is, from the thermodynamics point of view, highly unstable. This highly metastable nature of the structure maymore » provide the authors the opportunity to fabricate the above mentioned novel microstructure by partially graphitizing the cementite by appropriate subsequent thermal treatment. In the present paper, a laser melted ductile cast iron was heat treated by conventional heat treating method. The novel Fe-base alloy containing both the as-rapidly solidified eutectic cementite and the ultra-fine graphite was successfully fabricated as a consequence of the partial cementite-graphite phase transformation.« less