Papers by Author: Wan Jae Lee

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Authors: Hoo Soon Im, Jah Mahn Hur, Wan Jae Lee
Abstract: The dry-milling technique was used for mixing and crushing oxides and graphite powder to get homogeneous mixed powders. The weight ratio of ball-to-powder was 30:1 and argon gas was filled in jar. The carbon content was more 10~ 20wt% than the stoichiometric amount. The drymilling was carried for 20 hours. After milling, the mixed powders were reduced and carburized at 900~980°C for 3 hours flowing Ar gas in tube furnace. The dry-milled powders showed wide diffraction patterns of X-ray. The reactions of reduction and carburization were completed in 3 hours at 980°C. After the reactions, the mean size of WC particles was about 200 nm. The content of free carbon in WC/Co mixed powders decreased as the reaction temperature increased.
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Authors: Hoo Soon Im, Jah Mahn Hur, Wan Jae Lee
Abstract: Nano-sized WC particles in WC/Co composite powders were synthesized by mechanochemical method. The raw powders(WO3, Co3O4, VC, Cr3C2 and graphite) were mixed by planetary milling with ethyl alcohol for 30 hours. The compositions were WC-10 and -20 wt% Co after reduction and carburization. The powders of VC and Cr3C2 were added as the inhibitors to inhibit the formation of nano-sized WC particles. The direct reduction and carburization of the oxides with carbon were carried at 900°C for 1 to 3 hours under flowing Ar gas. The resultant powders were compacted and sintered at 1300~1360°C for 0.5 hour. The degree of the direct reduction and carburization of the mixed powders increased with the reaction time. The mean size of WC particles in WC/Co composite powders was about 16 nm, and the mean size of WC/Co composite powders was about 240 nm. In WC-10 and -20 wt% Co, the sintered compacts shrank by 16% and 24%, respectively. The mean sizes of WC in WC/Co composite powders were about 50 and 30 nm.
1177
Authors: Dong Ryoul Lee, Wan Jae Lee
Abstract: Direct reduction and carburization process was thought one of the best methods to make nano-sized WC powder. The oxide powders were mixed with graphite powder by ball milling in the compositions of WC-5,-10wt%Co. The mean size of mixed powders was about 250 nm after milling. The mixture was heated at the temperatures of 600~800°C for 5 hours in Ar. The reaction time of the reduction and carburization was decreased as heating temperatures and cobalt content increased. The mean size of WC/Co composite powders was about 260 nm after the reactions. And the mean size of WC grains in WC/Co composite powder was about 38 nm after the reaction at 800°C for 5 hours.
1185
Authors: Sun Yong Park, Wan Jae Lee
Abstract: To improve the mechanical properties of WC-Co cemented carbides, a dual composite was studied. The compositions of granule and matrix were nano-sized WC-6 wt% Co(granule) and normal sized WC-20 wt% Co(matrix), respectively. The granules were made by spray-drying method and sintered at 1380°C for 10 minutes in vacuum. After sieving, the granules were grouped to 50, 100 and 150 ㎛ and mixed with WC and Co powders as the volume fractions of granule to matrix were 50 to 50, 40 to 60 and 30 to 70. These compacts were sintered at 1380°C for 10 minutes in vacuum. The microstructure, transverse rupture strength and wear resistance were investigated. The relative densities of the dual composites were about 98% after sintering. The mean size of the WC grains in the dual composite was about 300~400 nm. As the volume fraction of the matrix in the dual composite increased, the transverse rupture strength increased and hardness decreased. The wear resistance of the dual composite increased with decreasing matrix volume fraction and increasing granule size.
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