Papers by Author: Kyu Hong Lee

Abstract: The present study is concerned with the improvement of hardness and wear resistance in (Cr3C2,CrB)/carbon steel surface composites fabricated by high-energy electron beam irradiation. Two kinds of powder mixtures, 50Cr3C2-50STS304 and 50CrB-50STS304 (wt.%), were placed on a plain carbon steel substrate, which was then irradiated with electron beam. The surface composite layer of 1.0~1.3 mm in thickness was successfully formed without defects, and contained a large amount (up to 58 vol.%) of Cr7C3 or Cr1.65Fe0.35B0.9 particles in the austenite or martensite matrix. The hardness and wear resistance of the surface composites were 2~3 times higher than those of the steel substrate according to hard particles. Particularly, the surface composite fabricated with CrB powders showed excellent wear resistance because selective wear of the matrix was considerably reduced.

Abstract: The present study aims at investigating the effects of microstructure on fracture toughness of two A356 Al alloys. These A356 alloys were fabricated by casting processes such as rheo-casting and casting-forging, and their mechanical properties and fracture toughness were analyzed in relation with microfracture mechanisms. All the cast A356 alloys contained eutectic Si particles mainly segregated along solidification cells, and the distribution of Si particles was modified by the casting-forging process. Microfracture observation results revealed that eutectic Si particles segregated along cells were cracked first, but that Al matrix played a role in blocking crack propagation. Tensile properties and fracture toughness of the cast-forged alloys having homogeneous distribution of eutectic Si particles were superior to those of the rheo-cast alloy.

Abstract: This study aims at correlating microstructure with hardness and corrosion resistance of surface alloyed materials fabricated with Fe-based metamorphic powders by an accelerated electron beam irradiation method. The surface alloyed materials contained 48 vol.% of hard Cr2B crystalline phases in the Cr0.19Fe0.7Ni0.11 matrix, and thus its hardness was 2.5 times greater than that of the steel substrate. The corrosion resistance of the surface alloyed materials was better than that of an STS304 stainless steel or coatings fabricated by high-velocity oxygen fuel spraying of Fe-based metamorphic powders because the Cr0.19Fe0.7Ni0.11 matrix of the surface alloyed layers and coating was selectively corroded, while Cr2B borides were retained inside pits. These findings suggested that the fabricated surface alloyed materials presented good application possibilities as excellent wear- and corrosion-resistant materials.