The surface carburization of tungsten heavy alloys has been studied by packing the sintered alloys with carbon black powder at various temperatures and for various times. When carburization temperature increased, the thickness of the WC layer formed increased and the surface hardness increased up to three times that of an untreated alloy. In all the specimens with a WC layer, an additional eta(η)-phase layer and a carbon segregation layer formed below the WC layer. The eta-phase formed during the formation of WC particles around tungsten particles and kept the surface hardness high. When carburization temperature or time increased, impact energy of heavy alloys decreased considerably by the carbon segregation effect. The formation of microcracks in thick WC layer as well as the carbon segregation were able to be suppressed by reducing carburization temperature. On carburizing at 600℃, the surface hardness increased by about 40% with reduction of impact energy only by about 15\%. Carburization at 800℃, on the other hand, resulted in the increase in hardness by about 80% with reduction of impact energy by about 80%.
