New bonding method of cemented carbide tip and carbon steel with WC-(20-60mass%)Co alloy powder as thermal stress relaxation layer was proposed by authors. These joints were frequently fractured near the interface between sintered WC-Co alloy and steel. In this study, microstructure and strength of sintered bonding area between WC-40mass%Co alloy powder and 0.23-0.96mass%aC steels have been investigated. It is recognized that the reaction layer including tl phase of Co3W3C and Fe3W3C particles are formed inside the sintered WC-40mass%Co alloy near the interface. The thickness of reaction layer decreased with the increase in carbon content of steels and is zero for carbon content of 0.55mass% or more. Four-point bending strength, which is increased with the increase in C content of steels, is approximately 1.3GPa for 0.96mass%C. The strength is strongly affected by the existence of η phase.
