The WC-Co cemented carbides have become one of most important tool materials because of their excellent properties. But their application range is not very wide because they are limited in size, simple in shape, and high in cost, et al. Therefore, it is greatly significant to explore the new techniques to connect cemented carbides and steels.Based on the analyses of the limited connecting joints and detrimental multicarbides (called as η phases, hereafter) in diffusion welding, and the lower connecting strength and the lower using temperature of brazing, the tungsten-inert-gas (TIG) welding of WC-30Co cemented carbide and the carbon steels was systematically investigated by means of scanning electron microscope, electron probe, transmission electron microscope, X-rays diffraction, bending strength, and hardness tester, and the formation mechanism of η phases was explored in using Fe-Ni alloys as the filling materials. Furthermore, a new technique was put forward and realized via adding carbon to the Fe-Ni and Fe-Co filling materials to control η phase formation. Consequently, the joint strength problem was solved and the application foundation of this new technique was established.The detail results were described as followed.1. η phase quantity formed decreased with the increase of Ni content in the filling materials. The effect of C content in the base steels on η phase formation at the boundaries between WC-30Co and the weld was not distinct.2. The η phase formation could be decreased and restrained through adding C and Ni to the filling materials. When adding 0.6wt%C to the Fe-Ni alloys, the Fe content in the y phase in WC-30Co, which is necessary to form η phase, increase from 35at% to 61at%.3. The suitable C addition in the Fe-Co alloy as filling materials would also restrain the formation of n phases. At the same other processing condition, the hardness of the Fe-Co-C weld is higher than that of the Fe-Ni-C weld, but η phase formation was easier.4. Whether ηphases form at the interface between WC-Co and the weld is related to the concentration ratio of [W] to [C] in the binding phases, instead of C concentration. In the specimens welded with Fe-Co-C alloys as filling alloys, n phases form when the ratio of [W] to [C] is lower than 0.31, and η phases do not form when the ratio is higher than 0.37.5. n phases found in the experiment are multicarbides in Fe3W3C or Co3W3C type, but the ratio of atoms are not strictly consistent with M6C. n phases exited in two types of microstructure. One is coarse, and their sizes are smaller and smaller with the increase of the distance from the interface. This type of n phases is consisted in several grains commonly, and contains untransformed WC particles. Another dispersal nucleates independently in the binding phase in the semi-fused zones. And their average size is in the range of 0.05-0.1 micrometers.6. At the interface between cemented carbides and the welds, the temperature is high and th composition is suitable for n phase formation. The coarse n phases could nucleate at the different boundaries of solid WC grains in the semi-fused zones, or at the different sites on a same boundary. And then they would growth into Y and WC phases at same time. During their growth, they might accumulate together and growth with solid WC grains under the washing effect of thermal conduction and stirring of liquid metals to form big particles.7. Big n. phases accumulated at the boundaries between WC-30Co and the welds would result in the bending fracture surface along the interface and the obvious decrease of bending strength.8. The highest average designed non-ratio bending stress and strength reaches to 940Mpa and 1352Mpa, respectively, for the joints welded with the filling alloy of Fe-55wt%Ni-0.6 wt%C. The best microstructure and properties could be gotten.