Near-Net Shape Forming of Functionally Graded Cemented Carbide, Sinter-Bonded on Steel

Abstract

The cemented carbide coating on steel is required to have a harder and thicker coated layer to improve durability However, it is difficult to coat a hard and thick coated layer firmly, because it promotes cracks and peels the coated layer due to the thermal stress originated by the thermal expansion mismatch. To overcome this problem and to improve the property of hard material, we studied a new FGM of steel coated with cemented carbide layers. Cobalt compositionally graded cemented carbide powders stacked on steel and sinter-bonded to the steel by a new sintering process, using pressurizing and direct resistance heating by pulse current We manufactured the desired structure and confirmed the stress relief, high bonding strength and distribution of functionality of coated layers between the surface with high wear resistance and with high toughness inside. We applied this FGM to a stabilizer blade for a well drilling tool. Although the blade has a curved surface, we successfully sinter-bond graded cemented carbide layers on the near-net-shaped steel substrate with a uniformly graded structure. To fabricate the stabilizer, the blades were welded to steel. The FGM blades with-stood the heat cycle of welding and were confirmed to have welding performance equivalent to conventional blades. Because the entire surface is covered with graded cemented carbide layers, this new stabilizer is expected to have excellent performance compare to the conventional stabilizer whose blades have cemented carbide tips that are inserted partially.