Welding characteristics of cemented carbide-2

       

General weldability characteristics

Cemented carbide contains a higher content of carbides and alloying elements. Although welding can be carried out, it is prone to hardened structure and cracks during welding. Effective process measures must be taken to obtain satisfactory welded joints. At present, the commonly used welding methods for welding hard alloy and steel in production include oxygen-acetylene flame brazing, vacuum brazing, arc welding, inert gas shielded welding, friction welding, plasma arc welding, vacuum diffusion welding and electron beam welding. Carbide and steel have the following characteristics when welding.

① The relationship between linear expansion coefficient and brazing cracks The size of cemented carbide is relatively small, and it is generally fixed on a relatively thick steel support material. Brazing is an effective welding method to connect cemented carbide and base metal. The linear expansion coefficient of cemented carbide (4.1～7.0×10-6/℃) is very different from the linear expansion coefficient of ordinary steel (12×10-6℃-1), and cemented carbide is only 1/3 of steel ~ 1/2. Both cemented carbide and steel expand freely when heated, but the shrinkage of steel is much greater than that of cemented carbide when cooled. At this time, the weld is under pressure, and the hard alloy surface is under tensile stress. If the residual stress is greater than the tensile strength of the cemented carbide, cracks may occur on the surface of the cemented carbide. This is one of the most important causes of cracks in hard alloy brazing.

② The relationship between hardness and crack sensitivity The hardness of cemented carbide is proportional to wear resistance and welding crack sensitivity. The higher the hardness of cemented carbide, the greater the possibility of cracks during brazing. Moreover, the cemented carbide used for general finishing or super finishing is prone to cracks during brazing. According to the hardness and strength of different grades of cemented carbide, the welding crack susceptibility of cemented carbide can be judged. The order from poor to good is as follows.

YG category YG3X, YG3, YG4, YG6X, YG6, YG8, YG11, YG15

YT type YT60, YT30, YW1, YT15 (YW2), YT14, YT5

The above two types of cemented carbides, from left to right, show that the hardness and wear resistance gradually decrease, while the strength and toughness increase, and the possibility of brazing cracks decreases.

③ Influence of welding residual stress. The residual stress in the welding area is a potential hazard. Although cracks may not be found on the cemented carbide workpiece immediately after welding, it is easy to produce cracks in the subsequent sharpening, storage or use process. Cause the tool to be scrapped. When the brazing area of the cemented carbide is larger, the residual welding stress generated is greater, and the possibility of cracks is greater.

When welding hard alloy tools, the welding residual stress must be minimized. Measures must be taken to reduce the brazing stress during welding. Measures such as lowering the brazing temperature, preheating and slow cooling before welding, selecting brazing filler metals with good plasticity, adding compensation gaskets, and improving the joint structure can be taken. When brazing large-area cemented carbide, no matter the strength, special measures should be taken to reduce welding stress and prevent cracks.

④ Oxidation problem When the cemented carbide is heated to above 800°C in the air, the surface of the cemented carbide begins to oxidize, forming a loose oxide layer, accompanied by decarburization. When heated to 950-1100°C, the surface layer will undergo rapid oxidation, and the formed oxide film will make the cemented carbide brittle and reduce the mechanical properties. The presence of surface oxide layer also reduces the strength and hardness of the weld. Taking measures during welding to minimize the oxidation of hard alloy welding parts is an important measure to improve welding quality.