Common Defects and Causes of Tungsten Carbide Sintering

Sintering refers to a process of converting powdery materials into dense alloy and is a very important step in the production process of cemented carbide. The tungsten carbide sintering process can be divided into four basic stages: removal of forming agent and pre-sintering stage, solid-phase sintering stage (800 ℃ - eutectic temperature), liquid phase sintering stage (eutectic temperature - sintering temperature), and cooling stage (sintering temperature - room temperature). However, because the sintering process is very complicated and the conditions are harsh, it is easy to produce defects and reduce the quality of the products. Common sintering defects and their causes are as follows:

1. Peeling

Cemented carbide with peeling defects is prone to burst to crack and chalk. The main reason for peeling is that the carbon-containing gas decomposes free carbon, resulting in a decrease in the local strength of the pressed products, resulting in peeling.

2. Pores

Pores refer to over 40 microns. The main reason for the generation of pores is that there are impurities in the sintered body that are not wetted by the solution metal, or there is serious segregation of the solid phase and the liquid phase, which may form pores.

3. Blistering

The blister will cause a convex surface on the cemented carbide, thereby reducing the performance of the tungsten carbide product. The main reasons for the formation of sintered bubbles are: 

    1) Air accumulates in the sintered body. During the process of sintering shrinkage, the sintered body appears liquid phase and densifies, which will prevent the air from being discharged, and then form slumped bubbles on the surface of the sintered body with the least resistance; 

    2) There is a chemical reaction that generates a large amount of gas in the sintered body, and the gas is concentrated in the sintered body, and the blister is naturally generated.

4. Deformation

The common deformation phenomena of cemented carbide are blister and concave. The main reasons for the deformation are uneven density distribution of the pressed compact. Severe carbon deficiency in the sintered body, unreasonable boat loading, and uneven backing plate.

5. Black center

The black center refers to the part with the loose organization on the alloy fracture. The main reason for black hearts is carburizing or decarburization.

6. Cracking

Crack is a relatively common phenomenon in the sintering process of cemented carbide. The main reasons for cracks are: 

    1) The pressure relaxation does not show immediately when the billet is dried, and the elastic recovery is faster during sintering; 

    2) The billet is partially oxidized when it is dried, and the thermal expansion of the oxidized part is different from that of the unoxidized part.

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