Common Defects and Causes of Cemented Carbide Sintering

Cemented carbide is a composite material made of high-hardness refractory metal carbides (such as tungsten carbide WC, titanium carbide TIC, etc.) as a matrix and sintered with metal binders (such as cobalt CO, nickel Ni, etc.) under high temperature and high pressure. This material has high hardness and good wear resistance and is widely used in many fields.

The sintering process of cemented carbide mainly includes the following stages:

Removal of forming agent and pre-sintering stage:

In this stage, the forming agent is gradually decomposed or vaporized and removed from the sintered body. At the same time, the addition of the forming agent may add carbon to the sintered body, and the amount of carbon addition varies with the type, quantity and sintering process of the forming agent.

The oxides on the surface of the powder are reduced, and hydrogen can reduce the oxides of cobalt and tungsten. When the forming agent is removed and sintering is carried out under vacuum conditions, the carbon-oxygen reaction is not strong.

The contact stress between the powder particles is gradually eliminated, the bonding metal powder begins to recover and recrystallize, surface diffusion begins to occur, and the strength of the compact is improved.

Solid phase sintering stage (800℃--eutectic temperature):

At the temperature before the liquid phase appears, the process of the previous stage continues, the solid phase reaction and diffusion intensify, the plastic flow is enhanced, and the sintered body shrinks significantly.

Liquid phase sintering stage (eutectic temperature--sintering temperature):

When the liquid phase appears in the sintered body, the shrinkage is completed quickly, followed by a crystallization transformation to form the basic organization and structure of the alloy.

Cooling stage (sintering temperature--room temperature):

The organization and phase composition of the alloy undergo certain changes with different cooling conditions. This feature can be used to heat treat the cemented carbide to improve its physical and mechanical properties.

These stages jointly determine the final performance and structure of the cemented carbide and are key steps in the preparation process of cemented carbide.

Sintering refers to a process of transforming powdered materials into dense bodies, which is a very important step in the production process of cemented carbide. The sintering process of cemented carbide 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); cooling stage (sintering temperature--room temperature). However, due to the complexity of the sintering process and the harsh conditions, it is easy to produce defects and reduce the quality of the product if you don't pay attention.

Common sintering defects and causes are as follows:

1. Peeling Cemented carbide with peeling defects is prone to bursting and powdering. The main reason for peeling is that the contact effect of cobalt in the compact causes the carbon-containing gas to decompose free carbon in it, resulting in a decrease in the local strength of the compact, resulting in peeling.

2. Holes Holes refer to pores with a size of more than 40 microns. The main reason for the formation of holes is that there are impurities in the sintered body that are not wetted by the molten metal or there is a serious segregation of the solid phase and the liquid phase, which may form holes.

3. Blisters Blisters will cause raised curved surfaces on the surface of cemented carbide, thereby reducing the performance of the product. The main reasons for blisters are:

1) Air gathers in the sintered body. During the sintering shrinkage process, the sintered body appears liquid phase and densification, which will cause the air to be unable to be discharged, and then form blisters on the surface of the sintered body with the least resistance; 2) There is a chemical reaction in the sintered body that generates a large amount of gas. The gas is concentrated in the sintered body, and blisters are naturally generated.

4. Deformation Common deformation phenomena of cemented carbide include blisters and concavities. The main reasons for deformation are: uneven density distribution of the block; serious local carbon deficiency in the sintered body; unreasonable loading of the boat; uneven pad.

5. Black heart Black heart refers to the part with loose structure on the alloy fracture. The main reason for black heart is: too low or too high carbon content.

6. Cracks Cracks are a common phenomenon in the sintering process of cemented carbide. The main reasons for the cracks are:

(1) Pressure relaxation does not appear immediately when the compact is drying, and elasticity recovers quickly during sintering;

(2) Part of the compact is severely oxidized when it is drying, and the thermal expansion of the oxidized area is different from that of the unoxidized area.