Analysis of Sintered Cemented Carbide Waste and Causes

Cemented carbide is a powder metallurgy product made of tungsten carbide micron powder of high hardness refractory metal as the main component, cobalt or nickel, molybdenum as binder, sintered in a vacuum furnace or hydrogen reduction furnace.

Sintering is a very critical step in cemented carbide. The so-called sintering is to heat the powder compact to a certain temperature, keep it for a certain time, and then cool it down to obtain the material with the required performance. The sintering process of cemented carbide is very complicated, and it is very easy to produce sintered waste if you don't pay attention.

So what is the reason for sintered waste alloy?

1. The first sintered waste of cemented carbide is peeling, that is, the surface of cemented carbide passes through the cracks on the edges, the shell or cracks, and in severe cases, a small thin skin like fish scales appears, cracks, and even powdering. The peeling is mainly due to the contact effect of cobalt in the compact, so that the carbon-containing gas decomposes free carbon in it, resulting in a decrease in the local strength of the compact, resulting in peeling.

2. The second most common pore in cemented carbide sintering waste is pores. Pores with a size of more than 40 microns are called pores. Factors that can cause blistering may form pores. In addition, when there are impurities in the sintered body that are not wetted by the molten metal, such as large pores such as "not pressed well", or there is a serious segregation of the solid phase and the liquid phase in the sintered body, pores may be formed.

3. The third most common pore in cemented carbide sintering waste is blistering. There are pores inside cemented carbide alloy products, and convex curved surfaces appear on the surface of the corresponding parts. This phenomenon is called blistering.
The main reason for the occurrence of blistering is that there is a relatively concentrated gas in the sintered body. There are usually two types: one is that the air gathers in the sintered body, and during the sintering shrinkage process, the air moves from the inside to the surface. If there are impurities of a certain size in the sintered body, such as alloy debris, iron chips, and cobalt chips, the air will concentrate here. When the liquid phase appears in the sintered body and it is densified, the air cannot be discharged, and blistering is formed on the surface of the sintered body with the least resistance.
The second is that there is a chemical reaction in the sintered body that generates a large amount of gas. When there are certain oxides in the sintered body, they are reduced and generate gas only after the liquid phase appears, which will cause the product to blister; WC-CO alloy is generally caused by the agglomeration of oxides in the mixture.

4. Uneven organization: Mixing

5. Deformation The irregular appearance change of the sintered body is called deformation. The main reasons for deformation are: uneven density distribution of the pressed block, because the density of the finished alloy is the same; serious local carbon deficiency in the sintered body, because the lack of carbon reduces the liquid phase relatively; unreasonable loading; uneven pad.

6. The loose organization area on the fracture of the blackheart alloy is called blackheart. The main reason: too low carbon content and inappropriately high carbon content. All factors that affect the carbon content of the sintered body will affect the formation of blackheart.

7. Cracks are also a common phenomenon of cemented carbide sintering waste. Suppression cracks: because the pressure relaxation does not appear immediately when the pressed block is dry, the elastic recovery is faster during sintering. Oxidation cracks: Because the briquette is partially severely oxidized when it is dried, and the thermal expansion of the oxidized part is different from that of the unoxidized part.

8. Overburning When the sintering temperature is too high or the insulation time is too long, the product will be overburned. Overburning of the product makes the grains coarser, the pores larger, and the alloy performance significantly reduced. The metallic luster of under-fired products is not obvious, and only re-burning is required.

Solution

Peeling: Improve the mixing and pressing process of the briquette to ensure uniform distribution of cobalt and carbon.
Holes: Reduce the introduction of impurities and avoid segregation of solid and liquid phases by optimizing the purity of raw materials and sintering process.
Blisters: Improve the sintering process, such as using vacuum sintering or controlling gas generation reactions to reduce gas aggregation.
Deformation: Ensure uniform density distribution of the briquette, reasonably design the boat and pad to avoid local carbon deficiency.
Black heart: Accurately control the carbon content to ensure that it is within the appropriate range.
Cracks: Optimize the drying process of the briquette, reduce oxidation, and ensure uniform distribution of pressure during sintering.

Other considerations for sintering alloys

Sintering temperature and time: Strictly control the sintering temperature and time to avoid over-burning or burn-through.
Raw material purity: Use high-purity raw materials to reduce the impact of impurities on the sintering process.
Sintering equipment: Regularly maintain the sintering equipment to ensure its normal operation and avoid sintering failure due to equipment failure.