Why is cemented carbide sintering waste produced?

Cemented carbide is a powder metallurgy product sintered in vacuum furnace or hydrogen reduction furnace with tungsten carbide micron powder of high hardness refractory metal as the main component and cobalt, nickel and molybdenum as the binder. Sintering is a very key step of cemented carbide. The so-called sintering is to heat the powder compact to a certain temperature, maintain it for a certain time, and then cool down, so as to obtain the materials with the required properties. The sintering process of cemented carbide is very complex. If you don't pay attention to it, it is very easy to produce sintering waste. Today we will share the common sintering waste and its causes.

1. The first problem of cemented carbide sintering waste is peeling

That is, the surface of the cemented carbide is cracked, warped or cracked through the edges, and in serious cases, there are fish scale small thin skin, burst crack, or even pulverization. Peeling is mainly due to the contact effect of cobalt in the compacts, which causes the carbon containing gas to decompose and dissociate carbon, resulting in the decrease of local strength of the compacts, resulting in peeling.

2. The second common problem of cemented carbide sintering waste is hole

Pores with more than 40 microns are called pores. The factors that can cause blister may form holes. In addition, holes may be formed when there are impurities in the sintered body that are not wetted by the soluble metal, such as large holes such as "not pressed well", or there are serious solid-phase and liquid-phase segregation in the sintered body.

3. The third common problem of cemented carbide sintering waste is blister

There are holes in cemented carbide products, and convex curved surfaces appear on the surface of corresponding parts. This phenomenon is called blister. The main reason for blister is that there is relatively concentrated gas in the sintered body. There are usually two kinds: one is that air accumulates in the sintering body. During the sintering shrinkage process, the air moves from the inside to the surface. If there are impurities with a certain size in the sintering body, such as alloy chips, iron chips and cobalt chips, the air will concentrate here. When the liquid phase appears in the sintering body and is densified, the air cannot be discharged, and blisters will be formed on the surface with the least resistance of the sintering body.

The second is that there is a chemical reaction to generate a large amount of gas in the sintering body. When there are some oxides in the sintering body, they are reduced to generate gas after the liquid phase appears, which will make the product Blister; WC-Co alloy is generally caused by oxide agglomeration in the mixture.

4. There is also uneven organization: mixing

5. Another is deformation

The irregular shape change of the sintered body is called deformation. The main causes of deformation are: the density distribution of compacts is uneven, because the density of alloy finished products is the same; The sintered body is seriously short of carbon locally, because the lack of carbon reduces the liquid phase relatively; Unreasonable loading; The backing plate is uneven.

6. Black heart

The loose area on the alloy fracture is called black core. Main reasons: low carbon content and inappropriate high carbon content. Various factors affecting the carbon content of sintered body will affect the formation of black core.

7. Crack is also a common phenomenon of cemented carbide sintering waste

Pressing crack: because the pressure relaxation is not immediately displayed when the briquette is dry, the elastic recovery is fast during sintering. Oxidation crack: part of the briquette is seriously oxidized when it is dry, and the thermal expansion of the oxidized part is different from that of the non oxidized part.

8. Over burning

When the sintering temperature is too high or the holding time is too long, the product will burn too much. The over burning of the product makes the grain coarser, the pores increase and the alloy properties decrease obviously. The metal luster of undercooked products is not obvious, and only need to be burned again.