Common Sintering Wastes and Causes

The main component of cemented carbide is micro-sized tungsten carbide powder of high hardness. Cemented carbide is the final product produced with powder metallurgy and sintered in a vacuum furnace or hydrogen reduction furnace. The process uses cobalt, nickel, or molybdenum as the binder.  Sintering is a very critical step in cemented carbide. The sintering process is to heat the powder compact to a certain temperature, keep it for a certain period of time, and then cool it down to obtain a material with the required characteristics. The sintering process of cemented carbide is very complicated, and it is easy to produce sintered waste if you make some mistakes. This article is going to talk about some common sintering wastes and what causes the waste. 

1. Peeling

The first common sintering waste is peeling. Peeling is when the surface of the cemented carbide appears with cracks on the edges and warping shells. Moreover, some appear small thin skins like fish scales, burst cracks, and even pulverization. The peeling is mainly due to the contact of cobalt in the compact, and then the carbon-containing gas decomposes free carbon in it, resulting in a decrease in the local strength of the compact, resulting in peeling.

2. Pores

The second most common sintering waste is obvious pores on the cemented carbide’s surface. Holes that have above 40 microns are called pores. Anything that can cause bubbles will cause pores on the surface. In addition, when there are impurities in the sintered body that are not wetted by the molten metal or the sintered body has a serious solid phase and the segregation of the liquid phase may cause pores.

3. Bubbles

Bubbles are when there are holes inside the cemented carbide and cause bulges on the surface of the corresponding parts. The main reason for bubbles is that the sintered body has relatively concentrated gas. Concentrated gas normally includes two types. 

4. Uneven structure caused by mixing different powders.

5. Deformation

The irregular shape of the sintered body is called deformation. The main reasons for the deformation include: the density distribution of the compacts is not uniform; the sintered body is severely deficient in carbon locally; the boat loading is unreasonable, and the backing plate is uneven.

6. Black Center

The loose area on the alloy fracture surface is called the black center. The cause of black center is too much carbon content or carbon content is not enough. All factors that affect the carbon content of the sintered body will affect the black center of the carbide.

7. Cracks 

Cracks are also a common phenomenon in cemented carbide sintered waste. There are two types of cracks, one is compression cracks, and the other is oxidation cracks.

8. Over burning

When the sintering temperature is too high or the holding time is too long, the product will be over-burned. The over-burning of the product makes the grains thicker, the pores increase, and the alloy properties decrease significantly. The metallic luster of under-fired products is not obvious, and it only needs to be re-fired.