The sintering process for cemented carbide is extremely complicated, and sintering waste is likely to occur if care is not taken.
Cemented carbide is a powder metallurgy product made by sintering micron-sized tungsten carbide powder, which is a high-hardness, high-melting-point metal, with cobalt, nickel, or molybdenum as a binder in a vacuum or hydrogen reduction furnace. Sintering is a very important step in cemented carbide. So-called sintering is a process in which a powder compact is heated to a specific temperature, held for a period of time and then cooled to obtain a material with the required properties. The sintering process for cemented carbide is extremely complicated, and sintering waste is likely to occur if care is not taken.
1. Initial chip off of sintered cemented carbide scrap
That is, the surface of cemented carbide has edge cracks, deformed shells or cracks, and in severe cases, small thin skins resembling fish scales, chips or even spray. The occurrence of delamination is mainly due to the contact action of cobalt in the briquette, the carbon gas will decompose the free carbon in it, which will reduce the local strength of the briquette and lead to delamination.
2. Second common hole for sintered carbide scrap
Pores larger than 40 microns are called pores. Factors that cause air bubbles can cause holes. In addition, pinholes may occur when there are impurities in the sintered body that do not get wet with the molten metal, such as large pores such as "unpressurized", or when there is severe segregation between the solid and liquid phases. .
3. A third common type of sintered carbide scrap is foaming.
There is a hole inside the cemented carbide product, and a raised curved surface appears on the surface of the corresponding part. This phenomenon is called blistering. The main reason for bubbles is that the sintered compact contains relatively concentrated gases. There are generally two types: one in which air accumulates in the sintered body, and the other in which air moves from the interior to the surface during the sintering shrinkage process. If there is a certain amount of impurities such as alloys, iron scraps, cobalt scraps, etc. in the sintered body, air will gather here. When the sintered body turns into a liquid phase and densifies, it becomes impossible to expel air. Smaller surfaces form vesicles.
Second, a chemical reaction occurs in the sintered body, generating a large amount of gas. When certain oxides are present in the sintered body, they are reduced after the liquid phase appears to outgas, resulting in foaming of the product. WC-CO alloys usually arise from the agglomeration of oxides in the mixture.
4. There is an unequal organization: a mix of subjects
5. Then there is the deformation
The irregular shape change of the sintered body is called deformation. The main reasons for deformation are: Uneven distribution of briquette density due to the same density of the finished alloy. Severe depletion of carbon in the sintered body, as the depletion of carbon causes the liquid phase to decrease relatively. unreasonable shipping; uneven support plates;
6. Black Heart
The loose areas of tissue that appear in alloy fractures are called black hearts. Main reasons: too little carbon and unreasonably high carbon. All factors affecting the carbon content of the sintered body affect the formation of black cores.
7. Cracks are also common in sintered carbide scrap
Compression cracking: The decompression of dry briquettes is not instantaneous, so elastic recovery occurs faster during sintering. Oxidation cracking: When the briquette dries, it will partially oxidize, and the thermal expansion of the oxidized and unoxidized parts will be different.
8. Over Fire
If the sintering temperature is too high or the holding time is too long, the product will overheat. If the product is overfired, the grains will thicken, the pores will expand, and the properties of the alloy will deteriorate significantly. The metallic luster of poorly fired products is not evident and simply needs to be annealed.
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