Sintering is a key process in the production of cemented carbide. And it plays a decisive role in the performance of the final product. The sintering of cemented carbide can also be said to be the phenomenon or process that occurs when the workpiece is heated at an appropriate temperature and atmosphere. The sintering (hydrogen) atmosphere in the sintering furnace is very important to ensure the normal progress of the sintering process and product quality. Therefore, it is necessary to understand the nature, function, influence, and control of the sintering atmosphere.
Hydrogen atmosphere analysis
The sintering atmosphere of cemented carbide was hydrogen first. Industrial hydrogen contains water. Even after purification, hydrogen with a dew point of -40°C still contains 5 parts per million of water. Water can decarburize WC and TiC and react with graphite boats and carbon-containing fillers:
WC+H2O=W+H2+Co (decarburization reaction)
TiC+H2O=TiO+H2+C (decarburization reaction)
C+H2O=CO+H2 (decarburization reaction）
Forming agent cracking product hydrocarbons decompose under the action of cobalt and some catalysis:
CH4=C+H2 (carburization reaction)
2CO=CO2+C (carburization reaction)
Because cobalt powder is extremely easy to oxidize, some fine-grained cobalt powder in the cemented carbide mixture exists in the form of oxide. Even the coarser particle surface will form an oxide film, and the hydrogen reduces the cobalt oxide during sintering:
Therefore, from the hydrogen atmosphere anlysis in the cemented carbide sintering furnace, the composition of the furnace atmosphere is complex and difficult to control. If the water content in hydrogen fluctuates greatly, it will be more difficult to control. Cemented carbide production usually uses hydrogen with a dew point of -40°C to obtain better products. Using hydrogen with a lower dew point, such as -70°C, will increase the production cost.
Hydrogen atmosphere control in the sintering furnace
Fill with carbon-containing alumina or graphite particles in the boat to avoid decarburization of water vapor in hydrogen. This results in a localized atmosphere that is substantially independent of the furnace atmosphere. That is, it creates a carburizing atmosphere around the sintered body. This atmosphere is suitable for sintering to ensure that the carbon content of the alloy is within the allowable range.
The amount of forming agent added will affect the atmosphere in the furnace. The amount of addition increases, the hydrocarbons in the furnace gas also increase, and the carbon of the sintered body also increases. The carburization of paraffin is less, and the carburization of buna rubber is more, so the amount of carburization should be well controlled. In the low-temperature sintering stage (800°C), the temperature rise rate is too fast, and the cracked products of the forming agent are too late to be taken away. This will increase the concentration of hydrocarbons in the furnace, which will cause carburization of the sintered body.
If the flow rate of hydrogen is too large, the decarburization reaction of carbides is easier to proceed to the right. Make the decarburization effect stronger. Excessive flow will reduce the concentration of hydrocarbons in the furnace. This reduces the amount of the sintered body by gas phase carburization. Insufficient hydrogen flow will increase the contact of molding agents such as hydrocarbons, which are decomposition products of buna rubber, with the sintered body. Cause products to produce “peeling”. The hydrogen flow rate should be determined according to the sintered body’s loading amount, the forming agent’s additional amount, the boat’s pushing speed, and the boat’s quality.