The atmosphere during sintering process is the keypoint for MIM technology, it decide the sintering result and the final performance of products. Today, we will talk about it, the Atmosphere of Sintering.
The Role of sintering atmosphere:
1) Dewaxing zone, remove the lubricant in the green body;
2) Reduce oxides and prevent oxidation;
3) Avoid product decarburization and carburization;
4) Avoid oxidation of products in the cooling zone;
5) Maintain a positive pressure in the furnace;
6)Maintain the consistency of sintering results.
The Classification of sintering atmosphere:
1) Oxidizing atmosphere: Pure Ag or Ag-oxide composite materials and sintering of oxide ceramics: Air;
2) Reducing atmosphere: Sintering atmosphere containing H2 or CO components: Hydrogen atmosphere for cemented carbide sintering, Hydrogen-containing atmosphere for iron-based and copper-based powder metallurgy parts(ammonia decomposition gas);
3) Inert or neutral atmosphere: Ar, He, N2, Vacuum;
4) Carburizing atmosphere: contains high components that cause carburization of the sintered body, such as CO, CH4, and Hydrocarbon gases;
5) Nitrogen-based atmosphere: With high nitrogen content sintering atmosphere: 10% H2+N2.
The Reforming Gas:
Using hydrocarbon gas(natural gas, petroleum gas, coke oven gas) as raw materials, using air or water vapor to react at high temperature, and the resulting H2, CO, CO2, and N2. A small aount of mixed gas of CH4 and H2O.
The Exothermic Gas:
When preparing the reforming gas, the raw material gas and air pass through the converter in a certain proportion. If the ratio of air to raw material gas is high, the heat released during the reaction is sufficient to maintain the reaction temperature of the converter, without the need for external to the reactor heating, the resulting conversion gas.
The Endothermic Gas:
When preparing reformed gas, if the ratio of air to raw gas is low, the heat released during the reaction is not enough to maintain the reaction temperature of the reformer, and the reactor needs to be supplied with heat from outside. The resulting reformed gas is called Endothermic Gas.
The Atmosphere Carbon Potential is the relative carbon content of the atmosphere, which is equivalent to the carbon content in the material when the atmosphere and the sintered material with a certain carbon reach a reaction equilibrium (no carburization, no decarburization) at a certain temperature.
And the Controllable Carbon Potential Atmosphere is the general term for the prepared gas medium introduced into the sintering system to control or adjust the carbon content of sintered steel.
The keys to control the amount of CO2 and H2O in the atmosphere:
1) Control of H2O amount-dew point
The Dew Point: The temperature at which water vapor in the atmosphere begins to condense into mist under standard atmospheric pressure. The more water content in the atmosphere, the higher the dew point. The dew point can be measured with a dew point meter: the water absorption conductivity measurement using LiCI.
2)Control the amount of CO2 and measured by infrared absorption analyzer.