The carbon potential in the process furnace atmosphere to ensure a good carburized case should be determined by the type of steel to be treated and the carburizing process temperature.
Generally, one would use an atmosphere carbon potential between the eutectoid line on the Iron-Carbon Equilibrium diagram at approximately 0.8% carbon and up to 1.2% carbon (maximum). If control of the carbon potential is not exercised, there are numerous problems that can occur on the resulting formed case. These include: retained austenite, grain-boundary oxidation, intergranular cracking, surface cracking, low surface hardness and carbide networking.
The carburizing furnace atmosphere is made up of either endothermic atmosphere carrier gas plus city gas as the enrichment gas OR nitrogen/methanol carrier gas plus city gas as the enrichment gas.
If one is satisfied that the carrier gas is being generated in a satisfactory manner and there are no problems occurring with the carrier gas in terms of dew point, yet problems are occurring at the process furnace, then the quality of the city gas (methane enrichment gas) should be verified. The following problems can occur as a result of the lack of control on the enrichment gas.
- Sooting: This problem is as a direct result of too much carbon in the furnace atmosphere, and it can be visibly seen precipitating out of the atmosphere. This condition would usually occur at carbon potentials that are approaching 1.6% or greater. This condition will cause the furnace refractory to become overloaded with carbon diffused into the refractory brick, which will lead to difficulties in atmosphere control. Additionally, the carburized surface of the steel will lead to a serious potential for the formation of retained austenite. The obvious remedy is to cut back the enrichment gas or to dilute the furnace atmosphere with air. The problem with air dilution is that there becomes a greater risk for the formation of grain-boundary oxides and surface oxides. Great care needs to be exercised when adding air into the enriched atmosphere so that one does not create the problem of oxide formation. If the furnace has been operated at high carbon potentials for extended periods of time, it will be necessary to burn out the carbon from the furnace refractory. Some of the modern-day furnace manufacturers will build the furnace with a built-in burnout system. This means that the furnace operator only has to go to the program mode for burnout, and the burnout will be completed automatically. With older-type furnaces, one simply reduces the furnace temperature to approximately 1600°F and removes any ferrous atmospheres that might be present within the process chamber. The doors of entry and exit to the furnace are opened and air is simply blown into the process chamber. The air can be supplied either by an external air blower or by lines of compressed air simply blowing into the process chamber. This will cause any carbon present in the refractory brick to ignite and burn. The temperature-control instrument should be observed because a rise in temperature will occur, and the temperature will continue rise until all of the refractory carbon is burned out. Generally, the burnout time would be approximately two to three hours.
- Low hardness: Low hardness can be caused by too low a carbon potential in the process furnace. Low carbon could be caused by too low a dew point in the process gas, too low a carbon potential and too slow a quench (if the atmosphere is within the required carbon potential). Too much residual retained austenite as a result of too slow a quench or too much carbon in the surface of the steel can also result in lower hardness. The remedy would be to check the furnace atmosphere carbon potential and adjust accordingly. Quench-medium temperature should also be checked to be sure that it is not too hot and causing a slack/slow quench.
- Grain-boundary oxidation: This can be caused by the presence of oxygen/moisture within the furnace atmosphere. The remedy would be to check the furnace for potential air leaks and to check the volume of dilution air being used and adjust accordingly. Close and careful control of the dilution air is necessary to reduce the risk of the grain-boundary oxide formation.
These are just some of the potential problems that can occur as a result of minimal control of the furnace atmosphere.