Troubleshooting the Process Furnace Atmosphere
If it has been ascertained that the endothermic generator is functioning correctly, then the next step is to observe the process furnace atmosphere conditions. The five common methods of testing and controlling the furnace atmosphere are as follows:
- Shim analysis, by weight or by controlled burning
- Three- or four-gas analyzer
- Dew point
- Oxygen probe
- CO2/infrared analyzer
Perhaps the most common method of testing the furnace atmosphere will be by using the dew-point method. This method is an old but tried and tested method of atmosphere control. It is usually supported by one or two other methods of control and uses the following formula:
Carbon potential = (Final weight - Original weight) / final weight x 100% + original C content
In order to ensure that no contamination is carried into the process furnace, it is necessary to ensure that the part is clean. Cutting fluids, cutting oils and lapping compounds may contain chlorides, sulfides, silicones and hydrocarbon compounds. These surface contaminants may lead to a serious disturbance in the furnace atmosphere as well as at the work surface. In addition to this, it must be ensured that the workpiece surface is free and clear of surface oxides prior to entry into the furnace. The presence of oxides on the workpiece surface will lead to non-uniform case formation (if carburizing). It is then necessary to ensure that the parts are pre-cleaned or degreased prior to entry into the process furnace.
If high dew points are being experienced in the process furnace atmosphere, then it is likely that air is present in the furnace atmosphere. There are many sources of air ingress into the furnace. First check that all door seals are not broken or damaged. The next area to observe would be all pneumatic cylinders that operate inner doors and elevators. If the furnace has an internal mechanical handling device, the external drive housing that is mounted onto the side of the furnace could be improperly sealed or the seal could be damaged. Another source of oxygen/air that is often overlooked and could contribute to high dew-point levels is the external air itself. High air humidity levels will contribute to high furnace dew-point levels. If high water-vapor levels are present within the furnace atmosphere, then there is a serious risk of causing the following reaction:
Fe + H2O ↔ FeO + H2
It can be seen that water vapor will cause serious surface oxidation to any alloy steel at an elevated temperature, and it will combine with carbon in the steel to form carbon monoxide and hydrogen as shown in the following reaction:
(C) + H2O ↔ CO + H2
A further effect of the presence of moisture within the furnace atmosphere will be to cause grain-boundary oxidation, which can have a serious adverse effect on the steel surface, particularly if there is no further machining to take place after the heat-treatment procedure.