Our troubleshooting discussion continues.
Part overheating is usually a result of the parts being too close together. It is known as hollow-cathode effect. The hollow-cathode effect can usually be seen during the process if one looks through the process sight glass. The particular area being subjected to the hollow-cathode effect can sometimes be seen glowing at a visible temperature. It can also be seen after the process is completed and the part is unloaded from the process chamber as a dark area on the part where the hollow cathode has occurred. It can be measured in terms of hardness. That area of the part will be lower in surface hardness than the rest of the component.
The part can also be overheated by process voltage and amperage variations during the process. This can be caused simply by an incorrect program process value being used in either the process controller or the PC (if the process control is by PC). The overheating could also be caused simply by an incorrect temperature value being used.
What is the correct process temperature to use? This will depend on:
- The steel composition being used for manufacture of the part
- The preheat-treatment tempering temperature
- The surface metallurgy required
A very important aspect of non-uniform process temperatures during the nitride procedure and in the process chamber is that it will cause case-depth variations on the component. Therefore, the loading of the process chamber is extremely important to ensure that hollow cathode does not occur. Further, it is important that the process thermocouples are placed in the load area that will present an accurate picture of the process temperature on the part surface. It is equally important that the process-control values are known to be “good values” from previous runs.
Loss of Nitriding
If, during the observation of the process conditions through the furnace sight glass, there are areas on the part being treated and there is no plasma glow sitting uniformly around the component, it simply means that the selected process pressure is incorrect and the value is too high. If this condition occurs, it is most important to rectify it because no nitriding effect will take place where no glow is seen on the component. This means that no case will be formed or, if it is, it will be very shallow. The manner in which this condition is corrected is simply to check that there are no leaks on the vessel (remember, the process is operating at partial-pressure conditions). If there are no leaks, reduce the operating pressure setting until the glow seam reappears on the area not previously covered by the plasma glow seam.
We will conclude this discussion next time in Part 6.