The nitriding process is perhaps one of the most misunderstood thermochemical surface-treatment processes practiced today.
In the first part of this article (August 2016) we covered two nitriding processes: gas and salt-bath nitriding. We looked at problems resulting from these two techniques. This article will focus on ion/plasma nitriding and its troubleshooting. We will also discuss some general nitriding concerns.
The formed nitrided case generally occurs as shown in Figure 1.
The actual construction of the case is dependent on the composition of the steel being treated. For example, the carbon content of the steel will contribute to how the compound layer (white layer) will form.
Generally, the compound layer should form approximately 50% of epsilon nitride and approximately 50% of gamma-prime nitrides. The stable nitrides will form with the nitride-forming elements beneath the compound layer. The real success of the nitriding process is the preheat treatment prior to finish machining and subsequent nitriding.
Ion nitriding is also known as glow-discharge nitriding or plasma nitriding (Fig. 2). The process is gaining a great deal of popularity in North America due to legislation on process effluents, European engineering specifications and a growing awareness of the process repeatability and metallurgical consistency due to computer control (Fig. 3).
It is necessary to understand that there are two power-system types: continuous DC power and pulsed DC power. Also, there are two types of hardware systems, which are cold-wall and hot-wall (Fig. 4).
Overheating is usually a result of the parts being too close together and 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 to be glowing at a visible temperature. After the process is completed and the part is unloaded from the chamber, it can be identified as a dark area on the part. It can also 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 that is being used for the part manufacture
- The preheat treatment tempering temperature
- The required surface metallurgy
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 to this, 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
During the observation of the process conditions through the furnace sight glass, if there are areas on the part being treated without uniform plasma glow, 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 be taking place where no glow is seen on the component. This means that no case will be formed or it will be very shallow. The manner in which this condition is corrected is 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.
This is usually seen on continuous-DC current systems but can also occur in the pulsed-DC systems (although less frequently). The cause is from too high a process voltage, so the remedy would be simply to reduce the process voltage until the discharge no longer continues to occur. The arc discharge is seen as a miniature “lightning strike” in the process chamber and will usually be attracted to sharp corners on the component, which will result in localized overheating and probable surface burn/localized melting. The remedy is to reduce the process voltage or change the process pressure.
This is usually seen at sharp corners. The most probable cause is “nitride networking.” This means that the corner that has chipped has been oversaturated with nitrogen. This condition can apply also to gas and salt-bath nitriding. The cause is that too much nitrogen is present in the corner due to the “corner effect.” Nitrogen is soluble in iron up to a value of approximately 7% by volume (maximum). When oversaturation occurs, the nitrogen has precipitated out of solution during the cooling stage of the process and has settled at the grain-boundary locations in the corner of the component (Fig. 5). The remedy is to reduce the nitrogen in the process or round off the corners of the component.
Other Nitriding Problems
Like part chipping, some nitriding issues can occur regardless of the process used.
Low Surface Hardness
This can be caused by low nitrogen availability with inadequate nitrogen in solution with the steel to form sufficient stable nitrides at the surface. Another condition that can cause low surface hardness is that the steel itself is too low in nitride-forming alloying elements. The remedy is to change either the steel that the component is manufactured from or increase the nitrogen and thus the nitride potential of the process gas.
The cause of this condition is usually a surface contaminant being carried into the process on the part surface. Simply check the manufacturing method for the type of coolant or cutting fluid used during the premachining operation and then verify the method of precleaning prior to the nitride procedure.
Some surface contaminants can be removed by sputter cleaning at the commencement of the ion-nitride process using hydrogen as the sputter-clean gas. If hydrogen is not aggressive enough, a blended mixture of hydrogen/argon can be used. Be cautious with the use of argon, however, because this gas has a high atomic weight and can cause surface etching. The maximum suggested volume of argon would be 10% with 90% hydrogen. Generally, the mixture ratio is 5% argon and 95% hydrogen.
Nitriding is a very useful process to develop properties for certain operations. It cannot take the place of other surface-hardening methods, and it is not without its processing challenges. We hope this two-part look at the different processes and troubleshooting suggestions can help you produce higher-quality nitrided parts.
- Some Practical Aspects of the Nitriding Process, McQuaid H.W. and Ketcham W. J., Transactions American Society of Steel Treaters, 1928
- Adolph Fry. US Patent 1,487,554 18 March 1924
- Practical Nitriding and Ferritic Nitrocarburizing, Chapter 17 Troubleshooting. Pye D., ASM International, 2003