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Arc Discharge

This is usually seen on continuous-DC current systems but can also occur in the pulsed-DC systems (although not too frequently on the pulsed-power system). 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 simply 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 simply to reduce the process voltage or change the process pressure.

Part Chipping

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 also apply to gas nitride and salt-bath nitride. 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. The remedy is simply to reduce the nitrogen in the process or round off the corners of the component.

Low Surface Hardness

This can be caused by low nitrogen availability with insufficient nitrogen in solution with the steel to form sufficient stable nitrides in the surface of the steel. Another condition that can cause this 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.

Surface Flaking

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 pre-machining operation and then check the method of pre-cleaning 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, then a blended mixture of hydrogen/argon can be used. However, be cautious with the use of argon because this gas has a high atomic weight and can cause surface etching. The maximum suggested volume of argon would be 10% argon with 90% hydrogen. Generally, the mixture ratio is 5% argon and 95% hydrogen.

References

1. McQuaid H.W. and Ketcham W.J., Some Practical Aspects of the Nitriding Process, Transactions American Society of Steel Treaters, 1928

2. Adolph Fry, US Patent 1,487,554, March 18, 1924

3. Pye, D., Practical Nitriding and Ferritic Nitrocarburizing, Chapter 17, Troubleshooting, ASM International, 2003