We continue our discussion from last week's part 2.
If it is seen that the nitrided case begins to peel off, this is usually indicative that surface decarburization is present on the surface of the component. The decarburization is as a direct result of
- Insufficient surface stock removal at the pre-nitride machining operations
- Decarburization has occurred at the pre-heat-treatment operation
The component should be considered to be scrap, and it is not recommended to be salvaged.
The surface of the steel is seen to be randomly “dimpled” over the affected surface. Once again, this problem can be associated with the presence of surface decarburization.
If the nitrided case is seen to be chipping, particularly at corners, it is usually indicative of what is known as nitride networking. This is an over-enriched area of nitrogen where very hard and brittle nitrogen precipitates form with the nitride-forming elements in the steel. This problem will usually occur when the nitriding potential of the process gas is too high. The remedy is to check the gas flow and dissociation and reduce the flow accordingly.
This can occur as a direct result of the presence of a surface contaminant on the component. Investigate the pre-cleaning method prior to nitriding and after pre-machining.
This problem is usually due to a low core hardness that is failing to support the nitrided case. Another possibility is that the formed case is too shallow, and this can be remedied by increasing the case depth. However, the increase of case depth should be cautioned. Check what the application of the workpiece is and what sort of load will be applied to the component.