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In NACE MR0175, is nitro-carburizing allowed?

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Question:

Will “Nitro-Carburizing” be allowed surface treatment with the same status of “Nitriding”? If it is allowed, would the NACE requirement be the same as Nitriding or it would be different?

Answer:

As ISO 15156 is written today, the acceptable surface treatment has been defined solely as nitriding below the lower critical temperature.
Additional response to query submitter: We believe that you have identified an area where our standard ISO 15156 does not accurately define what has been accepted practice in the Oil & Gas Industry.
It has been common practice to use salt bath, liquid or ion nitriding including nitrocarburizing and carbonitriding for a wide variety of applications.
The two key elements that have been constant through the years are that the maximum case depth has been defined as 0.15 mm (0.006”) and the process temperature is below the temperature where any new transformation products are formed; this is the lower critical temperature for the alloy being processed.
The only application problems that we are aware of are when the nitriding (and related) processes are applied to areas where the local yield strength is exceeded; in these areas the plastic deformation has resulted in local breaks or cracks through the hardened surface due to the reduced ductility that is associated with the higher hardness. The MP will propose a ballot to clarify the description of acceptable processes.
Note that this ballot will need to be successful before this is an approved process listed in ISO 15156.

This question is in relation to NACE MR0175/ISO 15156-3 Annex A.1.5.1

Reference: ISO 15156 Maintenance Panel Inquiry #2013-08

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