A number of systems have been seen by this writer where the furnace is simply loaded up with work to be processed, and the process retort closed up and sealed. The ammonia gas valve is turned on, and ammonia is simply allowed to flow into the process retort without any control or even checks on the ammonia gas volume.
Consistent and repeatable metallurgical and mechanical results demand consistent and repeatable process parameters if, for no other reason, the ammonia flow needs to be controlled in order not to achieve nitride networking. Nitride networking is simply a saturated solution of nitrogen in iron/steel. Nitride networks will generally commence at corners and grow according to the selected process temperature, process time at temperature and process-gas composition.
Nitride networks are very brittle and will cause a catastrophic failure. If one observes a photomicrograph that shows the compound layer and the diffusion layer with an oversaturation of nitrogen, one will see what appears to be white fingers probing into the diffusion zone from the compound layer. The probing fingers are caused by a “super-saturated solution of nitrogen in iron.”
This is why it will be necessary to control the flow rate of ammonia process gas into the nitride process chamber. As stated previously, the networks are very brittle and can cause the surface compound layer to peel off if they are not controlled by (at least) gas dissociation. This is determined by the measurement of the insoluble exhaust gases in relation to the soluble gases.
With the process of ion nitriding, the process-gas ratio can be controlled quite easily in relation of nitrogen to hydrogen. This is because one is using individual molecular gases of nitrogen and ammonia. The gas ratios can be easily and quickly changed and monitored by mass-flow control units.
The process of ion nitriding does offer a more effective method of changing the gas ratios to suit the material being processed. This will be done according to the steel being treated. This feature will allow the heat-treatment associate to change the process-gas ratios of nitrogen and hydrogen in order to create the surface metallurgy to suit the job application.
On the other hand, ammonia-gas decomposition can be controlled to some extent simply by enrichment or dilution. The dilution gas would be hydrogen and the enrichment gas is ammonia.
Both temperature control and temperature uniformity are necessary process-control parameters to ensure uniform surface metallurgy and especially mechanical properties. Spalling can be a very problematic area of potential process failure that will necessitate a scraping of the component. The cause of this occurrence is generally attributed to:
- Surface decarburization occurring during the pre-heat treatment of the component
- Surface oxidation occurring because of the lack of control of carbon potential during the pre-heat-treatment operation