How to Control the Cooling Temperature in a Nitriding Furnace?
Following gas nitriding nitrocarburizing processes, if the procedure involves cooling the furnace—under the protection of an ammonia atmosphere—to below 200°C before unloading, instances of furnace gas deflagration (rapid combustion) upon opening the furnace lid occur periodically, albeit to varying degrees of severity.
1. The primary atmosphere within a nitriding furnace consists of ammonia, nitrogen, hydrogen. Of these, both hydrogen ammonia are flammable gases. When mixed with air within a specific concentration range, they can ignite upon contact with an open flame (including sparks) when reaching their ignition temperature (above 510°C). In a sealed container, this reaction manifests as an explosion; in an open container, it manifests as deflagration.
2. At this stage, the furnace temperature has already dropped below 200°C. Upon opening the furnace lid—despite the influx of ambient air—gas combustion (deflagration) theoretically should occur in the absence of an external ignition source (open flame).
3. However, a critical issue arises here: hydrogen is a potent reducing gas. During the furnace cooling process, it reduces the iron nitrides—which have settled within the furnace chamber as fine particulates (dust)—back into iron powder. It is well known that minute particles of reduced iron powder undergo rapid oxidation generate intense heat upon contact with air; this causes their temperature to skyrocket, effectively turning them into sparks. Additionally, active carbon powder—which may have deposited during the nitrocarburizing process—can also oxidize upon contact with air, generating sparks. These sparks then ignite the "hydrogen ( ammonia)–air" mixture, resulting in the phenomenon of deflagration.
4. Preventive Measures:
(1) Before opening the furnace lid, purge the furnace interior with nitrogen gas to dilute displace the flammable gases; only proceed to open the lid once this purging is complete.
(2) Exercise extreme caution when opening the furnace lid. Operators should maintain a safe distance the furnace body allow the internal gases to dissipate naturally for a brief period. Alternatively, a crumpled piece of paper ( similar combustible material) can be ignited tossed into the furnace chamber to deliberately ignite any potentially residual flammable gases, thereby ensuring safety before proceeding to hoist out the treated parts.