Gas Nitriding and O2 Measurement


Gas Nitriding is a heat treatment process that diffuses nitrogen onto the surface of a metal to create a case-hardened layer. The nitrogen layer is added to the surface of steel parts using dissociated ammonia as the source. The ammonia is ‘cracked’ by introducing it to a furnace at temperatures typically between 550…570°C. The ammonia (NH3) dissociates into nitrogen and hydrogen, with the nitrogen diffusing onto the hot surface of the metal creating a nitride layer.

The presence of oxygen in this heated environment can cause explosion. To minimise the risk of explosion when introducing ammonia, the furnace must first be purged and oxygen levels decreased, typically to below 3-5%. The purge gas is generally nitrogen.

This part of the process is generally controlled by mass flow meter to determine how much nitrogen needs to be supplied based on the part surface area and furnace retort volume. Some Nitriding professionals also choose to measure the oxygen level in the furnace at this point as a 2nd reference value. Both from a safety point of view and also as part of a system to monitor the potential (Kn) in the furnace.