Gas Nitriding and O2 Measurement

An introduction to Gas Nitriding

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 Problem

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 volume1. 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 furnace2.

The Solution

SST Sensing’s GAP analyser is perfect for this application as it is suited to temperatures up to +700°C and it’s robust, stainless steel construction, along with flanged process connection allow it to be safely installed in the harshest environments. The LCD display shows operators the O2 level inside the furnace to allow them to safely introduce the ammonia to begin the nitriding process. The GAP also comes with up to 2 x 4…20mA outputs as well as MODBUS communications to automate the process.


Another benefit of the GAP sensor monitoring O2 in the furnace is that it can be used to detect leaks. The sensor will highlight rising O2 levels in a purged furnace if left for any period of time while not in use. The detection of these leaks can help as part of a preventative maintenance programme, keeping the plant up and running as efficiently as possible.

Suggested Sensor for Application – Gas Analysis Probe (GAP)

  • The GAP provides customers with a single gas flue or ventilation monitoring control system
  • Barometric pressure and temperature sensors included
  • RS485 MODBUS RTU as standard
  • User configurable relay and analog outputs
  • Easy to swap sensor module, no special tools
  • Calibrate in fresh air or cal gas via external calibration port
  • Various probe lengths available
Oxygen Analyser

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