Steels to be nitrided require a pre-heat-treatment procedure prior to final nitriding. When a hard surface has been formed on the steel, it is the core of the material that supports the newly formed case when load conditions are applied to the surface. It is known that martensite is the hardest phase and condition of steel, and it will form provided that the following conditions are present:
  • The presence of sufficient carbon
  • Appropriate austenitizing temperature
  • Fast enough cooling rate to transform austenite into martensite
Once the martensite has been formed, the steel will need to be tempered. The resulting metallurgy will be:
  • Tempered martensite
  • Possibly some retained austenite
  • Appropriate mechanical properties
The resulting core metallurgy and properties will now be able to support the newly formed stable nitrides in the surface of the steel. The required pre-tempering temperature should be approximately 50-75°F above the final nitriding temperature so as not to allow the nitride procedure to temper back the core properties of the steel.

Another aspect as to the selection of the final nitriding procedure temperature is the fact that the nitriding procedure will act as a temper on the core of the steel, thus decomposing any retained austenite that might be present as a result of incomplete transformation at the pre-austenitize and quench procedure. The nitriding process temperature will decompose the retained austenite and produce a more dimensionally stable core.

The temperature uniformity within the nitriding process chamber should be uniform, in the region of no more than a maximum deviation of +/-10°F. The reason for this is that if the temperature deviation is too great (particularly if the temperature is on the high side and exceeding +10°F), there is a very strong likelihood that the steel will have reduced core properties and not be able to adequately support the formed nitride case.

Temperature uniformity (as well as temperature accuracy) within the nitride process chamber is a mandatory requirement, particularly for aerospace, automotive, tooling and other temperature-sensitive engineered components.

Therefore, Temperature Uniformity Surveys (TUS) are a mandatory requirement. This is to ensure both temperature uniformity and temperature accuracy. TUSs should be performed anytime there has been a change in the equipment, such as rebrick, seal replacement, element replacement, thermocouple replacement, circulation fan replacement or any other structural change/repair on the process furnace.

It is the pre-heat treatment (particularly the pre-tempering temperature selection) that will give the appropriate core strength to support the load that might be applied to the final nitrided surface during its operation. Another facet that can be considered as an intermediate heat-treatment process step is that of cryogenic treatment. This will assist in the decomposition of any retained austenite and produce good dimensional stability. The time at the cryogenic temperature will be dependent on the size of the workpiece being treated and its material chemistry.