Stainless steels and other high temperature materials can pick up nitrogen if exposed to nitrogen-containing
atmospheres such as nitrogen, nitrogen mixtures and cracked ammonia. During nitrogen pick-up nitrides and other
brittle compounds of chromium, molybdenum, titanium, vanadium and aluminium are formed. Atmospheric
oxygen, even at relatively low levels, reduces the risk for nitridation. At low temperature, 400-600°C, a layer of
nitrides are formed at the steel surface; at higher temperatures nitrogen uptake and nitride formation occur
throughout the material. Nitridation i.e. nitride formation, causes chromium depletion and reduced oxidation
resistance in the same way as carburization. This can lead to catastrophically high oxidation rates on the outer
surface of equipment which is subjected to a nitriding atmosphere on the inside - for example the muffles in
annealing furnaces. Nitrogen pick-up can also cause embrittlement due to surface or internal nitride formation.
Nickel is the alloying element which provides the greatest protection against nitridation, due to the fact that nickel
does not form stable nitrides. This is illustrated by Figure 20 which shows the nitrided depth for some austenitic
high-temperature alloys after exposure to nitrogen with traces of oxygen at 825°C. If oxygen is present, i.e. in
oxidising conditions, strong oxide formers such as chromium and silicon are beneficial.
In view of the effect of nickel, it is inadvisable to use martensitic, ferritic-austenitic or ferritic stainless steels in
nitriding atmospheres at temperature above approximately 500°C. More suitable materials are austenitic stainless
steels or nickel-base alloys.
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