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One Minute Mentor: Nitriding in Fluidized-Bed Reactors at Low Temperatures
March 16, 2017
Source: ASM International
In nitriding treatment furnaces, the heating is done electrically, and the fluidizing gas composition is chosen to provide the optimum chemistry for the desired surface treatment (usually a mixture of NH3 + N2, with CH4 added when nitrocarburizing is desired). The granular solid is considered inert and does not react with the part being treated but is fundamental in establishing the fluidized bed and its outstanding properties.
For stainless steel nitriding, treatment temperature must be kept below 840°F to prevent the precipitation of chromium nitride. The lower treatment temperature increases the efficiency of the Cr2O3 barrier against nitriding; even after the acid treatment, there is still some oxide layer left at the steel surface.
The result is lower diffusion rates and lower nitriding potential (less dissociation of NH3, which is the mean supplier of atomic nitrogen needed for the nitriding treatment), as shown in the graph. The NH3 dissociation effect is controlled by changing the composition of the atmosphere, but the oxide layer would dissociate or become permeable only at much higher temperatures, which would cause the precipitation of nitrides and carbides in the case.
Volume 4D, Heat Treating of Irons and Steels ⇒ Heat Treated High-Alloy Steels ⇒ Nitriding of Stainless Steels ⇒ Thermochemical Nitriding Treatments
Materials Processing and Treatment | Heat Treating
Materials Processing and Treatment | Surface Engineering
Metals and Alloys | Carbon and Alloy Steels
Metals and Alloys | Stainless Steels