Cooling rate and Gas Nitriding 2

[eagertknow]

Hi,

I am looking forward to find a reference that can answer the question whether there is a link between the characteristics of microstructure (white layer and general)to cooling rate at the end of a two stage gas Nitriding cycle.
Any help will be greatly appreciated.

 

[Goahead]

The article on Gas Nitriding from ASM Metals Handbook, 8th edition vol.2 page 149 does not mention cooling rate as affecting microstructure.
Material, microstructure and hardness before gas nitriding, dissociation rate, temperatures and time all affect depth of case, microstructure and distortion.

http://www.welding-advisers.com/

 

[swall]

Cooling rate has no effect on the microstructure of the nitride case after it has been formed. Cooling steel from 1000F down to room temperature doesn't involve any phase transformations or other changes in microstructure.

 

[TVP]

Cooling rate will have an effect on what happens to the excess nitrogen atoms in the diffusion zone, but not on the compound layer (white layer). Fast cooling from above ~ 400 C will allow more nitrogen to be dissolved into solid solution, which will embrittle the steel. Slow cooling until 400 C will allow the nitrogen to form nitride particles (either with iron or with nitride forming elements like chromium, aluminum, and titanium). I have experienced problems with low ductility after nitriding because of this phenomenon. Quenching should not take place above ~ 400 C. Unfortunately, there is not a great deal of information in the ASM Handbooks about this particular aspect of nitriding. Try a keyword search of Scirus (

http://www.scirus.net)

or Google Scholar.

 

[swall]

TVP--very interesting. Gave you a star for that one.