Firearms Bluing: How hot is too hot?

[Mossyrock]

When it comes to firearms finishes, specifically the different bluing techniques, how hot is too hot? Normal hot tank bluing runs about 275-300 degrees, and nitre bluing (usually confined to small parts like pins and screws) goes around 600 degrees. I can't find a reference on the temp used for Carbonia or "charcoal" bluing, but I would imagine that they go at least 500 degrees. At what point, temperature wise, does the high temperature compromise the heat treatment of the steel?

 

[jungle]

300-350F will have no effect on the steels used in firearms or the heat treat, it may even serve to stress relieve steel if the temps are brought up and down slowly. 500F will get into the temper range and may reduce hardness at mid range rockwell numbers, it has been the cause of embrittlement to some steels held in this range for a while.
Charcoal blue is a form of case hardening done at temps around 1300F surface temp with a long slow cool down.
Carbonia is a gas furnace heat with an oil quench.

Check www.ronsgunshop.com for pics and descriptions.

 

[Jim K]

In the old factories, the carbona bluing was part of the heat treatment process, which is why it can't really be duplicated.

Jim