I do not think that the Hot Blue gets quite hot enough to fully anneal the frame. But it may do some damage in that regard.

Steels anneal at temps between 700-1000 F so the 300 degree bath of the hot bluing process won't hurt the steel's integrity.

However the sodium hydroxide used in the process can really do damage on bondings of that soft silver solder they used. Oh, and that silver solder melted and flowed at about 450 F which isn't enough to melt it, but enough to assist the sodium hydroxide in damaging the bond. Also playing a role in this is the expansion caused by the heating of the metals. the solder and the steel expand at different rates thus further stressing the bonds.

All in all, hot bluing a soldered gun of any kind is not really a good idea, especially an older one.

I've always read that it takes 1,450 F to truly "anneal" steel. The reference to the 700-1,000 degree range is more likely relevant to the temper of the steel.

you;re right, sorry, I was thinking "C" and typed "F" by mistake.

nice catch

As others have said, the temperature used in hot bluing is well below the lower critical temperature, and won't alter the grain structure or surface harness. Temperatures used in either a tempering cycle, or annealing cycle are really about the same. The difference is in the rate of cooling from those temps. I don't know enough about the chemical or mechanical reaction to the hot bluing process, and only know I don't like it.