Well I've foolishly (per Wonko) read through pages of internet "expert gun guy" verbiage about case coloring firearms. 95% is a repetition of the same stuff somebody smart must have once said, with no textbook or scholarly journal references.
The non-deep thinkin' version is that
traditional charcoal color case hardening IS a form of heat treatment, and,
DONE RIGHT, can increase the strength of the steel.
Still looking for numbers that are applicable to firearms
but Miller's post re: case hardened 1020 is noted and appreciated
And another but relevant topic. There have been periodic posts over on Trapshooters.com by guys who wanted to fancy up their case colored Krieghoffs with custom engraving...requiring annealing...then requiring re-heat treatment...leading to fractured receivers...leading to very unhappy customers
Heat treatment tutorial
http://navybmr.com/study%20material/14250a/14250A_ch2.pdf 
