American Machinery's Handbook. The Critical temperature at which pearlite is transformed to Austenite as it is being heated is also called the decalescence point. The temp at which it turns back to pearlite upon cooling is called the recalescence point. The recalescence point may be, depending upon the exact alloy, from 85° to 215° F Lower" than the decalescence point.
"These critical points have a direct relation to the hardening of steel. Unless a temperature sufficient to reach the decalescence point is obtained, so that pearlite is changed into austenite "No Hardening Action Can take Place and unless the steel is cooled suddenly before it reaches the recalescence point, thus preventing the changing back again from Austenite to Pearlite no hardening can take place"".
I put in over 35 years as a machinist, with some 25 of tham as a modelmaker where I wore out a vast number of files. I never had a file "Skate" on a part that was not Glass Hard.
Personally I would want no shotgun frame of mine quenched at a temperature lower than that recalescence point, regardless of who did it.
The Decalescence point seems to bottom out at about 1325°F with .85% carbon. I have not looked up the Recalescence points but I highly suspect Dr Gaddy was staying barely above them.1325°F - even the low limit of 85°F would equal to 1240°F. There may be just a bit more room to play with than that.
