Pulled from a post on 1911 forum:
The tempering temperature for 4140 steel to reach the typical 1911 slide's 40 Rockwell C is almost 800 degrees F. The frame is softer and can stand even higher temperature before softening further.
See tempering temperature vs hardness chart taken from here:
Heat Treating 4140 steel FAQ : anvilfire.com How-to.
SAE 4140 Steel Tempering
Tempering Temperature; Hardness Brinell; Hardness Rockwell; Tensile Strength
400°F 200°C 514 HB 55 HRc 297 KSI
500°F 260°C 477 HB 50 HRc 243 KSI
550°F 290°C 461 HB 48 HRc 235 KSI
600°F 320°C 444 HB 47 HRc 225 KSI
650°F 340°C 429 HB 46 HRc 217 KSI
700°F 370°C 415 HB 44 HRc 210 KSI
800°F 430°C 363 HB 39 HRc 182 KSI
900°F 480°C 331 HB 36 HRc 166 KSI
1000°F 540°C 293 HB 31 HRc 145 KSI