Look at the tempering charts for common hardenablr steel. If you're below 450F nothing happens. And gun parts are commonly tempered at a substantially higher temperature to avoid embrittlement problems. Ever see the edge of an overhard sear crumble? (A lot of PSI can develop on a sharp edge.) The big problem particularly with beginners (and open flames) is controlling the heat. Oh so easy to let the temperature get way higher than you want,
More like ~375 F, which is a pale straw color. Sear surfaces are kept harder than other parts, and are tempered at lower temperatures.
The claim that it's not likely to be a problem with a propane torch is completely bogus.
