Yep, if it is a hardened surface then you could and most likely will create a problem with too much heat. Propane or electric heat not much of a chance. The solder for low temps does not have much strength however.
That is some really bad advice for someone new to soldering, and who is working on a heat treated part. Propane torches absolutely can and do put out enough heat to damage steel temper, and a lot more than that.
Bcraig, it would be wise to avoid soldering on either the hammer or sear, even if your JB weld part doesn't work. To your question of how much heat is too much - on bare steel, as soon as you start seeing it change colors (yellow -> brown -> blue) you're in the range where it can affect temper. Normal tempering temps are between the pale yellow and solid blue color stages, depending on the steel and how hard the manufacturer wanted it to be. With harder surfaces like you're working with, the temper was probably done at lower temps, in the yellow->brown range. You
will exceed that if you try to solder to it.
