There's two approaches to putting a spot of weld on the cylinder bolt end, and the first is placing a heatsink close to it, so it blocks much of the heat from traveling into the thin section just behind the cam lump. I've done this with two copper blocks in a vise, and put a quick spot of weld on it, without losing the temper.
The other way, is to put a spot of weld on it, then harden it again by quenching, then polish the spring area bright, and temper it to a blue color, to get the temper back to where it should be. It should be drawn blue from the edge of the screw hole all the way out. The latch end can be brought to a brown, just a little harder.