Thanks Phil. Normalizing and stress relief I get.......... I was referring to the fact that at one point in mfgr wear surfaces like the primary extraction cam and the cocking ramp are heated and quenched for localized surface hardness. For wear resistance. And when these areas are subjected to extreme heat they will be "drawn back" to a softer state.
Since a Remmy is the standard everyone seems to use, I'll refer to those. I think you will find you are mistaken about the localized surface hardening of wear surfaces. The whole bolt is hardened, probably to 38-42Rc, and that is it. If anything gets harder, it is because it is work hardened through use. How much harder it gets is pretty minimal. Most like to have some separation in hardness between the mating parts of the receiver and bolt. A few points is generally sufficient.
As to TIG welding the part, I'd say if anything the area where the weld goes is actually harder when done than it was before you began. 4100 Series will go to around 47-50 max. 4300's go a good bit higher, maybe to 57 even. That's pretty hard. At 43, it's a challenge to even drill a hole in it, tapping is damn near impossible. And cutting is done with carbide or similar. Typically if you have a welded area on 4100 series steel, that area will eat tools. A drill will simply come out with a round end after it is burnt in place. Endmills really don't like that stuff, and sometimes even a file will skip over the surface. It's at the cost of being more brittle, but that's a relative term with 41xx and in the rear of the bolt, brittle isn't really a problem anyway.
Very little heat treatment is done by hand anymore. And in any case the terms are used differently all through the various industries.
Well, the terms are the same really, I use draw and normalize interchangeably even though technically that's two different processes that happen at the same time. I think for this discussion you get the idea.
Heat treat by hand is probably done more than you think. There's all sorts of reasons why it might be done less, most of which is that steels today are so tough and hard, you don't need to re-make the parts. Back when heat treat was done by hand for lots of this stuff, the parts broke cause the temps were not right. The steel wasn't as capable. They just didn't last. Now, the part outlives us if done properly.
I make cutters all the time at work. These are expected to live up to some serious requirements, and do. Millions of cuts on 1/16" stainless steel wire. Try that with a scissors. You do NOT make these parts making shortcuts. I did a set some time ago where I absolutely did not have one day to mess around, One broke and I needed product to go out. I heat treated the parts with a torch, drew them with a torch, and they lasted long enough that I had to sharpen them a couple times that day to finish the job. I then made parts with the same flat of steel, but this time did them properly. Correct temps, atmosphere, quenching. They've been in place for a year or more, and there's not a mark on em. The difference is night and day.
Yes, you can harden stuff with a torch, but there is a lot more to heat treat than just hardness. I'll guarantee you you won't get one of your A2 knives to 67Rc with a torch. The difference between an hour at 1725 and a few minutes at 1680 is night and day.
