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I use a torch on stuck bolts all the time. But there's no need to quench them; just heat the bolt head cherry red and put a wrench on it. The bolt will come out like it's screwed into warm butter. In fact, the bolt will get harder to turn as it cools off. You might think that there's a risk of twisting off the hot bolt, but I've never had that happen. I did this trick on some lug bolts that were rusted about halfway through. I tried taking the first one out cold and it twisted off like it was made of clay. Then I got out the hot wrench and removed the rest intact. Generally speaking, you should replace any bolt that you heat, since it will become annealed. But usually you're taking out a bolt that you would have destroyed anyway. I've also used the trick on seized disk brake bleeders. There's no way to heat the caliper, since it's full of fluid, not to mention the seals. Heat up the bleeder to cherry red, grab it with Vise Grips and it will come right out. Watch out for scalding hot brake fluid, though. Heating bolts works for aluminum as well as cast iron. I removed a broken screw out of an intake manifold using a small brazing tip and Vise Grips. I can't imagine any other way I could have extracted it.
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