torch - when to use, when to avoid?

[Bruno_300TE]

Hi everybody
Working on old cars can be frustrating, especially when dealing with frozen bolts, bushings, bearings etc. Often a torch comes handy in such situations: just heat up till the nuts, bolts and bushings etc come off easily. However, I wonder, when would you stay away from the torch because the integrity of the material may suffer? Does a wheel carrier degrade when heated up with a acetylene torch in order to get a frozen ball joint off?
Are there any golden rules to follow?
Any thoughts are welcome!
Bruno

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1992 300TE 137'000 miles