Originally Posted by
Notso_fastLane
It's probably not the thickness of the wall/weld.
After welding any metal (Ti, Al, or steel), unless it's non-structural, and very very few exceptions, there needs to be a post weld heat treat/stress relief, or the weld process itself actually weakens the metal locally.
Not really. IME most steels are just allowed to cool. Some alloys need preheat and stress relief but though not uncommon it's not that common either, especially in structural steel such as plates and beams, even stainless. I'm talking about welding in industrial construction and maintenance including refineries, steel mills, water towers, smokestacks, storage tanks, chemical plants and both fossil and nuclear power plants.
Lots of high pressure ASME code work on pressure vessels and boilers. Welding to the highest standards, often 100% X-ray. Mostly stick and heli-arc. In all positions; lots of overhead and quite a bit of mirror and blind welding--looking through the root from the other side. You can't roll a boiler superheat section or an MSR in a nuke.
I don't know the aircraft industry. I assume it's stringent.