Kitfox III, bent tube

[jrevens]

Dave has brought up some very good points... I'd like to expand on them a little if I may. I have both TIG & oxy/acet equipment, & used to be pretty competent with both. I don't do enough welding anymore to be really good, but I know people who are. Here are some comments from a friend who is -

"Way back in the dinosaur days when gas welding was the norm and TIG was trying to get a foot hold, TIG got a bad rap because folks were using 4130 rod and it cracked. The reason the TIG welds were cracking and the gas welds were not was because there is a lot more heat in gas welding and the 4130 tubing, as well as the 4130 filler would pretty much get annealed in the process, thereby defeating the benefit of using Cro-Moly tubing in the first place.

Today, the current thinking for not only aircraft, but anything including Cro-Moly tubing weldments, including NHRA top fuel as well as NASCAR is: Filler rod, ER70s-2 or slightly better ER80s-D2 mild steel TIG rod, and the use of the absolute least amount of heat needed to form a puddle. The rule of thumb is blue/yellow weld, if it's grey it's too hot. A minimum of 70 deg. F ambient temp. and dead still air. And no post weld heating, none. Old plans that call out 4130 rod get ignored."

The "no post weld heating" comment refers (I believe) specifically to the process he talks about in his second paragraph. I believe, especially with TIG or MIG welds and most especially with cluster welds and truss structures like we have in our aircraft, that post heating (with a flame) to relieve stresses is an often neglected but important process. The lack of that is what causes a welded structure to "spring" from it's desired shape when removed from a jig. Some of this built-in stress is also a potential crack inducer. We don't see that quite so much with a gas-welded structure since material furthur away from the actual weld is heated more. That's what I think I know about the subject.