John in CR wrote:Sounds like bogus 4130 to me, and/or you got 4140, which does require heat treatment. Get real stuff from an aircraft supply and use that larger diameter and double the wall thickness and be done with it.
If you already have the tubing, then shape the joint differently and/or use a reinforcing sleeve cut at an angle or V to spread the stress and heat related weakness along the axis of the boom instead of perpendicular to it.
Well, i *do* buy my 4130 from an aircraft supply house. The "V" cut reinforcing sleeve is a good idea - I have used perpendicular cut reinforcing sleeves before, and they just moved the stress around to somewhere else. The more I read about MIG welding 4130, and I am just beginning to understand this, the more I begin to think that I either need to preheat the joint or heat treat it afterward. Experts on welding forums either say "Use TIG on 4130, forget MIG" or "Yeah, you can use MIG on 4130 if you do this and that" However, the exact recipe for this and that is different every time I read about it.
Yes, I have gone to lengths to add reinforcement to joints. One of the joints (that failed, eventually) had a short internal tube welded onto the head tube, then the pedal boom was welded over that, so it functioned like the old cast lugs on frames. I can't remember if that one failed at the head tube, or just ahead of the reinforcement.
This has only been an issue with joints that are taking a lot of tension and repeated flex. The rest of the frame has always held together if they were a part of a triangulated assembly. I have a part on my new project that will be in the same kind of stress - which is reason #2 I am trying to learn more about this issue.