Originally Posted by
WizardOfBoz
If your bike can take joints that are only 30% of the strength of base metal, you don't need to heat treat. But because modern bike frames must already be beefy enough to keep stresses low to avoid fatigue failure, no modern frame will have the added mass to allow it to survive without proper heat treatment. As the OP found. I agree with the aerospace welder: the original bike welder may have done a great job (its certainly aesthetically adequate) and was let down by his heat-treating colleagues.
...with respect, take a look at the wall thickness of the aluminum tubing in the original pictures. Rad Bikes doesn't care about frame weight savings, because their products are all electric bikes.
The portion of the seat tube above the top tube is not an especially high stress area if properly designed and the seat post fit is a proper slip fit the length of the seat tube. Ideally, all it does is provide someplace to hang the clamp that keeps the seat post from slipping down or up in the tube once you have proper positioning. The stresses ought to be, properly, taken mainly by the seat post, and distributed along the length of the seat tube that are in contact with it.
Which is why you have so many of us questioning what is going on mechanically. It doesn't much matter how well you heat treat the frame if there is a constant rocking back and forth of the post and saddle, and the bulk of those forces are absorbed by that poor little aluminum alloy collar...it's gonna break off in the manner seen in the pictures.