Steel tubing usually gains ductility from overheating, as all cold-working is lost and the intercrystalline slip planes become larger. The development of overheat-cracks usually comes later, after the resulting coarser crystals spread fatigue cracks through the area of highest stress, over a great number of stress cycles. By then, even the annealed metal will appear to behave in a brittle fashion. In this way, increased ductility is associated with the development of cracks, even though a more ductile metal condition helps prevent stress cracking during any cold-working operation of manufacture or when exceeding the yield strength during severe abuse. Note that the yield strength is lower in annealed, ductile metal. Cracks resulting from slight crash deformations are due to both over-stress of the already-cold-worked metal and the subsequent cyclic overstress applied to areas left with uneven stress concentrations. The localized overstress indeed results from uneven residual stresses, only some of which are relieved (while others are created) during realignment operations. The metal remembers, and you can't fully stress-relieve a built lugged frame, so the frame's working life is shortened. Welded frames are often stress-relieved in their entirety through heat-treating, relieving undesireable residual stresses left from the welding. In some aluminum alloys, even a few steels, ductility may decrease from this process as alloying elements diffuse, making the metal stronger and harder, but this process must be carefully controlled in terms of temperature and duration, usually moreso than when a lugged frame is torched together.
David Snyder
Auburn, CA
> Isn't the key distinction here the magnitude of the bulge? The original
> post states the "slightest of bulges". My guess is that frames with a
> very slight bulge might never fail, while others with more extreme bulges
> are doomed to die if ridden hard. I've seen a number of frames with
> bulges that the owners never saw, that have been ridden for years and
> years without incident.
>
> Now when a crack starts (not just in the paint) that is the kiss of death.
> But the scarry thing is that if we all took a look at some of the bikes we
> owned really carefully, we would find things that we would rather not
> think about. And if you don't find distortion on many old frames from a
> crash, you may find distortion from overheating. That doesn't give much
> confidence either.
>
> Now the other thing to factor in is how the frame was built. A frame
> brazed at high temperature looses ductility and becomes more brittle -
> such a frame I suspect is much more likely to fail down the road. And a
> frame that has been brazed with silver and retains its ductility? Perhaps
> much less of an issue.
>
> Mike Kone in Boulder CO