Monkeylad writes:
>
> What I think you're describing is annealing, referred to sometimes
> as "cooking the tubes" for the layman, and is used to allow the grain
> structure of the metal to return to the state it was in before it was
> formed into tubing.
Not quite; annealing makes the steel as soft as it can be, which may or may
not be the state it was in previously. And I wouldn't call annealing
"cooking the tubes", because the main damage from high temperatures is
unrelated to annealing - oxidation, brass inclusions, intermetallic
compounds being the big problems I believe - though I've never actually
experience overheated steel myself ;^) Grain growth and recrystallization
happen at a certain rate, that is faster at higher temperatures but is never
instantaneous, so time-at-temperature can be as important as the peak temp
the steel was subjected to, and cooling rate can be more important still.
> The after affects of annealing is "softer" and
> more "brittle" tube.
I assume this is a typo and you meant "less brittle", or "more ductile",
right?
> As I understand
> it annealing happens with steel between 1400-1800 degrees. This
> would explain why it happens more in brass brazed frames.
Not quite; it's the cooling rate (namely, very slow) from above the transformation temperature (henceforth "TT") that defines annealing. If the steel never got above the TT (as with good silver brazing), then it can't be annealed, no matter what the cooling rate. If it did get above the TT, (roughly, brass-brazing temps), then fast cooling (quench) results in hard, brittle steel, and slow cooling results in soft, ductile steel. Fully annealed requires much slower cooling than any bike joint could ever see; it's done in a heat-treating oven. Important to note: the inherent stiffness (Young's Modulus) is not changed in any case.
Also good to remember: Steel quenched from above the TT can be much stronger than steel that was silver brazed and never reached TT. Bike tubing is thin enough that the air-quench and the self-quench (heat conducting to the cool steel outside the joint) of brass-brazed joints can result in a reasonably strong steel, often stronger than the tempered steel in a silver-brazed joint. Wait, did I just say brass-brazed frames are stronger than silver-brazed? No, because if you think about it, every brass-brazed joint has an area outside the joint that only got to silver-brazing temperatures, the "tempered zone" that is weaker than the brass-brazed part. And this tempered zone is larger and weaker for brass-brazed frames than for silver. But ironically it's the silver-brazing temperature that made the brass frame weaker than the silver frame!
If you've seen lots of crashed and/or fatigue-cracked frames, you may have noticed that brass frames often fail as much as a centimeter away from the lug edge, because that's where the steel only got up to silver brazing temps. Crashed silver frames usually buckle right at the edge of the lug, because their tempered zone is almost completely inside the lug.
A very quickly brass-brazed joint can be better in every way than a "cooked" silver joint, so we can't say for sure that silver is always better. I do believe it usually is at least a bit more durable though. Still, if built by a good brass brazer, a frame will last essentially forever for most riders unless it is built out of tubing that is too thin and flexible. If the tubing is thick enough for adequate stiffness, and that makes it thick enough that brass joints will last forever for a given rider, then can we really say that silver joints would have been better? Remember the ride is unchanged, since Young's modulus is a constant for all heat treatments of a given steel.
I hope I put enough "weasel words" (usually, most, etc.) in there to leave room for the exceptions: extremely high-mileage and/or extremely strong riders can break even good frames, and some people like light, flexible frames made of thin tubing. Silver brazed frames are also more repairable if they do fail - much easier to sweat the old tube out without overheating anything. I like silver. Confentes and Herses are brass brazed though, right? (Somebody will correct me if I got that wrong but there's plenty more examples of truly great brass-brazed frames.)
I hope if you remember one thing about this, that it's "It ain't that simple!" Heat treating of steel is a huge field that even the experts don't know everything about.
Mark Bulgier
Seattle, Wa
USA