On Tue, 5 Mar 2002 00:52:29 -0800 Mark Bulgier <mark@bulgier.net> writes:
[ snip ]
>
> Cuevas was vocal about his belief that brass brazing was proper for
753. He
> must have sent Reynolds a silver-brazed sample to get certified, but
then he
> made all his frames (as far as I know) with brass. I have done
instrumented
> crush testing of frames, and I can tell you for sure that
> quenched-and-tempered low-alloy steels like Prestige and 753 can be
strong
> enough despite their light weight, if the heat is kept to a minimum.
In
> many cases the thinner heat-treated tube (753) was stronger than the
thicker
> normalized tube (531). But brass brazing on the thin unbutted part,
like
> the Cuevas shifter boss, is asking for trouble. The initial crack may
well
> have even started during brazing.
>
> Since the middle of the tube where you're sitting usually hasn't been
heated
> much, at most only a tiny cable housing guide that's almost certainly
> silvered on, it's safe to say: Garth, go ahead and sit on your top
tube.
> Just don't quote me out of context... ;^)
>
Hello Mark, and everyone:
I would like to add about $.02 worth to this discussion. Not about any particular tubing, but about the brazing/soldering process, and metalurgy. It has been my understanding (and I have studied metalurgy a little while at the University), that it's not so much the heating (unless one gets the metal really red-orange in color, and for a long period of time), but how quickly the metal is allowed to cool. It is better to let the heated area cool as slowly as possible, which allows the crystal structure of the metal to return to near it's original state. If there is a draft (not a beer, but a breeze) in the room, and the heat affected area cools too quickly, the crystal structure of the metal is locked (so to speak) in a heat altered state. The heat affected area will then have a different crystal structure (different size of crystals) than the area that hasn't been heated, and there will be a dividing line between the two different crystal structures, which is where the metal will crack or break. I learned to use anything available to shield the area from outside air, allowing the metal to cool as slow as possible. What makes the metal brittle is quick cooling, that's what you do to temper the stuff, you dip it in water or oil to lock the crystal structure into it's heat altered state.
I don't pretend to be an expert in the field of metalurgy, but this is some what I have retained throughout both my work and school experiences. If I am even slightly wrong, please feel free to correct me, so that others will not be misled.
Happy cycling (and sitting on top tubes),
"Bicycle Mark" Perkins Fresno Cycling Club - Historian Fresno, California, U.S.A.
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