In a message dated 3/5/02 6:18:34 PM Pacific Standard Time, bicyclemark@juno.com writes:
<< It has been my understanding (and I have studied metallurgy 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. >>
I have a mid 80s Columbus catalogue that I will quote from. On the second
page of text, under "Technology and Research" after very specific discussion
as to the alloys and what they do for the steel, "As a consequence, Columbus'
Cr-Mo steel loses a mere 7% mechanical strength after approximately 5 minutes
heating at 700 degrees C."
On the last page of text under "How to Make the Best Use of Columbus Tubes,
"3. As a general rule, the welding zone should not be overheated to prevent
steel embrittlement. The melting point of the brazing alloy should not exceed
650-700 degrees C." The recommended rods are "AG 9 BS 1845 - AG 10 BS 1845 -
AG 11 BS 1845 - LAG 50 CD ni DIN 8513 - LAG 40 CD ni 8513".
Brass flows between 825 and 850 degrees C. Silver flows around 650 degrees C.
Columbus clearly recommends the use of silver brazing temperatures and
alloys. My own experience is that when using a frame alignment table to cold
set frames that I built using silver braze, it was a lot of effort to move
them the very small amounts necessary for perfection. When cold setting
Italian and Belgian production frames, I was astounded at how little effort
it took to move them a lot! My experience in setting production frames is
limited to probably 20 something, not one of them provided the resistance to
setting that my own silvered frames did. This experience is more objective
than subjective and is not particularly theoretical, but has formed my
opinion that silver is vastly superior in practice. I fully allow for those
rare master brazers who can get it hot fast, flow the brass, and then cool it
slowly to get a frame that is superior to overheated production frames, but
I've never had one of their frames on an alignment table to check the feel.
The "lore" that went along with cycling in the 70s was that production frames
had a life expectancy of less than a year. The average top pro would use them
up and sell them off after just a few months. I wasn't there, so for me it's
hearsay, and I'd sure like to hear more about that!
Stevan Thomas
Alameda, CA