Don Gillies wrote:
> Well, I don't believe that 531 tubing can 'get soft', but In theory
> a heat-treated frame like 753 could possibly 'get soft', and here's
> why:
>
> My understanding is that 753 tubing was just thin 531 tubing that
> has been heat-treated, i.e. heated to high temperature an then
> quenched to rapidly freeze the grain structure in the random pattern
> obtained at high temperature, thereby stiffening the tubing:
>
> Well, it turns out that the frame is annealing itself all the time,
> from the heat at room temperature. If you have ever looked at a
> piece of glass that is 100 years old, [snipped]
Don, I don't think that's right. I'm not a metallurgist but I did have some Strength of Materials course work and lab work long ago and it focused primarily on steels. I learned that there is a threshold temperature (more than one actually but it's complicated), below which no tempering occurs. (I do believe the word you're looking for is tempering not annealing, by the way.)
Do you have evidence of this slow room-temperature tempering happening in steel? The reference to glass is not convincing, the materials are quite different. Steel has phases in different crystal structures (body-centered cubic and face-centered cubic) that are stable at different temperatures, and there's a Transformation Temperature that must be crossed. It's binary: either you're above the TT or you're not.
Also sorry if it seems like quibbling but when you say the heat-treatment stiffens the tube, that is definitely not true. The inherent stiffness of the material, the modulus of elasticity or Young's Modulus, remains constant for all heat treat states. It is a common myth that 753, Prestige et. al. are stiffer. In the same wall thickness and diameter they are exactly the same stiffness as non-heat-treated 531; but since they were typically sold in a thinner gauge, they are in fact less stiff than the normal non-heat-treated tubes.
Mark Bulgier
Seattle, WA USA