Heat treating does not increase the stiffness of steel. All steel no matter the alloy or heat treat has the same modulus of elasticity (it can change slightly with high percentage alloys). Heat treating increases the tensile strength and hardness. Your 2nd point is also incorrect. Annealing does make steel softer, but it does not increase the fatigue limit. Think of a steel spring, very high tensile material, very high fatigue limit. Tubing used for a bike frame should be considered a spring.
Your comment on glass has no bearing with this discussion. Glass is plastic with no crystillyn structure. Most steel is made of crystals. Bike frames do not anneal during riding. Even if they did the stiffness would not change. What does happen during use is very small cracks can start forming. After so many flexes these cracks can grow and then the tube will fail. At some point in time when the crack goes clear through the tube the frame stiffness will decrease. In no other case will the stiffness decrease from riding!
Jim Merz Big Sur CA
On Tue, Jul 6, 2010 at 12:34 PM, donald gillies <firstname.lastname@example.org> 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:
> The opposite of Quenching metal is annealing, i.e. heating metal to a
> high temperature and then slowly cooling it back to normal
> temperature. That reorganizes the grain structure making the frame
> less-stiff but more resilient and increasing the fatigue limit.
> 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, you can see ripples and lumps in the
> glass. This is not from a poor manufacturing process 100 years ago,
> in fact, 100 years ago the glass was perfectly smooth, but in the
> intervening 100 years the glass has "gone soft", i.e. it has
> integrated enough thermal vibrations into its grain structure to melt
> slightly and change shape, i.e. it has annealed, slightly.
> The same thing is happening to Reynolds 753 all the time. The more
> heat the frame gets, the more likely the frame will 'anneal'. In
> fact, the degree of annealing is probably equal, almost, to the
> integral of all the heat the frame has absorbed since it was
> originally heat-treated.
> The conclusion is inescapable. First of all, don't buy any Reynolds
> 753 frames from Texas or Arizona! And secondly, when not in use, you
> should store your reynolds 753 bicycle in a refrigerated meat locker!
> Nothing less preserve the ride characteristics of your frame and keep
> it from going soft !!! :-) :-)
> - Don :-) Gillies
> San Diego, CA, USA