Just read this on retrobike:
You can only respray/ oven bake a steel frame so many times before the steel turns to mush...
The reason:
The same heat treatment that helps give the steel it's strength can be overdone. The steel starts to lose it's zing.
Is baking frames for paint really that stressful to steel? I find it very hard to believe....
I think there's a point you can heat it up to where it starts to lose strength but I don't think people bake paint at that sort of temperature
I think the temperature you'd need to take it to for that to happen would blister the paint.
If that's the case why doesn't your oven & hob in the kitchen turn to mush also...
I have heard that each shot blast remives metal so the frame is a bit thinner each time
this was confirmd by some one on here who could quote the weight of there frame after blastng for 3 resprays
the heat thing looks like rubbish. I know a bit about materials. Either the process puts in enough heat to change the metal and knackers it or it doesn't. I think repeating the process is irrelevant
Nah. Heat treatment won't affect a tube's stiffness so it won't lose it's 'zing' or whatever they meant by that. It affects strength uts but paint process temps shouldn't be like heat treatments. I'm not a frame baker or metalurgist though.
I heard a story about pro roadies complaining to the team mechanics that the frames were losing stiffness as the season went on. He got them repainted, later the Pros were really happy with their 'new' frames.
Thinking out loud here, at work we often specify that steel (beams, columns etc) should not be burn cut in certain areas as the burning process can change the carbon content within the steel and make it (more ) brittle.
I can't see how the above could be applied to a baking process, other than the material used for the welds will be slightly different, which may cause cracking at these points due to each material cooling at different rates.
Could also be talking out of my arse.
The heat treatment thing is bulls**t, you'd need to get it a lot hotter than a paint drying oven to alter the temper, or any other mechanical properties of the metal.
As for 'shot' blasting... if the frame is media blasted with the correct blasting media, by someone who knows what they are doing it'll be fine. If it is shot-peened with ball bearings by a shaved monkey then it might not do it much good.
I'm no expert, but I do have an Engineering degree, and did quite a few modules on material science..
The temperatures used to bake a paint or powder coat finish onto a frame is MUCH lower than that used to heat treat a steel frame. I'd not worry about it myself...
I'm not a materials scientist but I have baked a cake before. Baking something for the second time makes it more brittle, see a cake vs a biscuit. Maybe this is what the retro bike post meant. A more biscuity frame.
Since when TF were steel frames heat treated? Alu yes, but steel?
(some) steel frames do get heat-treated
I had a frame fall apart after a respray and oven bake.
Mind you, it was a Raleigh Dynatech, and tbh the rewelding of the new back end probbly created enough heat to melt the glue. 
That was fun; built it all up, took it out for a test spin, got a few yards down the road -BONG! Downtube separated from the BB shell...
The process is called annealing, look it up on google / wiki. The sort of temps that frames are baked at will not have any effect on stregth (or "Zing" what ever that may be??).
It's a process used to "reset" the abnormal molecular structure which the steel may have been subject to in it's forming / finishing process. With regards frame building it will be standard process after welding.
If that's the case why doesn't your oven & hob in the kitchen turn to mush also...
how many drop-offs to flat have you done on your oven? 
No idea what temperature paint is baked at but I imagine it would be too low to have any significant effect on the properties of the steel.
I think TheBrick may be on to something with the biscuit thing though!
As I thought, no metallugists on STW tonight.... 
At least freeagent makes some sense though, thankyou.
Biscuits? Really? I reckon you just dry them out a bit more baking the second time. I'd be willing to bet this isn't a factor when heating a steel frame to temperatures required to bake paint 
.
how many drop-offs to flat have you done on your oven?
Like freeagent, not a metallurgist, but have an engineering degree with several modules on materials science. I agree with the general gist of this thread - temperatures involved in baking paint are far too low to have any affect on the material properties of steel (I'd go and look up relevant temperatures to prove my point, but just a quick think about orders of magnitude makes it clear that the temperature difference is so large as to make that a complete waste of time).
There are just so many myths about materials science and physics floating around in the cycling community that I'm actually inclined to disbelieve anything stated as well know fact about the science of cycling on a cycling forum.
Biscuits? Really?
Yes, Biscuits.
[img]http://www.the-biscuit-van.co.uk/images/biscuit-fox's-crinkles-butter.jpg[/img]
69er - MemberAs I thought, no metallugists on STW tonight...
we prefer the term 'materials scientist' these days...
materials scientist do it on different scales.
Nah, metallurgist sounds a bit more old school and proper!
I'll take a guess at this.
Assuming it is Cr-Mo steel then you'll be fine. The strength of this steel is based on the strengthening effects of Cr and Mo. I don't think there's any real heat treatment process used to gain the strength (hence why it is used in high temp creep resistant applications). Throwing it in an oven at <300degC won't do a thing.
Fanatic278 (metallurgist)
Nah, materials scientist is too handwavey and vague, even if that's what the degree title was. Metallurgist sets thoae of us who do proper stuff apart from the weirdos tinkering around with semiconductors and other such rubbish.
After a quench you'd look at 250ish degrees c for a stress relieving temper, that would be enough to drop the hardness by a few vickers and up the toughness a bit, nothing scary though. I'd doubt paint-baking would be much above 150-180. Enough to mess with precipitation hardening al grades if left long enough maybe, but steel should be reet, might even help if there was no post weld heat treat and it's not an air hardening grade like 853
Fanatic I think you'll find cr mo steels are heat treated. I think its the difference between reynolds 525 and 725, or some other number combination. Out of interest what area of metallurgy do you work in.
I have a physics degree with a few materials modules.
A quick check on Wikipedia reveals 4130 is normalized (above roughly 800degC). There might be some post weld heat treatment, but that would still be about 650degC.
Cr is the main strengthening element, stabalised by the Mo (to prevent forming Cr carbides). As long as the temp is less than 300degC there won't be a difference.
Degree in materials engineering to be precise. But spent most of my career as a welding engineer in the oil & gas industry. Still had to revert to Wikepedia and my old lecture notes though!
As an engineer I'd be happy enough with freeagents explanation.
I did work with a "metallurgist" a couple of years ago... he was a shocking engineer and a bit of a cock. I much prefer materials scientists or applied physicists for sensible answers to tricky engineering questions....
I'm not doupting for a moment that a paint bake of the frame would change anything
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