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Not quite; tempering means to try to get some toughness back after the quench leaves the steel in a hard but brittle state. Tempering weakens the steel but if done right leaves the steel still stronger than it was before the whole quenched-and-tempered heat treating. The process has many variables they play with to get the properties they want. 753 almost certainly changed the particulars of its heat-treatment over the years, so you probably can't say anything categorically true for all 753.
I would say those elongation numbers are wrong, because not enough information is given for them to be really right. It depends a lot on the exact metallurgy, which can vary hugely within those categories. Say sticking within 4130, between annealed on one end, and quenched (not tempered) at the other extreme, you'd see a vast range of elongation numbers. Add in the effects of cold-working on top of the heat-treat factors, it gets complicated. I'm not saying I understand it!

Not exactly, though it is a good one to look at. But the real limits to what you can get away with there depend on other factors too, like the diameter-to-wall ratio, which when it's high leads to catastrophic crumpling failure. Imagine trying to put a smooth curve in a beer can, with no localized kinking. You may also need to deal with the rate at which the metal work-hardens as you indent it.

Jim Merz, who made more 753 frames than most anyone (in the US anyway, not counting Trek), told me you'd need to soften the steel in the vicinity of the indent. Sorry if I'm mis-remembering what he said, and I don't know if he said he actually tried that, or if he was conjecturing. Sounds do-able to me though, if you won't be too sad if it fails, and/or if the frame is worthless to you without the increased tire clearance. Heating it to roughly the silver-brazing temperature range and holding it there a while would temper it more, trading a loss of strength for more ductility. Heating it to red hot, roughly brass brazing temperature, then letting it cool naturally in room temperature still air, would result in "normalized" steel, which contrary to popular belief is actually a pretty decent combo of strength and ductility in these low-alloy steels. It is essentially turning 753 into 531 "as-delivered", though maybe with a bit less strength than 531 due to 531 having cold-work done to it after normalizing. Also there would be a relatively soft tempered zone around the fringes of the part you got red-hot. Counter-intuitive, but the part that only got to silver temperature. That over-tempered area will be the weakest part of the tube, more like the strength of 531 after brazing, which is weaker than as-delivered. Having that weak tempered zone of the HAZ can be OK if it falls out past any "features" like changes in thickness, such as you find at the end of a lug point, a bad place to put the weakest part of the HAZ. For example if there's a chainstay bridge near where you heated the stay to soften it, that discontinuity will likely be a stress-riser that will cause it to fatigue crack there. With such thin tubing, and not particularly strong compared to modern super-steels, fatigue is likely to end up being the frame's cause of death eventually.

Think hard about the shape of your indent also; there's a right way and a wrong way! In my opinion, you can't do much better than the indent that older Columbus SL/SP used, sort of a leaf or boat shape, with a fairly sharp crease down the middle. Normally we avoid sharp creases but in this case the sharp crease down the middle doesn't cause problems, and it leaves the steel in a pretty mellow shape bending-wise, elsewhere where it does matter. Many nice old bikes with indents like that survive from 70, 80 or more years ago. Look at the indents in a Barra aluminum, shaped like the SL indent, and Barras aren't known for breaking there. (They break, but somewhere else!) About the worst you can do is a transverse indent such as you get by hammering a pipe against the stay. A flat spot is pretty bad too — Reynolds tried that with the original shape of their 531 SL chainstays, many of which broke. Later 531SL with the round-oval-round chainstays are more reliable.

Let us know if you try it, how it turned out. A lot of people with 753 frames want to know the answer to your question. I indented a ton of Tange Prestige stays, a similar quenched-and-tempered low-alloy steel, but Prestige had a softer heat-treat (more tempered) than 753. I suspect Prestige chainstays may have been even softer than the main triangle, specifically to allow the builder to manipulate the shape.

Mark B