Well done Mark, bang on I think.
Reynolds 531 was steel with *main* alloying agents in 5:3:1 ratio (Manganese,Molybdenum,Silica). It is allowed to cool slowly so is soft. 753 is the same metal but cooled rapidly by immersion in oil, it is much harder (Higher Ultimate Tensile Stress), so the tubes are made thinner, giving the same frame strength for less weight. So 753 is just thin 531 slam dunked in oil to cool.
The price... In gaining strength 753 loses ductility, so when shock loaded tends to snap, rather than streach as 531 would. Thats why, I'm told, you can't cold set you rear dropouts from say 126mm to 130mm if it's 753 as it snaps when yanked.
During brazing the hot torch/braze reheats the tubes (over the HAZ)which then cool slowly. Thats why lower melting pt silver braze and quick torch work is needed so the 753 does not revert to 531 (with very thin walls). Damaged 753 often can't be repaired because the torch re-heats the tubes, weakening them too much.
I'm told a 753 tubeset is a mix, forks are always 531 because 753 is just too risky on the fork. Also, when Holdsworth started using 753 on the Professional the team suffered from snaps. Jock Kerr snapped a frame (right chainstay I think) during a late 70's Milk Race and was not a happy camper. You 'll get some techy stuff on the reynolds website.
Norman (I'll suffer the weight of 531) Glasgow
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