Butted tubes are annealed several times in the butting process, there's a limit to how much cold working a tube can take, as cold work results in work hardening, so Reynolds selects particularly ductile grades of steel for butted tubes. The problem with the graph is that is shows only one spot for HAZ. Fusion welding is not a directly comparable process, and may affect what kinds of temperatures the HAZ point of reference was seeing. One would expect that it would show a state similar to as-quenched. Otherwise the heat treatment schedule of 631/853 is pretty much a mystery due to being proprietary.

631/853 will air harden if heated in excess of 850C. While Reynolds currently says that silver at 700C has a reduced propensity to anneal 853, the problem here is that air cooling grades of steel basically don't anneal in normal air. They must be cooled slowly and gradually, generally in a temperature controlled furnace, as to not quench the steel. Normal steels require the rapid thermal conduction of a quench medium like oil or water, and will anneal in air or a bucket of sand as an insulator. We often say anneal, but what we really mean here is temper. Once you reach anneal/quench temperature (brass brazing/welding), the original heat treatment is basically completely lost. Below this temperature is tempering which merely softens the heat treatment. What we're really concerned with here is tempering. Reynolds doesn't give us much on the heat treatment schedule since it's proprietary, however temperatures below 850C will soften 853.

While you might be able to effectively reduce 725 to 525 with a torch, 631 gets bumped up to something not quite 853 when heated to austenitization temps. Annealing or normalizing 853 into 631 would normally be a many hours long process of gradual cooling in a furnace. The trick here is to destroy the heat treatment with excessive tempering, which will reduce hardness and strength, but should also increase ductility a significant degree. While with a heat treated non-air-hardening steel like 753, you could go above 700C, and overtemper it and getting your certification rejected by Reynolds, then go even higher to 900C (brass) and anneal it with air cooling, do brass temperatures with 631 and it will harden with air cooling. Softening 853 without a heat treating furnace requires tempering as hot as possible without causing austenitization or even partial austenitization. Basically bringing it up past silver temps, but below bronze temps. Even then, it's uncertain how much crimping you can get away with. It's going to be 853 with a soft temper, not normalized/annealed 631.