People often wonder about the temperatures used to differentiate brazing from soldering. Why 450°C (840°F)? Is there some significance to these "exact" numbers?
In reality, the answer is "no." The numbers selected have no specific metallurgical meaning other than that they represent a convenient demarcation point between the melting temperatures of zinc (420°C/787°F) and aluminum (660°C/1220°F).
A number of years ago, the definition of brazing specified a temperature of 800°F as the demarcation point between "brazing" above that temperature, and "soldering" below that temperature. But of course, since the world continues to "get smaller," so to speak, via closer international communications and via faster air travel, it was decided by the brazing community that the temperature for brazing really should be specified using standard metric terms, since metric was the world standard. In so doing, they agreed to declare that the "standard" minimum liquidus temperature for a BFM would now use a conveniently rounded-up metric number of 450°C, rather than the previously used 800°F (420°C), and in one quick step brazing jumped from 800°F up to 840°F (officially, 450°C). Of course, it's possible that the brazing community could have said: "Let's use 500°C as the demarcation temperature," and that would have probably been fine too.
Thus, this choice of temperature at which soldering ends and brazing begins is really just a number "pulled out of the air" – between the melting points of zinc and aluminum – to provide a basis for being able to differentiate between the two processes.
Brazing used to be known as "hard soldering," and "soft soldering" was the term used for what we merely call soldering today. In many places, especially among some of the older jewelry makers, the term hard soldering and soft soldering is still being used.
So now you know "the rest of the story" and hopefully understand that the temperatures used to separate soldering from brazing do not have anything to do with a specific metallurgical reaction occurring at 450°C (840°F) temperature, but it was merely agreed upon for the convenience of differentiating between the two processes.