Important: The 450C (840F) refers only to the published “liquidus” temperature of the filler metal. What’s that? By definition, the “liquidus temperature” of a metal is the temperature at which, when being cooled, that liquid metal will begin to solidify. Thus, at temperatures higher than the “liquidus” temp of any metal, that particular metal should still be liquid, i.e., in its molten state. It should be free-flowing.
Looking, for instance, at pure copper, it has a liquidus temp of 1083C/1981F (please note that those temps are exact, and should be stated as such). When it is heated to a temperature above its liquidus the copper metal should be fully molten, and, if it is being used as a brazing filler metal (BFM), it could then flow into a joint that needs to be brazed together. At temperatures lower than its liquidus, the copper would not be fully liquid, and thus would not be able to flow.
Note: Remember, the “liquidus” temperature is a temperature, and not a “state of being” (liquid, solid, etc.). I often hear people say: “When the filler metal went liquidus…..” No, that is wrong! That person SHOULD have said: “When that filler metal became liquid after it was heated above its liquidus temp….” Please use the term correctly.
Recommendation: Always heat the brazing filler metal (BFM) to a temperature higher than its liquidus, and try to add enough extra heat to help make that liquid-metal even more free-flowing, so that it can easily move into a joint being brazed. For all metals other than aluminum I like to suggest that the BFM be heated to about 100F (50C) above its published liquidus. This will allow enough extra heating to make the BFM quite fluid so that it can easily flow into a joint being brazed. You cannot use this same temp range for aluminum, otherwise, you might melt the base metals, since the melting points of the aluminum BFMs are fairly close to the melting points of the aluminum base metals being joined.
Please remember that all thermocouples, furnace instruments, BFM compositions, etc., have “tolerances” to them, which must be taken into account. I remember one person who complained that he couldn’t get the BFM to flow, even though, according to his furnace charts, he had heated the furnace to almost 5F above the published liquidus temp of the BFM. He didn’t understand that he needed to take all these tolerances into account, and thus, needed to braze at a temp much higher than the published liquidus temp to ensure that the BFM would indeed flow as desired.
Important Thing To Remember From This: The temperature used in the definition of brazing is merely defining the physical melting characteristic of a brazing filler metal (BFM), and is not saying or suggesting anything about recommended brazing temps that someone should use when they are brazing. It is merely telling you that the actual brazing filler metal (BFM) must have a “liquidus” temp above 450C (840F) in order to be classified as a brazing filler metal.