If there is oxidation film on the surface of molten lead in time of Brazing, its wetting is not to be good and uneven. So it promotes the wetting and flowing by removing oxidation film using Flux.
Flux Type Basic Material Package Unit (Kg)
HF1A Paste Cu alloy, Carbon steel, Low alloy steel (Low temp.)  
HF6A Paste Cu alloy, Carbon steel, Low alloy steel(high temp.) 0.5, 1
HF7A Paste Ni-Cr steel, Low alloy steel, STEEL, Carbon steel, Cu alloy 0.5