Silver soldering can be used on cast iron. The presence of carbon in the form of graphic can make proper bonding difficult. Heating, shot peening or chemical cleaners are possible methods of surface preparation.
Preparing the Surface
The main obstacle to silver soldering cast iron is carbon on the surface of the material. Normal procedures with flux will not remove the carbon. The oxidising part of flame -- outside the blue zone -- from a torch can be used to heat the metal to a red-hot state to burn off the carbon. After the metal has cooled, it can be cleaned with a stiff wire brush and treated in the normal manner
Basics of Silver Soldering
Silver soldering is sometimes referred to silver brazing or hard soldering. The process is done at temperatures above 420 degrees C, which is much hotter than soft soldering and produces much stronger joints due to the physical changes in metal. Silver soldering only fills in a small space between the two pieces being joined. The gap should be no more than a few microns wide. This allows the solder to flow into the gap by capillary action
Tips for Silver Soldering
The parts need to be at the same temperature to allow the solder to flow smoothly. If parts of two thickness are being joined, heat the thick part first. A strip of solder may be cut and placed on the lip of the edges to be joined to simplify the process. Quenching is not recommended as heat can be trapped in cavities and result in steam and possibly weaken the part.