Joining Cast Iron
The main difficulty associated with silver soldering cast iron is due to the presence of carbon on the surface which prevents the alloy "wetting" onto the iron. It must be removed. The flux is not capable of doing so.
The best technique is to heat the cast iron up to a red heat with the oxidizing part of the flame (outside the blue zone!) This burns the carbon from the surface.
Allow it to cool.
Clean the components with a stiff wire brush.
Braze as normal using HT5 flux.