Keith,
The thing to remember is if you melt the base metal then you are going to have to follow a 'procedure' to deal with the cast iron. It is full of carbon that will move around and change state...if it is heated enough to melt.
To get around all this metallurgy, you merely repair it without melting the base material...you braze it. It gets hot, but it doesn't become molten, that's the key.
Stuart
